WEBVTT 00:00:00.000 --> 00:00:02.475 align:middle line:90% [SQUEAKING] 00:00:02.475 --> 00:00:04.455 align:middle line:90% [RUSTLING] 00:00:04.455 --> 00:00:06.435 align:middle line:90% [CLICKING] 00:00:06.435 --> 00:00:16.415 align:middle line:90% 00:00:16.415 --> 00:00:19.220 align:middle line:84% ANA BELL: All right, let's get started. 00:00:19.220 --> 00:00:23.980 align:middle line:84% So today's lecture is lecture three out of four 00:00:23.980 --> 00:00:26.170 align:middle line:84% on the idea of object-oriented programming 00:00:26.170 --> 00:00:30.250 align:middle line:84% and creating our own object types through Python classes. 00:00:30.250 --> 00:00:31.750 align:middle line:84% The majority of today's lecture will 00:00:31.750 --> 00:00:33.130 align:middle line:90% be on this idea of inheritance. 00:00:33.130 --> 00:00:35.470 align:middle line:84% But before we get there, I'd like 00:00:35.470 --> 00:00:37.870 align:middle line:84% to do a little bit of a recap of the big things 00:00:37.870 --> 00:00:39.880 align:middle line:90% that we've seen already. 00:00:39.880 --> 00:00:42.460 align:middle line:84% And along the way, we'll be writing 00:00:42.460 --> 00:00:45.790 align:middle line:84% a new data type for something more abstract, 00:00:45.790 --> 00:00:48.850 align:middle line:84% an animal class, more abstract than what we've seen before. 00:00:48.850 --> 00:00:51.110 align:middle line:84% But then, when we get to the idea of inheritance, 00:00:51.110 --> 00:00:52.840 align:middle line:84% we'll build upon this animal class 00:00:52.840 --> 00:00:56.030 align:middle line:90% with some more animal objects. 00:00:56.030 --> 00:01:00.260 align:middle line:84% So the big idea behind creating our own data types 00:01:00.260 --> 00:01:03.290 align:middle line:84% is that we want to mimic what's going on in real life, right? 00:01:03.290 --> 00:01:05.300 align:middle line:84% And in real life, we basically have 00:01:05.300 --> 00:01:09.200 align:middle line:84% all of these different objects in the world, right? 00:01:09.200 --> 00:01:12.470 align:middle line:84% But these objects can be grouped according 00:01:12.470 --> 00:01:13.790 align:middle line:90% to some categories, right? 00:01:13.790 --> 00:01:17.180 align:middle line:84% So in this particular slide, I've got six different objects. 00:01:17.180 --> 00:01:19.340 align:middle line:84% But the three on the left can kind of 00:01:19.340 --> 00:01:20.660 align:middle line:90% be grouped together, right? 00:01:20.660 --> 00:01:23.510 align:middle line:84% We know that they are a kind of cat. 00:01:23.510 --> 00:01:26.960 align:middle line:84% And as such, we know that all these cats, 00:01:26.960 --> 00:01:29.390 align:middle line:84% we can describe them using some common properties 00:01:29.390 --> 00:01:30.480 align:middle line:90% and common behaviors. 00:01:30.480 --> 00:01:34.070 align:middle line:84% So for these cats, I would say that all these cats 00:01:34.070 --> 00:01:37.530 align:middle line:84% have a name an age and a color associated with them, right? 00:01:37.530 --> 00:01:40.672 align:middle line:84% So I know that all cats will therefore generally 00:01:40.672 --> 00:01:42.380 align:middle line:84% have a name and age and a color, and then 00:01:42.380 --> 00:01:45.290 align:middle line:90% some similar set of behaviors. 00:01:45.290 --> 00:01:48.110 align:middle line:84% The items on the right, those three objects, 00:01:48.110 --> 00:01:49.940 align:middle line:84% I know they can be categorized together. 00:01:49.940 --> 00:01:51.470 align:middle line:84% Let's say that they're wild rabbits. 00:01:51.470 --> 00:01:53.178 align:middle line:84% And let's say that, for the wild rabbits, 00:01:53.178 --> 00:01:54.660 align:middle line:84% I don't actually give them a name. 00:01:54.660 --> 00:01:56.940 align:middle line:84% So I would categorize them, again, 00:01:56.940 --> 00:02:02.100 align:middle line:84% using common properties, just an age and a color, no name. 00:02:02.100 --> 00:02:04.290 align:middle line:84% And then those three objects on the right 00:02:04.290 --> 00:02:06.180 align:middle line:84% also have a common set of behaviors 00:02:06.180 --> 00:02:08.440 align:middle line:84% different than the objects on the left. 00:02:08.440 --> 00:02:12.460 align:middle line:84% And so we're trying to mimic the idea of these categorizations 00:02:12.460 --> 00:02:15.510 align:middle line:84% and data types that we see in real life. 00:02:15.510 --> 00:02:21.570 align:middle line:84% OK, so a little bit of a recap, when we define our own data 00:02:21.570 --> 00:02:25.600 align:middle line:84% type in Python, we decide on a bunch of attributes. 00:02:25.600 --> 00:02:29.430 align:middle line:84% And attributes can either be data or they can be procedures. 00:02:29.430 --> 00:02:34.320 align:middle line:84% The data is, you think of them as what variables make up 00:02:34.320 --> 00:02:35.070 align:middle line:90% your object. 00:02:35.070 --> 00:02:36.780 align:middle line:84% And this is something that you decide. 00:02:36.780 --> 00:02:38.160 align:middle line:84% So for a coordinate object, we've 00:02:38.160 --> 00:02:40.470 align:middle line:84% seen this example a lot of times, 00:02:40.470 --> 00:02:42.450 align:middle line:90% we decided on x and y values. 00:02:42.450 --> 00:02:44.940 align:middle line:84% For this more abstract idea of an animal, 00:02:44.940 --> 00:02:47.760 align:middle line:84% well, we can just say that we can describe 00:02:47.760 --> 00:02:50.580 align:middle line:84% an animal by its age, so how long 00:02:50.580 --> 00:02:54.160 align:middle line:90% it's been alive since birth. 00:02:54.160 --> 00:02:55.990 align:middle line:84% In terms of procedural attributes, 00:02:55.990 --> 00:03:00.100 align:middle line:84% these we implemented using methods in Python classes. 00:03:00.100 --> 00:03:02.800 align:middle line:84% And the idea behind these is just, 00:03:02.800 --> 00:03:04.840 align:middle line:84% how can somebody or somebody who's 00:03:04.840 --> 00:03:07.510 align:middle line:84% creating an object of this type manipulate the object? 00:03:07.510 --> 00:03:10.340 align:middle line:84% What are some ways to interface with this object? 00:03:10.340 --> 00:03:13.850 align:middle line:84% So our coordinate class, one of the more interesting things 00:03:13.850 --> 00:03:15.830 align:middle line:84% was to find a distance between a coordinate 00:03:15.830 --> 00:03:16.850 align:middle line:90% and another coordinate. 00:03:16.850 --> 00:03:19.160 align:middle line:84% But some of the simpler things were 00:03:19.160 --> 00:03:22.160 align:middle line:84% to just get the value of the x-coordinate, the y-coordinate, 00:03:22.160 --> 00:03:23.270 align:middle line:90% and things like that. 00:03:23.270 --> 00:03:26.180 align:middle line:84% For our animal class that we're implementing today, 00:03:26.180 --> 00:03:28.440 align:middle line:84% it's going to be a little bit more abstract. 00:03:28.440 --> 00:03:32.105 align:middle line:84% But one of the simplest things is to just say, hey, tell me 00:03:32.105 --> 00:03:33.230 align:middle line:90% how long you've been alive. 00:03:33.230 --> 00:03:36.680 align:middle line:84% That's basically just grabbing the value 00:03:36.680 --> 00:03:41.090 align:middle line:84% of the attribute, the H. So here, 00:03:41.090 --> 00:03:45.230 align:middle line:84% we're defining our data object, right? 00:03:45.230 --> 00:03:48.080 align:middle line:84% The class keyword tells Python we're 00:03:48.080 --> 00:03:49.370 align:middle line:90% creating our new data type. 00:03:49.370 --> 00:03:51.150 align:middle line:84% This is the name of our data type. 00:03:51.150 --> 00:03:55.130 align:middle line:84% So the type of this thing that we're creating is Animal. 00:03:55.130 --> 00:03:59.960 align:middle line:84% In parentheses here, animal's parent 00:03:59.960 --> 00:04:02.690 align:middle line:90% is the generic Python object. 00:04:02.690 --> 00:04:04.370 align:middle line:84% And later in today's lecture, we're 00:04:04.370 --> 00:04:06.328 align:middle line:84% going to see what happens when we put something 00:04:06.328 --> 00:04:07.560 align:middle line:90% else in those parentheses. 00:04:07.560 --> 00:04:10.160 align:middle line:84% So the parent of a class that we create 00:04:10.160 --> 00:04:13.360 align:middle line:84% is something other than just the generic Python object. 00:04:13.360 --> 00:04:15.490 align:middle line:84% And then the very first method that we always 00:04:15.490 --> 00:04:18.940 align:middle line:84% write in our new object definition is the init method. 00:04:18.940 --> 00:04:20.470 align:middle line:84% This tells Python, how do you create 00:04:20.470 --> 00:04:24.640 align:middle line:84% an object of this type, a very basic information 00:04:24.640 --> 00:04:26.230 align:middle line:90% that Python needs to know. 00:04:26.230 --> 00:04:29.530 align:middle line:84% So the init method is a special dunder method, 00:04:29.530 --> 00:04:32.550 align:middle line:84% double underscore, init, double underscore. 00:04:32.550 --> 00:04:34.240 align:middle line:90% And by now, you're familiar. 00:04:34.240 --> 00:04:36.390 align:middle line:84% The first parameter of every single method 00:04:36.390 --> 00:04:39.970 align:middle line:84% that we define inside a class is this thing called self. 00:04:39.970 --> 00:04:45.250 align:middle line:84% And remember, self is a variable, right? 00:04:45.250 --> 00:04:50.530 align:middle line:84% It's a variable name that allows us to talk about an object 00:04:50.530 --> 00:04:53.410 align:middle line:84% without having created one yet because all we're doing here 00:04:53.410 --> 00:04:55.240 align:middle line:90% is defining the class, right? 00:04:55.240 --> 00:04:57.530 align:middle line:84% We don't have actual objects created. 00:04:57.530 --> 00:04:59.830 align:middle line:84% And so this method here, the init method 00:04:59.830 --> 00:05:01.300 align:middle line:84% and all the other methods, are run 00:05:01.300 --> 00:05:02.830 align:middle line:90% on an object of type animal. 00:05:02.830 --> 00:05:04.970 align:middle line:84% But we don't have that object yet. 00:05:04.970 --> 00:05:09.190 align:middle line:84% So the first parameter will be that object 00:05:09.190 --> 00:05:11.470 align:middle line:90% in this abstract sort of way. 00:05:11.470 --> 00:05:15.410 align:middle line:84% And then you can put other parameters in that list. 00:05:15.410 --> 00:05:18.160 align:middle line:84% And so we say that, when we create a new animal object, 00:05:18.160 --> 00:05:22.800 align:middle line:84% we're going to initialize it by its age, so some number. 00:05:22.800 --> 00:05:26.040 align:middle line:84% Within the init, what do we usually do? 00:05:26.040 --> 00:05:30.750 align:middle line:84% Well, we usually initialize all the data attributes, also 00:05:30.750 --> 00:05:32.950 align:middle line:90% called instance variables. 00:05:32.950 --> 00:05:35.040 align:middle line:84% So here, how many data attributes 00:05:35.040 --> 00:05:39.750 align:middle line:90% do I have for the animal class? 00:05:39.750 --> 00:05:41.430 align:middle line:90% Two, exactly, yeah, two. 00:05:41.430 --> 00:05:45.000 align:middle line:84% The first one, self.age, is a data attribute. 00:05:45.000 --> 00:05:46.440 align:middle line:84% And we know it's a data attribute 00:05:46.440 --> 00:05:49.710 align:middle line:84% because we have that self appearing again, right? 00:05:49.710 --> 00:05:53.490 align:middle line:84% If it was just a variable name, like age or years 00:05:53.490 --> 00:05:56.100 align:middle line:84% or time or something like that, it would just 00:05:56.100 --> 00:05:57.630 align:middle line:90% be a regular old variable. 00:05:57.630 --> 00:05:59.670 align:middle line:84% And as soon as that init method ended, 00:05:59.670 --> 00:06:01.270 align:middle line:90% that variable would go away. 00:06:01.270 --> 00:06:03.930 align:middle line:84% But the fact that we've initialized this variable 00:06:03.930 --> 00:06:06.510 align:middle line:84% using self. tells Python, hey, this 00:06:06.510 --> 00:06:09.900 align:middle line:84% is a variable that I want to persist for as long 00:06:09.900 --> 00:06:12.120 align:middle line:90% as this object exists in memory. 00:06:12.120 --> 00:06:14.640 align:middle line:90% So it's an instance variable. 00:06:14.640 --> 00:06:16.740 align:middle line:84% So self.age equals age will create this data 00:06:16.740 --> 00:06:21.210 align:middle line:84% attribute, age, and assign it to the parameter passed in age. 00:06:21.210 --> 00:06:24.330 align:middle line:84% Now, self.name is also a data attribute. 00:06:24.330 --> 00:06:26.850 align:middle line:84% It's just not being passed in the parameter list. 00:06:26.850 --> 00:06:27.780 align:middle line:90% And that's OK. 00:06:27.780 --> 00:06:31.510 align:middle line:84% Not everything has to be passed into the parameter list. 00:06:31.510 --> 00:06:33.960 align:middle line:84% So here, what we're effectively doing 00:06:33.960 --> 00:06:36.150 align:middle line:84% is saying, when we create a new animal object, 00:06:36.150 --> 00:06:39.840 align:middle line:84% we have to tell it the age, how long it's been alive for. 00:06:39.840 --> 00:06:43.900 align:middle line:84% But then the name data attribute is always going to be none. 00:06:43.900 --> 00:06:47.700 align:middle line:84% So there's an absence of a value for the name for every animal 00:06:47.700 --> 00:06:49.860 align:middle line:90% we create right off the bat. 00:06:49.860 --> 00:06:54.095 align:middle line:84% OK, everyone OK with me so far here? 00:06:54.095 --> 00:06:56.720 align:middle line:84% AUDIENCE: What's the purpose of defining self.name equals None? 00:06:56.720 --> 00:06:58.790 align:middle line:84% ANA BELL: What's the purpose of defining this to be none? 00:06:58.790 --> 00:07:00.873 align:middle line:84% Well, later on, I'm going to add some methods that 00:07:00.873 --> 00:07:03.740 align:middle line:84% allow you to give a name to an animal if you'd like. 00:07:03.740 --> 00:07:07.230 align:middle line:84% But again, this is a design choice that I made. 00:07:07.230 --> 00:07:11.990 align:middle line:84% So yeah, you might not make the same design choices. 00:07:11.990 --> 00:07:14.590 align:middle line:84% So that's the definition for my class, 00:07:14.590 --> 00:07:16.360 align:middle line:90% just these four lines of code. 00:07:16.360 --> 00:07:20.440 align:middle line:84% And then down here, we saw, in the past couple lectures, 00:07:20.440 --> 00:07:22.660 align:middle line:84% how to create actual new objects, right? 00:07:22.660 --> 00:07:27.700 align:middle line:84% So this is where the action actually happens, right? 00:07:27.700 --> 00:07:30.340 align:middle line:84% So here, I'm creating a new animal object. 00:07:30.340 --> 00:07:34.600 align:middle line:84% A variable, myanimal, is bound to that animal object, right? 00:07:34.600 --> 00:07:36.310 align:middle line:90% So that's my variable name. 00:07:36.310 --> 00:07:37.880 align:middle line:84% You can name it anything you want. 00:07:37.880 --> 00:07:41.150 align:middle line:84% And then you're telling Python to create a new animal 00:07:41.150 --> 00:07:43.720 align:middle line:84% object simply by invoking the name of the class, 00:07:43.720 --> 00:07:45.820 align:middle line:84% and then passing in all the parameters 00:07:45.820 --> 00:07:49.390 align:middle line:84% that we're expecting here, except for self because self 00:07:49.390 --> 00:07:51.790 align:middle line:90% becomes this thing. 00:07:51.790 --> 00:07:54.550 align:middle line:84% If I were to draw a box around Animal(3), that is the self. 00:07:54.550 --> 00:07:58.670 align:middle line:84% That is this object that I just created. 00:07:58.670 --> 00:08:01.100 align:middle line:90% OK, so that's the init method. 00:08:01.100 --> 00:08:03.560 align:middle line:84% Last lecture, we saw some dunder methods. 00:08:03.560 --> 00:08:06.770 align:middle line:84% And I think I said, probably the second method 00:08:06.770 --> 00:08:08.810 align:middle line:84% you'd ever want to implement for a new class 00:08:08.810 --> 00:08:12.030 align:middle line:90% is this dunder str method. 00:08:12.030 --> 00:08:15.170 align:middle line:84% Now, the dunder str method tells Python 00:08:15.170 --> 00:08:21.230 align:middle line:84% how Python should print an object of type animal, right? 00:08:21.230 --> 00:08:22.940 align:middle line:84% Because initially, right off the bat, 00:08:22.940 --> 00:08:25.850 align:middle line:84% if we didn't implement this dunder str method, 00:08:25.850 --> 00:08:30.320 align:middle line:84% Python would default to the str method of the generic Python 00:08:30.320 --> 00:08:32.510 align:middle line:84% object, which just tells us the memory 00:08:32.510 --> 00:08:35.039 align:middle line:84% location this object has been created at, 00:08:35.039 --> 00:08:36.500 align:middle line:90% which is not very useful. 00:08:36.500 --> 00:08:39.919 align:middle line:84% When we print an animal object, and again, my design choice 00:08:39.919 --> 00:08:44.720 align:middle line:84% is to say I'm going to print animal, colon, 00:08:44.720 --> 00:08:47.930 align:middle line:84% the name of that animal, colon, and the age of that animal, 00:08:47.930 --> 00:08:49.970 align:middle line:90% again, my design choice, right? 00:08:49.970 --> 00:08:51.740 align:middle line:84% And remember, the dunder str method 00:08:51.740 --> 00:08:53.930 align:middle line:84% returns the string you want to print out. 00:08:53.930 --> 00:08:58.940 align:middle line:84% It doesn't print it out straight up. 00:08:58.940 --> 00:09:00.297 align:middle line:90% Everyone OK with that so far? 00:09:00.297 --> 00:09:01.130 align:middle line:90% It should be review. 00:09:01.130 --> 00:09:03.740 align:middle line:84% OK, so then the other things that we 00:09:03.740 --> 00:09:06.170 align:middle line:84% want to include in our class, and this is something 00:09:06.170 --> 00:09:10.070 align:middle line:84% that I included no matter what the language you're 00:09:10.070 --> 00:09:14.810 align:middle line:84% working with, is these things called getters and setters. 00:09:14.810 --> 00:09:17.900 align:middle line:84% So getters are these two right here. 00:09:17.900 --> 00:09:20.420 align:middle line:84% Getters are basically very simple functions 00:09:20.420 --> 00:09:23.480 align:middle line:84% that return the values of the data 00:09:23.480 --> 00:09:26.090 align:middle line:90% attributes that this object has. 00:09:26.090 --> 00:09:29.420 align:middle line:84% This object just has two, an age and a name, 00:09:29.420 --> 00:09:32.220 align:middle line:84% because they were defined using self.age and self.name. 00:09:32.220 --> 00:09:36.360 align:middle line:84% So here's a getter to just tell me the value of self.age. 00:09:36.360 --> 00:09:38.570 align:middle line:84% So all it does is return self.age. 00:09:38.570 --> 00:09:43.910 align:middle line:84% And name, all it does is return self.name, very, very simple. 00:09:43.910 --> 00:09:48.800 align:middle line:84% Setters, same idea, except that now, we're allowing someone 00:09:48.800 --> 00:09:51.830 align:middle line:84% using our class to set the values of these data 00:09:51.830 --> 00:09:55.040 align:middle line:84% attributes through these methods, right? 00:09:55.040 --> 00:09:57.620 align:middle line:84% So here, all it's doing is taking 00:09:57.620 --> 00:09:59.960 align:middle line:84% in a parameter for the thing you want to change 00:09:59.960 --> 00:10:01.910 align:middle line:90% the age or the name to, right? 00:10:01.910 --> 00:10:04.580 align:middle line:84% And all it does is say, well, self.age 00:10:04.580 --> 00:10:09.420 align:middle line:84% is going to be equal to the thing you passed in, OK? 00:10:09.420 --> 00:10:10.660 align:middle line:90% That's the age. 00:10:10.660 --> 00:10:13.020 align:middle line:84% So we're changing this to a different number. 00:10:13.020 --> 00:10:16.080 align:middle line:84% And then the set name is changing the name data 00:10:16.080 --> 00:10:17.890 align:middle line:90% attribute to a different string. 00:10:17.890 --> 00:10:21.090 align:middle line:84% And here, I'm using this default parameter 00:10:21.090 --> 00:10:25.530 align:middle line:84% that we talked about way back when we 00:10:25.530 --> 00:10:26.970 align:middle line:90% talked about functions, right? 00:10:26.970 --> 00:10:29.475 align:middle line:84% So if you don't pass in an actual string value, 00:10:29.475 --> 00:10:31.890 align:middle line:84% we'll default to the empty string. 00:10:31.890 --> 00:10:34.470 align:middle line:90% 00:10:34.470 --> 00:10:38.320 align:middle line:84% So let me show you how this works. 00:10:38.320 --> 00:10:41.190 align:middle line:84% So this is my animal class exactly as in the slides. 00:10:41.190 --> 00:10:44.840 align:middle line:84% I've got my init str and my two getters and setters. 00:10:44.840 --> 00:10:47.420 align:middle line:84% And then I've got two animals being created here, right? 00:10:47.420 --> 00:10:50.990 align:middle line:84% So here's a print for animal with age four. 00:10:50.990 --> 00:10:53.000 align:middle line:84% And here's a print of animal with age six. 00:10:53.000 --> 00:10:56.240 align:middle line:84% So if I run these, it should print animal, colon, none, 00:10:56.240 --> 00:10:58.910 align:middle line:84% because I didn't set the name to anything for these two, 00:10:58.910 --> 00:11:01.700 align:middle line:84% and then their respective ages, right? 00:11:01.700 --> 00:11:06.080 align:middle line:84% So this is using the str method on a. 00:11:06.080 --> 00:11:09.878 align:middle line:84% And this is using the str method on b. 00:11:09.878 --> 00:11:13.940 align:middle line:84% OK, and then, we can access, of course, 00:11:13.940 --> 00:11:16.560 align:middle line:84% using dot notation, all of our data attributes. 00:11:16.560 --> 00:11:19.070 align:middle line:84% So here, I'm accessing the age directly. 00:11:19.070 --> 00:11:21.800 align:middle line:84% But since the getter, get_age, just 00:11:21.800 --> 00:11:24.050 align:middle line:84% returns for me the value of that data attribute, 00:11:24.050 --> 00:11:25.920 align:middle line:84% these will actually print the same thing. 00:11:25.920 --> 00:11:27.720 align:middle line:84% So I'm just going to comment these out. 00:11:27.720 --> 00:11:30.920 align:middle line:84% So if I'm accessing a's age through either the data 00:11:30.920 --> 00:11:33.200 align:middle line:84% attribute directly or through the getter method, 00:11:33.200 --> 00:11:38.850 align:middle line:84% it will print four for both, pretty straightforward. 00:11:38.850 --> 00:11:41.070 align:middle line:84% And then we can do some things like this. 00:11:41.070 --> 00:11:43.590 align:middle line:84% So I can call the set_name method. 00:11:43.590 --> 00:11:46.080 align:middle line:84% So here, I'm passing an actual name for it. 00:11:46.080 --> 00:11:50.480 align:middle line:84% And then I can print the name or I can use the getter 00:11:50.480 --> 00:11:52.040 align:middle line:90% to print the name, right? 00:11:52.040 --> 00:11:54.470 align:middle line:84% So if I run that, you'll see, the name 00:11:54.470 --> 00:11:57.930 align:middle line:84% has now been changed for object a. 00:11:57.930 --> 00:12:01.300 align:middle line:84% Then, if I run the print method on a, then it prints animal, 00:12:01.300 --> 00:12:03.640 align:middle line:84% colon, the new name that I just set it to, fluffy. 00:12:03.640 --> 00:12:06.880 align:middle line:84% And then the age has been unchanged. 00:12:06.880 --> 00:12:09.340 align:middle line:84% If I run set_name without a parameter, 00:12:09.340 --> 00:12:11.680 align:middle line:84% it will revert to that default parameter 00:12:11.680 --> 00:12:14.610 align:middle line:84% for the name, which is the empty string. 00:12:14.610 --> 00:12:18.028 align:middle line:84% So the new name of my animal a will just be an empty string. 00:12:18.028 --> 00:12:19.820 align:middle line:84% So it's just going to be colon with nothing 00:12:19.820 --> 00:12:23.630 align:middle line:84% in there, so no space or anything, just nothing. 00:12:23.630 --> 00:12:26.210 align:middle line:90% 00:12:26.210 --> 00:12:29.700 align:middle line:90% OK, everyone all right so far? 00:12:29.700 --> 00:12:32.900 align:middle line:90% Hopefully, a little review. 00:12:32.900 --> 00:12:38.840 align:middle line:84% So we saw that we can actually grab the exact same value 00:12:38.840 --> 00:12:44.090 align:middle line:84% for the age by accessing the age data attribute directly 00:12:44.090 --> 00:12:48.350 align:middle line:84% using dot notation or our getter that we wrote. 00:12:48.350 --> 00:12:53.660 align:middle line:84% One of these is better than the other in terms of style 00:12:53.660 --> 00:12:58.010 align:middle line:84% and in terms of good coding practices 00:12:58.010 --> 00:13:01.080 align:middle line:84% and in terms of writing code that's easy to read, 00:13:01.080 --> 00:13:04.400 align:middle line:84% easy to modify, robust, things like that. 00:13:04.400 --> 00:13:08.000 align:middle line:84% The one that is better to use is the one 00:13:08.000 --> 00:13:10.220 align:middle line:90% that accesses the method. 00:13:10.220 --> 00:13:11.750 align:middle line:90% Both are using dot notation. 00:13:11.750 --> 00:13:15.350 align:middle line:84% But the first one is actually accessing the internals 00:13:15.350 --> 00:13:19.940 align:middle line:90% of my class definition, right? 00:13:19.940 --> 00:13:22.850 align:middle line:84% In order to know the value of my data attribute 00:13:22.850 --> 00:13:25.610 align:middle line:84% as someone who's just using this code for an animal class, 00:13:25.610 --> 00:13:29.840 align:middle line:84% I'd have to actually go in and read the init method 00:13:29.840 --> 00:13:32.582 align:middle line:84% to know these data attributes that are being initialized. 00:13:32.582 --> 00:13:33.540 align:middle line:90% I don't know about you. 00:13:33.540 --> 00:13:36.290 align:middle line:84% But I actually-- let's take an example of a list, something 00:13:36.290 --> 00:13:37.310 align:middle line:90% we've used a lot. 00:13:37.310 --> 00:13:41.150 align:middle line:84% Have you ever gone into the definition of the list class 00:13:41.150 --> 00:13:43.910 align:middle line:84% to see the data attributes that are being initialized? 00:13:43.910 --> 00:13:45.230 align:middle line:90% I haven't, right? 00:13:45.230 --> 00:13:47.720 align:middle line:84% All we've been doing is working with methods 00:13:47.720 --> 00:13:50.990 align:middle line:84% that allow us to make changes to lists, to do operations 00:13:50.990 --> 00:13:52.320 align:middle line:90% on lists, and things like that. 00:13:52.320 --> 00:13:55.480 align:middle line:84% So the internal workings of the list class is hidden from us. 00:13:55.480 --> 00:13:57.230 align:middle line:84% And that's just the way we like it, right? 00:13:57.230 --> 00:14:00.020 align:middle line:84% I don't care how the list is actually implemented. 00:14:00.020 --> 00:14:02.660 align:middle line:84% And the same thing should happen here, right? 00:14:02.660 --> 00:14:06.030 align:middle line:84% I shouldn't care how I implement the animal class. 00:14:06.030 --> 00:14:12.850 align:middle line:84% I shouldn't care what instance variables they're using. 00:14:12.850 --> 00:14:15.400 align:middle line:90% So let me show you why. 00:14:15.400 --> 00:14:17.430 align:middle line:84% So if someone who's writing the animal class 00:14:17.430 --> 00:14:21.120 align:middle line:84% decides in the future that age was a strange variable name 00:14:21.120 --> 00:14:26.250 align:middle line:84% to use and they decide to change the variable associated 00:14:26.250 --> 00:14:29.400 align:middle line:84% with how long this animal has been alive to, to be years, 00:14:29.400 --> 00:14:30.150 align:middle line:90% right? 00:14:30.150 --> 00:14:36.330 align:middle line:84% So here, I've got self.years equals age. 00:14:36.330 --> 00:14:40.650 align:middle line:84% That's the only change I've made to my animal class. 00:14:40.650 --> 00:14:43.800 align:middle line:84% So I've made the design decision to change this data attribute 00:14:43.800 --> 00:14:45.000 align:middle line:90% to be years. 00:14:45.000 --> 00:14:48.090 align:middle line:84% And then, of course, since I'm making this class, 00:14:48.090 --> 00:14:50.160 align:middle line:84% I need to make sure all my getters and setters 00:14:50.160 --> 00:14:52.900 align:middle line:84% and everything still works with this new data attribute. 00:14:52.900 --> 00:14:57.660 align:middle line:84% So my get_age will return self.years, right? 00:14:57.660 --> 00:15:00.090 align:middle line:84% I'm returning this variable, a data 00:15:00.090 --> 00:15:02.840 align:middle line:90% attribute that I've changed. 00:15:02.840 --> 00:15:06.450 align:middle line:90% Well, this is the full code. 00:15:06.450 --> 00:15:09.290 align:middle line:84% So you can see the changed data attribute here. 00:15:09.290 --> 00:15:11.480 align:middle line:90% I'm using self.years equals age. 00:15:11.480 --> 00:15:16.580 align:middle line:84% And then my getter is going to return self.years. 00:15:16.580 --> 00:15:20.280 align:middle line:84% And my setter is going to set self.years. 00:15:20.280 --> 00:15:24.210 align:middle line:84% Well, this implementation should be 00:15:24.210 --> 00:15:27.720 align:middle line:84% hidden from me, somebody who is just 00:15:27.720 --> 00:15:31.040 align:middle line:84% trying to create a bunch of animals in their code. 00:15:31.040 --> 00:15:37.300 align:middle line:84% So this code down here will work if I use my method, right? 00:15:37.300 --> 00:15:40.480 align:middle line:84% Because the method should still work no matter 00:15:40.480 --> 00:15:43.960 align:middle line:84% what the data attribute is called, age or years 00:15:43.960 --> 00:15:45.920 align:middle line:90% or time or whatever. 00:15:45.920 --> 00:15:49.770 align:middle line:84% But if I had code that accessed that data attribute directly, 00:15:49.770 --> 00:15:50.820 align:middle line:90% it doesn't work anymore. 00:15:50.820 --> 00:15:53.660 align:middle line:84% It throws an error because, surprise, that 00:15:53.660 --> 00:15:56.720 align:middle line:90% attribute no longer exists. 00:15:56.720 --> 00:16:03.370 align:middle line:84% So it's much better style and more 00:16:03.370 --> 00:16:08.080 align:middle line:84% robust to use only getters and setters, only methods, 00:16:08.080 --> 00:16:10.750 align:middle line:84% to make changes and to manipulate the objects. 00:16:10.750 --> 00:16:12.340 align:middle line:84% You should never ever really have 00:16:12.340 --> 00:16:17.130 align:middle line:84% to use the data attributes, right? 00:16:17.130 --> 00:16:18.420 align:middle line:90% Questions about that? 00:16:18.420 --> 00:16:21.130 align:middle line:90% 00:16:21.130 --> 00:16:23.860 align:middle line:84% OK, good, because that's something 00:16:23.860 --> 00:16:27.610 align:middle line:84% that you'll have to keep in mind on the quiz next Monday, 00:16:27.610 --> 00:16:30.200 align:middle line:90% not using data attributes. 00:16:30.200 --> 00:16:37.320 align:middle line:84% All right, so having said that, Python 00:16:37.320 --> 00:16:40.000 align:middle line:84% does allow you to do a bunch of questionable stuff. 00:16:40.000 --> 00:16:43.020 align:middle line:84% So, first of all, it allows you, as we just 00:16:43.020 --> 00:16:45.420 align:middle line:84% saw, to access the data attribute 00:16:45.420 --> 00:16:48.190 align:middle line:84% of a particular instance that you create. 00:16:48.190 --> 00:16:49.240 align:middle line:90% So you create an object. 00:16:49.240 --> 00:16:51.750 align:middle line:84% It's a very specific animal with a specific age. 00:16:51.750 --> 00:16:55.860 align:middle line:84% You can just use dot notation to access the value of all 00:16:55.860 --> 00:16:57.960 align:middle line:90% of these data attributes. 00:16:57.960 --> 00:17:03.210 align:middle line:84% Fine, we'll mess ourselves up in the future 00:17:03.210 --> 00:17:04.920 align:middle line:90% because maybe this won't work. 00:17:04.920 --> 00:17:06.640 align:middle line:90% But it's not so bad. 00:17:06.640 --> 00:17:10.109 align:middle line:84% However, Python also allows you to change 00:17:10.109 --> 00:17:12.960 align:middle line:84% the value of a data attribute outside of the class 00:17:12.960 --> 00:17:14.109 align:middle line:90% definition. 00:17:14.109 --> 00:17:17.339 align:middle line:84% So this is code we write not within the class. 00:17:17.339 --> 00:17:20.440 align:middle line:84% It's code we write as somebody who's using the class. 00:17:20.440 --> 00:17:21.730 align:middle line:90% So what does this mean? 00:17:21.730 --> 00:17:25.260 align:middle line:84% Well, now, I'm going to set the age data attribute 00:17:25.260 --> 00:17:28.830 align:middle line:84% to be whatever I want outside the class definition, right? 00:17:28.830 --> 00:17:33.215 align:middle line:84% I could even set it to a dictionary if I wanted to. 00:17:33.215 --> 00:17:34.840 align:middle line:84% In this particular case, I'm setting it 00:17:34.840 --> 00:17:36.220 align:middle line:90% to a string infinite. 00:17:36.220 --> 00:17:40.690 align:middle line:84% But if I do this, then I risk code on this animal class 00:17:40.690 --> 00:17:42.880 align:middle line:84% not working further on because maybe they 00:17:42.880 --> 00:17:45.310 align:middle line:84% assume that the age is always a number. 00:17:45.310 --> 00:17:47.170 align:middle line:84% And so a different method I might run 00:17:47.170 --> 00:17:50.175 align:middle line:84% will not work anymore if I happen to set it to the string. 00:17:50.175 --> 00:17:52.830 align:middle line:90% 00:17:52.830 --> 00:17:55.560 align:middle line:84% And then one other thing Python actually allows you to do 00:17:55.560 --> 00:18:00.270 align:middle line:84% is to add data attributes to instances. 00:18:00.270 --> 00:18:04.100 align:middle line:84% So now, the problem with this is that, let's say 00:18:04.100 --> 00:18:06.470 align:middle line:84% I create a whole bunch of instances of animals, right? 00:18:06.470 --> 00:18:07.850 align:middle line:90% This animal's got age four. 00:18:07.850 --> 00:18:09.110 align:middle line:90% This animal's got age six. 00:18:09.110 --> 00:18:10.700 align:middle line:90% This animal's got age five. 00:18:10.700 --> 00:18:13.880 align:middle line:84% And then one of these animals, I decide to add a new data 00:18:13.880 --> 00:18:14.630 align:middle line:90% attribute to it. 00:18:14.630 --> 00:18:16.320 align:middle line:84% Like, only one of these instances 00:18:16.320 --> 00:18:19.700 align:middle line:84% now has three data attributes associated with it, a name, 00:18:19.700 --> 00:18:21.605 align:middle line:90% an age, and now the size. 00:18:21.605 --> 00:18:24.110 align:middle line:90% 00:18:24.110 --> 00:18:26.390 align:middle line:84% All the other animal instances I've created only 00:18:26.390 --> 00:18:28.310 align:middle line:84% have a name and an age associated with them. 00:18:28.310 --> 00:18:31.470 align:middle line:84% Just this one happens to have this extra data attribute. 00:18:31.470 --> 00:18:34.580 align:middle line:84% So now, the whole reason why we're creating our own data 00:18:34.580 --> 00:18:36.530 align:middle line:90% types was to be consistent. 00:18:36.530 --> 00:18:38.870 align:middle line:84% To bundle the specific set of data 00:18:38.870 --> 00:18:42.230 align:middle line:84% and specific set of behaviors together flies out the window 00:18:42.230 --> 00:18:45.770 align:middle line:84% because now I have one instance that now has this extra data 00:18:45.770 --> 00:18:47.420 align:middle line:90% attribute associated with it. 00:18:47.420 --> 00:18:49.340 align:middle line:90% And nobody else does, right? 00:18:49.340 --> 00:18:54.860 align:middle line:84% So all that consistency has gone out the window. 00:18:54.860 --> 00:19:00.020 align:middle line:84% So never ever do any of these outside of the class 00:19:00.020 --> 00:19:00.600 align:middle line:90% definition. 00:19:00.600 --> 00:19:03.530 align:middle line:84% It's totally OK to access data attributes while you're 00:19:03.530 --> 00:19:07.130 align:middle line:84% defining a class, but not OK to do 00:19:07.130 --> 00:19:09.170 align:middle line:84% any of these outside of the class definition, 00:19:09.170 --> 00:19:12.380 align:middle line:84% even though Python allows you to do it. 00:19:12.380 --> 00:19:14.570 align:middle line:84% OK, so one of the things I wanted 00:19:14.570 --> 00:19:16.760 align:middle line:84% to show you in this lecture is something 00:19:16.760 --> 00:19:18.080 align:middle line:90% we haven't really seen so far. 00:19:18.080 --> 00:19:21.530 align:middle line:84% And that's actually just working with objects that we create. 00:19:21.530 --> 00:19:23.930 align:middle line:84% Yes, when we created fractions and coordinates, 00:19:23.930 --> 00:19:25.670 align:middle line:84% we just created a whole bunch of objects 00:19:25.670 --> 00:19:30.080 align:middle line:84% and then printed the numerators or printed the object 00:19:30.080 --> 00:19:31.760 align:middle line:90% or multiplied them together. 00:19:31.760 --> 00:19:35.120 align:middle line:84% But we never actually wrote nice functions that 00:19:35.120 --> 00:19:37.370 align:middle line:90% work with objects of our type. 00:19:37.370 --> 00:19:41.040 align:middle line:84% So one of the things I wanted to show you is how to do that. 00:19:41.040 --> 00:19:44.330 align:middle line:84% So here's a function that creates 00:19:44.330 --> 00:19:47.070 align:middle line:90% a dictionary out of a list. 00:19:47.070 --> 00:19:52.590 align:middle line:84% So the input here is going to be a list of whatever I want. 00:19:52.590 --> 00:19:56.610 align:middle line:84% And the function, what I would like it to do 00:19:56.610 --> 00:20:01.440 align:middle line:84% is to pick up from the list only numbers that are 00:20:01.440 --> 00:20:04.360 align:middle line:90% non-negative and just integers. 00:20:04.360 --> 00:20:06.690 align:middle line:84% So in this particular case, I would like my function 00:20:06.690 --> 00:20:10.960 align:middle line:84% to pick up the 2, the 5, and the 0, ignoring everything else. 00:20:10.960 --> 00:20:13.420 align:middle line:84% And I would like to create a dictionary out 00:20:13.420 --> 00:20:14.830 align:middle line:90% of these numbers. 00:20:14.830 --> 00:20:17.020 align:middle line:84% And what the dictionary should do 00:20:17.020 --> 00:20:19.340 align:middle line:84% is map each one of these numbers, 00:20:19.340 --> 00:20:22.340 align:middle line:90% so the 2, the 5, and the 0. 00:20:22.340 --> 00:20:24.260 align:middle line:90% These would be my keys. 00:20:24.260 --> 00:20:34.180 align:middle line:84% And they should be mapped to animal objects with these ages. 00:20:34.180 --> 00:20:37.360 align:middle line:90% So that's an animal of age two. 00:20:37.360 --> 00:20:39.850 align:middle line:84% And this is an animal with age five. 00:20:39.850 --> 00:20:43.990 align:middle line:84% And this should be an animal with age zero, right? 00:20:43.990 --> 00:20:47.380 align:middle line:90% So my key types are ints. 00:20:47.380 --> 00:20:50.290 align:middle line:84% And the values associated with the keys, 00:20:50.290 --> 00:20:54.410 align:middle line:84% the type should be animal, this object that I just created, 00:20:54.410 --> 00:20:54.910 align:middle line:90% all right? 00:20:54.910 --> 00:20:57.710 align:middle line:84% So the code is pretty straightforward. 00:20:57.710 --> 00:21:00.490 align:middle line:84% We just have a little loop that goes through each element one 00:21:00.490 --> 00:21:01.780 align:middle line:90% at a time in my list. 00:21:01.780 --> 00:21:04.270 align:middle line:84% That's for n and L. And then I'm just 00:21:04.270 --> 00:21:06.730 align:middle line:84% going to do something to the elements that 00:21:06.730 --> 00:21:11.530 align:middle line:84% are integers and greater or equal to zero, non-negative. 00:21:11.530 --> 00:21:14.590 align:middle line:84% So that will extract only the 2, the 5, and the 0 00:21:14.590 --> 00:21:18.220 align:middle line:84% as we go through the loop over the elements in L. 00:21:18.220 --> 00:21:23.210 align:middle line:84% And then the key line here is this one in red. 00:21:23.210 --> 00:21:28.370 align:middle line:84% I'm going to say, this line just adds an entry to my dictionary, 00:21:28.370 --> 00:21:29.150 align:middle line:90% right? 00:21:29.150 --> 00:21:31.702 align:middle line:84% So this is the syntax for putting something 00:21:31.702 --> 00:21:32.660 align:middle line:90% in a dictionary, right? 00:21:32.660 --> 00:21:35.630 align:middle line:84% There's no append or plus in a dictionary or anything 00:21:35.630 --> 00:21:36.390 align:middle line:90% like that. 00:21:36.390 --> 00:21:38.780 align:middle line:90% It's just straight up indexing. 00:21:38.780 --> 00:21:43.640 align:middle line:84% The key you want is n, so either a 2, a 5, or a 0. 00:21:43.640 --> 00:21:46.460 align:middle line:84% And the value I want to associate with that key 00:21:46.460 --> 00:21:51.050 align:middle line:84% is an animal with age whatever this is, 2, 5 or 0, 00:21:51.050 --> 00:21:54.320 align:middle line:90% so exactly what I wrote here. 00:21:54.320 --> 00:21:56.590 align:middle line:90% Everyone OK so far? 00:21:56.590 --> 00:21:59.970 align:middle line:84% All right, the loop goes through to the end of the list. 00:21:59.970 --> 00:22:01.830 align:middle line:84% And then we've created our dictionary. 00:22:01.830 --> 00:22:03.940 align:middle line:90% And we're done. 00:22:03.940 --> 00:22:06.340 align:middle line:84% As we're writing this code, how would we debug it 00:22:06.340 --> 00:22:08.410 align:middle line:84% or how would we check to see that it worked? 00:22:08.410 --> 00:22:11.110 align:middle line:84% Well, the instinct is to say, OK, well, 00:22:11.110 --> 00:22:12.950 align:middle line:84% let me check to see if this function worked. 00:22:12.950 --> 00:22:19.030 align:middle line:84% So here, this line, animals equals animal_dict(L) will run 00:22:19.030 --> 00:22:21.420 align:middle line:90% this function. 00:22:21.420 --> 00:22:24.480 align:middle line:84% And it runs it on this L. And at the end, 00:22:24.480 --> 00:22:28.810 align:middle line:84% it returns a dictionary, something that looks like this. 00:22:28.810 --> 00:22:34.230 align:middle line:84% So our instinct is to just print that return to dictionary. 00:22:34.230 --> 00:22:37.140 align:middle line:84% But if we were to print that, and you can actually 00:22:37.140 --> 00:22:39.450 align:middle line:84% run the code in Python, if you print that, 00:22:39.450 --> 00:22:41.590 align:middle line:90% you get something like this. 00:22:41.590 --> 00:22:43.380 align:middle line:84% And that's because Python doesn't 00:22:43.380 --> 00:22:46.050 align:middle line:84% dig through elements of dictionaries 00:22:46.050 --> 00:22:51.540 align:middle line:84% or even elements of lists to run the print method recursively. 00:22:51.540 --> 00:22:53.760 align:middle line:84% It just runs the print method top level. 00:22:53.760 --> 00:22:58.350 align:middle line:84% And the problem is, it knows how to print integers just fine. 00:22:58.350 --> 00:23:00.930 align:middle line:84% But it doesn't know how to print a dictionary where 00:23:00.930 --> 00:23:03.210 align:middle line:90% the values are animal objects. 00:23:03.210 --> 00:23:05.280 align:middle line:84% And so we run into the same problem where, 00:23:05.280 --> 00:23:07.620 align:middle line:84% now, the value associated with key 2 00:23:07.620 --> 00:23:10.290 align:middle line:84% is this animal object at that memory location. 00:23:10.290 --> 00:23:14.130 align:middle line:84% But how do I know that I didn't screw up my-- 00:23:14.130 --> 00:23:16.950 align:middle line:84% I created an animal with age five 00:23:16.950 --> 00:23:20.440 align:middle line:84% where it should have been two, right? 00:23:20.440 --> 00:23:22.900 align:middle line:84% So the solution, and you'll probably 00:23:22.900 --> 00:23:26.950 align:middle line:84% encounter this on the next quiz if you're debugging your code, 00:23:26.950 --> 00:23:32.350 align:middle line:84% the solution is to just iterate through the dictionary in such 00:23:32.350 --> 00:23:34.900 align:middle line:84% a way that you run that print statement directly 00:23:34.900 --> 00:23:36.580 align:middle line:90% on an object of type animal. 00:23:36.580 --> 00:23:38.140 align:middle line:84% Python knows how to do that, right? 00:23:38.140 --> 00:23:40.960 align:middle line:84% We told it the str method, right? 00:23:40.960 --> 00:23:43.040 align:middle line:90% We have an str method here. 00:23:43.040 --> 00:23:46.030 align:middle line:84% So it knows how to run the print directly on an animal object. 00:23:46.030 --> 00:23:48.460 align:middle line:84% It just doesn't know how to run the print where 00:23:48.460 --> 00:23:51.260 align:middle line:84% the value of a dictionary is an animal object. 00:23:51.260 --> 00:23:54.970 align:middle line:84% So let's replace this print of the dictionary 00:23:54.970 --> 00:23:57.230 align:middle line:90% with a little loop. 00:23:57.230 --> 00:24:02.270 align:middle line:84% It goes through this, the dictionary's items, right? 00:24:02.270 --> 00:24:04.040 align:middle line:90% So n is going to be my key. 00:24:04.040 --> 00:24:07.590 align:middle line:84% And a is going to be the value associated with that key. 00:24:07.590 --> 00:24:09.660 align:middle line:84% And I've just got the print statement here. 00:24:09.660 --> 00:24:12.630 align:middle line:84% So I'm using an F string here that prints key 00:24:12.630 --> 00:24:15.240 align:middle line:84% and whatever value that key is with val, 00:24:15.240 --> 00:24:18.180 align:middle line:90% whatever value that is, right? 00:24:18.180 --> 00:24:20.940 align:middle line:84% So now, the print statement is being run directly 00:24:20.940 --> 00:24:23.160 align:middle line:90% on an object of type, animal. 00:24:23.160 --> 00:24:26.580 align:middle line:84% And now, the result of this loop will 00:24:26.580 --> 00:24:29.430 align:middle line:90% be this, so key 2 with value. 00:24:29.430 --> 00:24:34.740 align:middle line:84% And then it uses the print statement on my animal object. 00:24:34.740 --> 00:24:35.730 align:middle line:90% Does that make sense? 00:24:35.730 --> 00:24:38.862 align:middle line:90% Everyone OK so far? 00:24:38.862 --> 00:24:41.090 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:24:41.090 --> 00:24:42.090 align:middle line:90% ANA BELL: Yeah, exactly. 00:24:42.090 --> 00:24:44.423 align:middle line:84% It's converting the stuff in the dictionary with strings 00:24:44.423 --> 00:24:47.430 align:middle line:84% because my print statement is being run directly 00:24:47.430 --> 00:24:49.583 align:middle line:90% on that object of type animal. 00:24:49.583 --> 00:24:50.750 align:middle line:90% And it knows how to do that. 00:24:50.750 --> 00:24:52.446 align:middle line:90% I implemented the dunder str. 00:24:52.446 --> 00:24:56.550 align:middle line:90% 00:24:56.550 --> 00:24:59.520 align:middle line:90% Everyone OK? 00:24:59.520 --> 00:25:02.640 align:middle line:90% OK, so let's have you try this. 00:25:02.640 --> 00:25:04.490 align:middle line:84% Let's have you write a little code. 00:25:04.490 --> 00:25:08.400 align:middle line:84% So this function, it's going to be very similar. 00:25:08.400 --> 00:25:09.650 align:middle line:90% We're not making dictionaries. 00:25:09.650 --> 00:25:10.650 align:middle line:90% You'll be making a list. 00:25:10.650 --> 00:25:13.220 align:middle line:84% But you'll encounter the same problem. 00:25:13.220 --> 00:25:17.030 align:middle line:84% The input here is going to be two lists of the same length. 00:25:17.030 --> 00:25:18.770 align:middle line:90% One list has numbers. 00:25:18.770 --> 00:25:21.180 align:middle line:90% One list has strings. 00:25:21.180 --> 00:25:24.410 align:middle line:84% And what I'd like you to do is create for me a new list. 00:25:24.410 --> 00:25:27.950 align:middle line:84% And the new list is going to have animal objects, 00:25:27.950 --> 00:25:30.710 align:middle line:90% where you match index by index. 00:25:30.710 --> 00:25:34.070 align:middle line:84% So the resulting animal object at index 0 00:25:34.070 --> 00:25:37.520 align:middle line:84% will basically create for me a new animal with age two 00:25:37.520 --> 00:25:40.630 align:middle line:90% and named blobfish, right? 00:25:40.630 --> 00:25:44.320 align:middle line:84% The animal, object and the resulting list at index 1 00:25:44.320 --> 00:25:46.990 align:middle line:84% will be with age five and name crazyant. 00:25:46.990 --> 00:25:49.090 align:middle line:84% And then the animal object at index 2 00:25:49.090 --> 00:25:51.580 align:middle line:84% will be age one and named parafox. 00:25:51.580 --> 00:25:54.580 align:middle line:84% So we're just doing the same thing index by index, 00:25:54.580 --> 00:25:56.740 align:middle line:84% where you create a new animal object 00:25:56.740 --> 00:26:02.620 align:middle line:84% with the age, this value, one at a time. 00:26:02.620 --> 00:26:05.572 align:middle line:84% And you set the name to be this value one at a time, 00:26:05.572 --> 00:26:06.655 align:middle line:90% and then return that list. 00:26:06.655 --> 00:26:10.310 align:middle line:90% 00:26:10.310 --> 00:26:15.780 align:middle line:90% So that should be line 79. 00:26:15.780 --> 00:26:18.720 align:middle line:90% 00:26:18.720 --> 00:26:21.180 align:middle line:90% OK, who has a start for me? 00:26:21.180 --> 00:26:24.350 align:middle line:90% 00:26:24.350 --> 00:26:26.900 align:middle line:90% OK, should we call it L3? 00:26:26.900 --> 00:26:28.210 align:middle line:90% OK. 00:26:28.210 --> 00:26:35.760 align:middle line:84% AUDIENCE: But then for n in L1, [INAUDIBLE] having it 00:26:35.760 --> 00:26:41.610 align:middle line:90% go through L2 of n. 00:26:41.610 --> 00:26:44.520 align:middle line:84% ANA BELL: Yes, but then if you're doing L2 of n, 00:26:44.520 --> 00:26:47.200 align:middle line:84% then this should be the index, right? 00:26:47.200 --> 00:26:49.990 align:middle line:84% So how do I make this be the index instead of the element 00:26:49.990 --> 00:26:51.750 align:middle line:90% directly? 00:26:51.750 --> 00:26:54.510 align:middle line:90% Yeah, yeah. 00:26:54.510 --> 00:26:59.878 align:middle line:84% AUDIENCE: Well maybe n of, over the range column? 00:26:59.878 --> 00:27:01.170 align:middle line:90% ANA BELL: Yeah, exactly, right? 00:27:01.170 --> 00:27:03.087 align:middle line:84% So instead of looking at the element directly, 00:27:03.087 --> 00:27:05.020 align:middle line:90% let's just look at the range. 00:27:05.020 --> 00:27:08.580 align:middle line:84% So for i in, range, and then we need to do len. 00:27:08.580 --> 00:27:11.620 align:middle line:84% Pick one of the lists because they're the same length. 00:27:11.620 --> 00:27:17.730 align:middle line:84% So now, i is 0, 1, 2, 3, 4, 5, all the index values. 00:27:17.730 --> 00:27:21.030 align:middle line:90% 00:27:21.030 --> 00:27:33.640 align:middle line:84% AUDIENCE: Well, and you can make L3 [INAUDIBLE] L1 at index i 00:27:33.640 --> 00:27:36.080 align:middle line:90% equal to L2 at index i. 00:27:36.080 --> 00:27:41.270 align:middle line:84% ANA BELL: So L1 at index i, so I need to create 00:27:41.270 --> 00:27:43.190 align:middle line:90% an animal with that age, right? 00:27:43.190 --> 00:27:45.215 align:middle line:90% So let's do this. 00:27:45.215 --> 00:27:49.210 align:middle line:90% 00:27:49.210 --> 00:27:54.230 align:middle line:84% Age equals L1 at index i, just to save it as a variable. 00:27:54.230 --> 00:27:57.130 align:middle line:90% And name equals L2 at index i. 00:27:57.130 --> 00:28:00.110 align:middle line:90% Do we agree? 00:28:00.110 --> 00:28:04.090 align:middle line:84% So now that I have age and name stored in these variables, 00:28:04.090 --> 00:28:10.680 align:middle line:84% how do I make an animal object with that age? 00:28:10.680 --> 00:28:11.310 align:middle line:90% Yeah. 00:28:11.310 --> 00:28:17.180 align:middle line:84% AUDIENCE: You could say L3 at i equals-- 00:28:17.180 --> 00:28:20.190 align:middle line:84% well, I know you'd call it animal.h. 00:28:20.190 --> 00:28:22.980 align:middle line:84% ANA BELL: Well, the init method creates for me 00:28:22.980 --> 00:28:25.720 align:middle line:90% an animal with that age, right? 00:28:25.720 --> 00:28:28.330 align:middle line:84% So when we just create a new animal object, we just 00:28:28.330 --> 00:28:31.320 align:middle line:90% pass in that age, right? 00:28:31.320 --> 00:28:38.380 align:middle line:84% Like, the constructor requires the age of the animal, right? 00:28:38.380 --> 00:28:40.660 align:middle line:84% So when we construct a new animal object, 00:28:40.660 --> 00:28:43.550 align:middle line:84% we just invoke the name of our animal. 00:28:43.550 --> 00:28:44.050 align:middle line:90% Where is it? 00:28:44.050 --> 00:28:46.180 align:middle line:90% Here, right? 00:28:46.180 --> 00:28:48.910 align:middle line:84% Or-- sorry, our animal type, our animal class. 00:28:48.910 --> 00:28:50.620 align:middle line:84% And then we pass in the age that we 00:28:50.620 --> 00:28:53.700 align:middle line:84% want to create this animal with, right? 00:28:53.700 --> 00:28:57.090 align:middle line:84% And that, according to the init method, 00:28:57.090 --> 00:29:00.090 align:middle line:84% creates self.age to be whatever is 00:29:00.090 --> 00:29:05.230 align:middle line:90% passed in, and the name, None. 00:29:05.230 --> 00:29:06.550 align:middle line:90% So we're halfway there. 00:29:06.550 --> 00:29:10.750 align:middle line:84% We've created an animal object with the age that we want. 00:29:10.750 --> 00:29:14.545 align:middle line:84% But the name data attribute for this object is None. 00:29:14.545 --> 00:29:17.160 align:middle line:90% 00:29:17.160 --> 00:29:18.375 align:middle line:90% Everyone with me so far? 00:29:18.375 --> 00:29:22.010 align:middle line:90% 00:29:22.010 --> 00:29:24.840 align:middle line:84% So how do we make the name of this animal 00:29:24.840 --> 00:29:28.850 align:middle line:84% object be the one that we saved from that L2 list? 00:29:28.850 --> 00:29:32.630 align:middle line:90% 00:29:32.630 --> 00:29:34.560 align:middle line:84% AUDIENCE: You can use the setter function. 00:29:34.560 --> 00:29:35.580 align:middle line:90% ANA BELL: Yeah, exactly. 00:29:35.580 --> 00:29:38.610 align:middle line:84% We can use the setter function, yeah, set name right here, 00:29:38.610 --> 00:29:39.120 align:middle line:90% right? 00:29:39.120 --> 00:29:41.070 align:middle line:84% Don't access the attribute directly. 00:29:41.070 --> 00:29:42.705 align:middle line:84% But yeah, we can use a setter function. 00:29:42.705 --> 00:29:46.570 align:middle line:90% 00:29:46.570 --> 00:29:49.210 align:middle line:84% So this created for me that new animal, right? 00:29:49.210 --> 00:29:53.500 align:middle line:84% But I need to actually save that animal somehow, right? 00:29:53.500 --> 00:29:55.520 align:middle line:84% Because I need to reference it later. 00:29:55.520 --> 00:29:57.010 align:middle line:90% So let me do this. 00:29:57.010 --> 00:29:58.900 align:middle line:90% a equals animal with that age. 00:29:58.900 --> 00:30:01.780 align:middle line:84% And then we run the setter function on this object, a, 00:30:01.780 --> 00:30:02.350 align:middle line:90% right? 00:30:02.350 --> 00:30:06.750 align:middle line:90% set_name-- it's just a function. 00:30:06.750 --> 00:30:09.070 align:middle line:84% And what name do we want to set it at, this thing here? 00:30:09.070 --> 00:30:12.120 align:middle line:84% So name here is this variable that we 00:30:12.120 --> 00:30:13.635 align:middle line:90% extracted from the L2 list. 00:30:13.635 --> 00:30:18.180 align:middle line:90% 00:30:18.180 --> 00:30:21.110 align:middle line:90% Everyone OK so far? 00:30:21.110 --> 00:30:26.780 align:middle line:84% So now, what I have is, object a is a variable that's 00:30:26.780 --> 00:30:30.655 align:middle line:90% bound to an animal object. 00:30:30.655 --> 00:30:33.340 align:middle line:84% The age was set when we first created it. 00:30:33.340 --> 00:30:36.775 align:middle line:84% And the name, we just set through the setter function. 00:30:36.775 --> 00:30:39.350 align:middle line:90% 00:30:39.350 --> 00:30:43.400 align:middle line:84% And now, we should just put it in my list. 00:30:43.400 --> 00:30:45.140 align:middle line:84% My list is originally empty, right? 00:30:45.140 --> 00:30:49.860 align:middle line:84% So now, I don't have a bunch of elements to add it to. 00:30:49.860 --> 00:30:58.527 align:middle line:84% So let's just append it to L3, like that, right? 00:30:58.527 --> 00:31:01.110 align:middle line:84% I mean, theoretically, I could have created an empty list that 00:31:01.110 --> 00:31:02.768 align:middle line:90% was three elements long. 00:31:02.768 --> 00:31:03.810 align:middle line:90% And then I could do L3.i. 00:31:03.810 --> 00:31:06.680 align:middle line:90% 00:31:06.680 --> 00:31:07.720 align:middle line:90% But this works, too. 00:31:07.720 --> 00:31:12.500 align:middle line:90% 00:31:12.500 --> 00:31:19.850 align:middle line:84% And then at the end, let's return L3, right? 00:31:19.850 --> 00:31:21.806 align:middle line:90% Questions about this? 00:31:21.806 --> 00:31:22.775 align:middle line:90% This is all right? 00:31:22.775 --> 00:31:30.510 align:middle line:90% 00:31:30.510 --> 00:31:32.910 align:middle line:84% OK, so if we run it and we just print 00:31:32.910 --> 00:31:34.950 align:middle line:84% the list with these animal objects, 00:31:34.950 --> 00:31:37.650 align:middle line:84% we run the same problem as that dictionary one, right? 00:31:37.650 --> 00:31:41.340 align:middle line:84% You see I've got a bunch of memory locations here? 00:31:41.340 --> 00:31:44.970 align:middle line:84% So to test that I did it right, instead of printing 00:31:44.970 --> 00:31:48.060 align:middle line:84% the list, let's iterate through our list, 00:31:48.060 --> 00:31:51.960 align:middle line:84% through this little for loop, and just run the print method 00:31:51.960 --> 00:31:56.370 align:middle line:90% directly on my object, right? 00:31:56.370 --> 00:31:58.050 align:middle line:84% So now, if I run that, it should just 00:31:58.050 --> 00:32:00.640 align:middle line:84% run the print statement directly on each of these animals, 00:32:00.640 --> 00:32:01.140 align:middle line:90% right? 00:32:01.140 --> 00:32:02.940 align:middle line:90% So that's correct, I think. 00:32:02.940 --> 00:32:06.000 align:middle line:90% 00:32:06.000 --> 00:32:07.540 align:middle line:90% Does that make sense? 00:32:07.540 --> 00:32:08.110 align:middle line:90% Yeah. 00:32:08.110 --> 00:32:11.090 align:middle line:84% AUDIENCE: What did you change to make it bigger? 00:32:11.090 --> 00:32:14.150 align:middle line:84% ANA BELL: Oh, so instead of printing the list, this thing, 00:32:14.150 --> 00:32:17.258 align:middle line:84% I looped through my list and printed the elements. 00:32:17.258 --> 00:32:17.800 align:middle line:90% AUDIENCE: Oh. 00:32:17.800 --> 00:32:19.680 align:middle line:90% So that's not in the function. 00:32:19.680 --> 00:32:21.388 align:middle line:84% ANA BELL: That's not in the function, no. 00:32:21.388 --> 00:32:23.155 align:middle line:84% That's just, yeah, that's outside. 00:32:23.155 --> 00:32:25.530 align:middle line:84% But this is something pretty common that you'll run into. 00:32:25.530 --> 00:32:28.530 align:middle line:84% You'll make a list or dictionary or some structure 00:32:28.530 --> 00:32:31.210 align:middle line:84% or tuple or something like that with objects of your type. 00:32:31.210 --> 00:32:33.085 align:middle line:84% And when you run the print statement directly 00:32:33.085 --> 00:32:36.400 align:middle line:84% on that structure, it doesn't go deeper than top level. 00:32:36.400 --> 00:32:38.880 align:middle line:84% And so it prints that uninformative stuff. 00:32:38.880 --> 00:32:44.940 align:middle line:90% 00:32:44.940 --> 00:32:51.180 align:middle line:84% OK, so in this example, we saw that it's better 00:32:51.180 --> 00:32:54.960 align:middle line:84% to access the attributes through getters and setters. 00:32:54.960 --> 00:32:58.410 align:middle line:84% So in addition to the init, the str method, 00:32:58.410 --> 00:33:00.390 align:middle line:84% writing getters and setters to have 00:33:00.390 --> 00:33:02.820 align:middle line:84% a consistent way of accessing and modifying these data 00:33:02.820 --> 00:33:04.420 align:middle line:90% attributes is really important. 00:33:04.420 --> 00:33:07.230 align:middle line:84% And then you can even impose restrictions, something like, 00:33:07.230 --> 00:33:08.550 align:middle line:90% the types have to be this. 00:33:08.550 --> 00:33:11.080 align:middle line:84% Or maybe the age can't be a negative number 00:33:11.080 --> 00:33:12.130 align:middle line:90% or something like that. 00:33:12.130 --> 00:33:17.500 align:middle line:84% And it allows a lot more consistent use of the object. 00:33:17.500 --> 00:33:19.980 align:middle line:84% So now, let's move on to hierarchies, OK? 00:33:19.980 --> 00:33:23.470 align:middle line:84% And this is where we're going to talk about inheritance. 00:33:23.470 --> 00:33:28.750 align:middle line:84% So there's something like maybe 28 objects on this slide, 00:33:28.750 --> 00:33:29.250 align:middle line:90% right? 00:33:29.250 --> 00:33:31.208 align:middle line:84% There's the six we encountered at the beginning 00:33:31.208 --> 00:33:33.250 align:middle line:90% of this lecture and 22 up there. 00:33:33.250 --> 00:33:37.030 align:middle line:84% So there's 28 separate objects on this slide. 00:33:37.030 --> 00:33:41.070 align:middle line:84% And all of these objects, we could say, are of type animal, 00:33:41.070 --> 00:33:41.670 align:middle line:90% right? 00:33:41.670 --> 00:33:46.000 align:middle line:84% Because by our definition, the attributes for an animal 00:33:46.000 --> 00:33:47.930 align:middle line:90% is how long they've been alive. 00:33:47.930 --> 00:33:52.010 align:middle line:84% And these all are objects that have been alive for some time. 00:33:52.010 --> 00:33:56.470 align:middle line:84% But in addition to having the attribute for how long 00:33:56.470 --> 00:34:00.820 align:middle line:84% they've been alive and an unknown name, we can actually 00:34:00.820 --> 00:34:02.860 align:middle line:84% then create separate categories, right? 00:34:02.860 --> 00:34:06.970 align:middle line:84% And each one of these boxes that I've created 00:34:06.970 --> 00:34:10.389 align:middle line:84% is a different subset of animal, right? 00:34:10.389 --> 00:34:14.320 align:middle line:84% We'll call it a subclass or a child of an animal class. 00:34:14.320 --> 00:34:18.400 align:middle line:84% And that's because they will bring about different data 00:34:18.400 --> 00:34:22.989 align:middle line:84% attributes in addition to what an animal's data attributes 00:34:22.989 --> 00:34:23.800 align:middle line:90% are. 00:34:23.800 --> 00:34:27.070 align:middle line:84% And they will bring about different behaviors in addition 00:34:27.070 --> 00:34:30.820 align:middle line:84% to the behaviors of our really generic animal object, right? 00:34:30.820 --> 00:34:34.840 align:middle line:84% So the things a cat can do, a rabbit might not be able to do. 00:34:34.840 --> 00:34:36.962 align:middle line:84% And the things a person can do, a cat won't do 00:34:36.962 --> 00:34:38.170 align:middle line:90% and a rabbit can't do, right? 00:34:38.170 --> 00:34:39.790 align:middle line:90% So they're all animals. 00:34:39.790 --> 00:34:44.590 align:middle line:84% But they all are going to have additional data attributes 00:34:44.590 --> 00:34:47.530 align:middle line:84% and additional behaviors that are different in these three 00:34:47.530 --> 00:34:49.520 align:middle line:90% categories, right? 00:34:49.520 --> 00:34:52.639 align:middle line:84% So I might say something like, the cat has a name, an age, 00:34:52.639 --> 00:34:54.440 align:middle line:90% and a pattern or a color. 00:34:54.440 --> 00:34:56.058 align:middle line:84% The rabbit, again, I said are wild. 00:34:56.058 --> 00:34:57.350 align:middle line:90% So maybe they don't get a name. 00:34:57.350 --> 00:34:59.870 align:middle line:84% But they'll have a color or pattern. 00:34:59.870 --> 00:35:03.870 align:middle line:84% And then the age, of course, from the animal, people, 00:35:03.870 --> 00:35:06.350 align:middle line:84% of course, have the-- the person object has 00:35:06.350 --> 00:35:08.945 align:middle line:90% the age that comes from animal. 00:35:08.945 --> 00:35:11.570 align:middle line:84% But in addition, they might have a list of friends or something 00:35:11.570 --> 00:35:14.028 align:middle line:84% associated, something like that associated with them right, 00:35:14.028 --> 00:35:17.258 align:middle line:84% and a list of friends, something a cat doesn't have, 00:35:17.258 --> 00:35:18.800 align:middle line:84% something that a rabbit doesn't have. 00:35:18.800 --> 00:35:20.510 align:middle line:90% So you see what I mean. 00:35:20.510 --> 00:35:21.920 align:middle line:90% And we can even go further. 00:35:21.920 --> 00:35:24.530 align:middle line:84% We can say, well, if I take my person object, 00:35:24.530 --> 00:35:26.780 align:middle line:84% I can now sub-categorize that as well 00:35:26.780 --> 00:35:29.000 align:middle line:84% and say, well, this is a student class. 00:35:29.000 --> 00:35:30.920 align:middle line:84% And then this student class, I would 00:35:30.920 --> 00:35:33.120 align:middle line:90% say a student is a person. 00:35:33.120 --> 00:35:34.940 align:middle line:84% So all the data attributes and all the 00:35:34.940 --> 00:35:37.940 align:middle line:84% behaviors that a person has, the student also has, 00:35:37.940 --> 00:35:39.440 align:middle line:84% and, of course, all the animal stuff 00:35:39.440 --> 00:35:41.600 align:middle line:90% because a person is an animal. 00:35:41.600 --> 00:35:46.250 align:middle line:84% So for example, let's say an animal is a generic object. 00:35:46.250 --> 00:35:47.070 align:middle line:90% It doesn't speak. 00:35:47.070 --> 00:35:50.330 align:middle line:84% But let's say a person gets the behavior to speak, right? 00:35:50.330 --> 00:35:52.760 align:middle line:84% So for speaking, I might just print Hello 00:35:52.760 --> 00:35:55.130 align:middle line:84% to the screen or something simple like that. 00:35:55.130 --> 00:35:57.150 align:middle line:90% A student is a person. 00:35:57.150 --> 00:36:02.390 align:middle line:84% So maybe they also get something like their age, 00:36:02.390 --> 00:36:05.210 align:middle line:84% the name, and maybe a list of friends associated with them. 00:36:05.210 --> 00:36:09.472 align:middle line:84% But a student might also have a major or a favorite subject 00:36:09.472 --> 00:36:11.180 align:middle line:84% in school associated with them, something 00:36:11.180 --> 00:36:12.830 align:middle line:84% that a person doesn't have, right? 00:36:12.830 --> 00:36:15.170 align:middle line:84% So that's a new data attribute associated 00:36:15.170 --> 00:36:17.970 align:middle line:84% with a student that's not associated with a person. 00:36:17.970 --> 00:36:20.810 align:middle line:84% A student might also have different behaviors, 00:36:20.810 --> 00:36:23.250 align:middle line:84% like tell me your favorite subject in school, 00:36:23.250 --> 00:36:24.090 align:middle line:90% things like that. 00:36:24.090 --> 00:36:26.640 align:middle line:84% Or it might override behaviors of a person. 00:36:26.640 --> 00:36:30.170 align:middle line:84% So if a person speaks, says, hello, prints hello 00:36:30.170 --> 00:36:33.255 align:middle line:84% to the screen, we can say, hey, if I ask the student to speak, 00:36:33.255 --> 00:36:34.880 align:middle line:84% they might say, I have homework instead 00:36:34.880 --> 00:36:37.210 align:middle line:90% or something like that, right? 00:36:37.210 --> 00:36:42.010 align:middle line:84% So what we're trying to do is take those relationships 00:36:42.010 --> 00:36:43.910 align:middle line:90% and implement them in code. 00:36:43.910 --> 00:36:48.970 align:middle line:84% So here, I've got an animal class, which is my base class. 00:36:48.970 --> 00:36:53.110 align:middle line:84% It's going to be my, also called parent class or super-class. 00:36:53.110 --> 00:36:56.448 align:middle line:84% And then anything that an animal has, 00:36:56.448 --> 00:36:58.990 align:middle line:84% all the data attributes and all the behaviors of that animal, 00:36:58.990 --> 00:37:01.720 align:middle line:84% will be inherited by person, cat, and rabbit. 00:37:01.720 --> 00:37:03.778 align:middle line:84% So anything-- so a person is an animal. 00:37:03.778 --> 00:37:04.570 align:middle line:90% A cat is an animal. 00:37:04.570 --> 00:37:05.710 align:middle line:90% A rabbit is an animal. 00:37:05.710 --> 00:37:08.350 align:middle line:84% So everything they have, all these three subtypes 00:37:08.350 --> 00:37:09.430 align:middle line:90% will have as well. 00:37:09.430 --> 00:37:13.190 align:middle line:84% But all these subtypes will be different amongst themselves, 00:37:13.190 --> 00:37:13.690 align:middle line:90% right? 00:37:13.690 --> 00:37:16.030 align:middle line:84% A person will have an ability to speak, maybe 00:37:16.030 --> 00:37:17.290 align:middle line:90% print hello to the screen. 00:37:17.290 --> 00:37:19.248 align:middle line:84% A cat could also have the ability to speak. 00:37:19.248 --> 00:37:21.040 align:middle line:84% But maybe they'll print meow to the screen. 00:37:21.040 --> 00:37:24.310 align:middle line:84% A rabbit won't even have the ability to speak at all. 00:37:24.310 --> 00:37:26.890 align:middle line:84% A person might have a list of friends, 00:37:26.890 --> 00:37:29.960 align:middle line:84% whereas a cat won't, a rabbit won't things, like that. 00:37:29.960 --> 00:37:32.110 align:middle line:84% So we can either add more information. 00:37:32.110 --> 00:37:34.300 align:middle line:84% Like, list of friends was an example of that. 00:37:34.300 --> 00:37:35.560 align:middle line:90% We can add more behavior. 00:37:35.560 --> 00:37:38.940 align:middle line:84% Like, the ability to speak is an example of that. 00:37:38.940 --> 00:37:42.210 align:middle line:84% And an example of overriding behavior, like I mentioned, 00:37:42.210 --> 00:37:45.750 align:middle line:84% is let's say we have a subclass student of person. 00:37:45.750 --> 00:37:51.780 align:middle line:84% If a person's speak method said to print Hello to the screen, 00:37:51.780 --> 00:37:55.440 align:middle line:84% we can override that behavior through a speak method 00:37:55.440 --> 00:37:58.410 align:middle line:84% inside student, where you don't just print hello to the screen. 00:37:58.410 --> 00:38:01.670 align:middle line:90% You can print, I have homework. 00:38:01.670 --> 00:38:07.050 align:middle line:84% So let's try to start implementing this relationship. 00:38:07.050 --> 00:38:09.110 align:middle line:90% This is just our animal class. 00:38:09.110 --> 00:38:10.370 align:middle line:90% There's nothing new here. 00:38:10.370 --> 00:38:11.990 align:middle line:84% I'm just doing a little refresher 00:38:11.990 --> 00:38:13.710 align:middle line:90% on what this class looks like. 00:38:13.710 --> 00:38:16.040 align:middle line:84% So we've got our init, where we initialize 00:38:16.040 --> 00:38:18.650 align:middle line:90% an age and a name that's None. 00:38:18.650 --> 00:38:20.780 align:middle line:84% We've got two getters, two setters, 00:38:20.780 --> 00:38:23.955 align:middle line:84% and this str method that prints animal, colon, name, colon, 00:38:23.955 --> 00:38:24.455 align:middle line:90% age. 00:38:24.455 --> 00:38:28.770 align:middle line:90% 00:38:28.770 --> 00:38:32.140 align:middle line:84% So yeah, OK, this animal class inherits from object, 00:38:32.140 --> 00:38:34.240 align:middle line:90% so the generic Python object. 00:38:34.240 --> 00:38:36.870 align:middle line:84% And now, let's work on the subclass, cat. 00:38:36.870 --> 00:38:39.930 align:middle line:84% So when I create my subclass cat, 00:38:39.930 --> 00:38:42.780 align:middle line:84% the way I tell Python that this cat is an animal 00:38:42.780 --> 00:38:46.770 align:middle line:84% is by putting in the parentheses here the name of the type 00:38:46.770 --> 00:38:50.940 align:middle line:84% that I want this class to inherit from. 00:38:50.940 --> 00:38:55.030 align:middle line:90% So a cat is an animal. 00:38:55.030 --> 00:38:58.090 align:middle line:84% Now, one of the things I kept coming back to 00:38:58.090 --> 00:39:00.550 align:middle line:84% is, any time you create a new data type, 00:39:00.550 --> 00:39:03.180 align:middle line:90% you have to have an init method. 00:39:03.180 --> 00:39:06.960 align:middle line:84% This doesn't specifically have an init method, right? 00:39:06.960 --> 00:39:09.340 align:middle line:84% I've just got two other methods here. 00:39:09.340 --> 00:39:11.790 align:middle line:84% So you might think that it's missing. 00:39:11.790 --> 00:39:16.380 align:middle line:84% But it's actually not because as soon as you put another data 00:39:16.380 --> 00:39:21.030 align:middle line:84% type here in the parentheses, so that cat is an animal, 00:39:21.030 --> 00:39:24.150 align:middle line:84% think of it like Python going into the animal class, 00:39:24.150 --> 00:39:27.990 align:middle line:84% copying and pasting everything that's part of the animal class 00:39:27.990 --> 00:39:30.240 align:middle line:84% or copying everything that's part of that animal class 00:39:30.240 --> 00:39:33.270 align:middle line:90% and pasting it inside cat. 00:39:33.270 --> 00:39:36.840 align:middle line:84% So since I don't have an init method specifically defined 00:39:36.840 --> 00:39:39.960 align:middle line:84% in cat, Python will say, oh, we'll 00:39:39.960 --> 00:39:43.760 align:middle line:84% just use the init method of your parent animal. 00:39:43.760 --> 00:39:45.730 align:middle line:84% So the way we create a cat is going 00:39:45.730 --> 00:39:48.940 align:middle line:84% to be exactly the same way we create an animal, 00:39:48.940 --> 00:39:51.760 align:middle line:84% except that the name is going to be cat as my object 00:39:51.760 --> 00:39:54.310 align:middle line:90% type instead of animal. 00:39:54.310 --> 00:39:56.360 align:middle line:84% But we just pass it in one thing, 00:39:56.360 --> 00:39:59.440 align:middle line:90% which is the age of this cat. 00:39:59.440 --> 00:40:01.785 align:middle line:84% So since we're copying and pasting everything-- yeah, 00:40:01.785 --> 00:40:02.285 align:middle line:90% question. 00:40:02.285 --> 00:40:06.190 align:middle line:84% AUDIENCE: The parent class of animal [INAUDIBLE] 00:40:06.190 --> 00:40:07.430 align:middle line:90% is that also [INAUDIBLE]? 00:40:07.430 --> 00:40:08.600 align:middle line:90% ANA BELL: Yes, exactly. 00:40:08.600 --> 00:40:10.820 align:middle line:84% So the parent class of animal is object. 00:40:10.820 --> 00:40:12.890 align:middle line:84% So cat will also be a Python object. 00:40:12.890 --> 00:40:15.890 align:middle line:84% But that's super generic stuff, like binding a variable name 00:40:15.890 --> 00:40:17.540 align:middle line:84% to this object, things like that. 00:40:17.540 --> 00:40:20.310 align:middle line:90% 00:40:20.310 --> 00:40:22.760 align:middle line:84% So not only does the init get copied in, 00:40:22.760 --> 00:40:26.240 align:middle line:84% but every single data attribute, age and name, 00:40:26.240 --> 00:40:29.550 align:middle line:84% every single way that that data attribute gets created, 00:40:29.550 --> 00:40:32.330 align:middle line:84% so the self.age is going to be a data attribute of cat. 00:40:32.330 --> 00:40:33.830 align:middle line:84% And it's going to be set to whatever 00:40:33.830 --> 00:40:36.110 align:middle line:84% is passed in as a parameter. self.name 00:40:36.110 --> 00:40:38.780 align:middle line:84% will be initialized to none, just like for animal. 00:40:38.780 --> 00:40:40.850 align:middle line:84% I've got my two getters, my two setters 00:40:40.850 --> 00:40:42.650 align:middle line:90% that also work with cats. 00:40:42.650 --> 00:40:46.100 align:middle line:84% And then, the str method of animal 00:40:46.100 --> 00:40:49.400 align:middle line:90% will also be inherited in here. 00:40:49.400 --> 00:40:51.200 align:middle line:90% But now, we notice one thing. 00:40:51.200 --> 00:40:52.820 align:middle line:84% And that's, we have an str method 00:40:52.820 --> 00:40:54.440 align:middle line:90% defined in the animal class. 00:40:54.440 --> 00:40:59.210 align:middle line:84% But then, in my cat class, I define an str method as well, 00:40:59.210 --> 00:40:59.930 align:middle line:90% right? 00:40:59.930 --> 00:41:04.770 align:middle line:84% So that's called overriding your parents's class. 00:41:04.770 --> 00:41:07.860 align:middle line:84% And when we create an object of type cat, 00:41:07.860 --> 00:41:11.610 align:middle line:84% if this object has a method that has 00:41:11.610 --> 00:41:15.960 align:middle line:84% the same name as their parent, we use this method. 00:41:15.960 --> 00:41:17.850 align:middle line:84% There's no reason to go up to your parent 00:41:17.850 --> 00:41:19.260 align:middle line:90% to ask for their method. 00:41:19.260 --> 00:41:23.650 align:middle line:84% We use the one that is for this object. 00:41:23.650 --> 00:41:28.140 align:middle line:84% And cat, in addition to having everything that animal has, 00:41:28.140 --> 00:41:32.100 align:middle line:84% implements a new behavior, which is the ability to speak. 00:41:32.100 --> 00:41:35.310 align:middle line:84% And all it does is print meow to the screen. 00:41:35.310 --> 00:41:39.080 align:middle line:90% OK, so let's look at some code. 00:41:39.080 --> 00:41:40.570 align:middle line:90% So here's my cat. 00:41:40.570 --> 00:41:43.090 align:middle line:84% So I created a new cat object the same way 00:41:43.090 --> 00:41:44.170 align:middle line:90% I would create an animal. 00:41:44.170 --> 00:41:48.760 align:middle line:84% But I'm invoking the name of this class, cat. 00:41:48.760 --> 00:41:50.320 align:middle line:84% The way I create an animal is just 00:41:50.320 --> 00:41:53.470 align:middle line:84% by passing in the age of this thing, right? 00:41:53.470 --> 00:41:56.080 align:middle line:84% So here, I'm creating a cat whose age is five. 00:41:56.080 --> 00:41:58.450 align:middle line:84% The name of this cat is None, right? 00:41:58.450 --> 00:42:03.140 align:middle line:84% Because that's what the init method of animal does. 00:42:03.140 --> 00:42:06.790 align:middle line:84% But I can run the methods on animal on my cat object 00:42:06.790 --> 00:42:08.440 align:middle line:90% because a cat is an animal. 00:42:08.440 --> 00:42:10.840 align:middle line:84% So all the methods that work with animals 00:42:10.840 --> 00:42:13.030 align:middle line:84% will work with an object of type cat. 00:42:13.030 --> 00:42:15.670 align:middle line:84% So, here, I can just run the set_name method 00:42:15.670 --> 00:42:17.200 align:middle line:90% on my cat object. 00:42:17.200 --> 00:42:20.320 align:middle line:84% Even though the method is not explicitly defined in here, 00:42:20.320 --> 00:42:22.730 align:middle line:90% it's defined in my parent. 00:42:22.730 --> 00:42:24.670 align:middle line:84% So if I set the name to fluffy and then I 00:42:24.670 --> 00:42:27.670 align:middle line:84% print the cat object, it's going to print, 00:42:27.670 --> 00:42:30.415 align:middle line:84% it's a cat, colon, the name, colon, the age. 00:42:30.415 --> 00:42:32.990 align:middle line:90% 00:42:32.990 --> 00:42:36.560 align:middle line:84% speak is just going to print meow to the screen. 00:42:36.560 --> 00:42:38.985 align:middle line:84% We can do the getter methods as well. 00:42:38.985 --> 00:42:41.360 align:middle line:84% So all of these methods that were implemented with animal 00:42:41.360 --> 00:42:43.290 align:middle line:90% work with cats as well. 00:42:43.290 --> 00:42:48.710 align:middle line:84% Now, in here, object a was created up here 00:42:48.710 --> 00:42:52.010 align:middle line:90% when we talked about animals. 00:42:52.010 --> 00:42:56.480 align:middle line:84% It's an animal object because it was created using the animal 00:42:56.480 --> 00:42:58.910 align:middle line:90% invocation here. 00:42:58.910 --> 00:43:02.630 align:middle line:84% Does the animal class have a method to speak? 00:43:02.630 --> 00:43:03.380 align:middle line:90% No. 00:43:03.380 --> 00:43:06.350 align:middle line:84% So if I actually run this, it'll give me an error, right? 00:43:06.350 --> 00:43:07.880 align:middle line:84% It just says there's no attribute 00:43:07.880 --> 00:43:09.330 align:middle line:90% speak, which makes sense. 00:43:09.330 --> 00:43:10.250 align:middle line:90% I never defined that. 00:43:10.250 --> 00:43:12.455 align:middle line:84% I defined that in your child, not the parent. 00:43:12.455 --> 00:43:15.960 align:middle line:90% 00:43:15.960 --> 00:43:17.400 align:middle line:90% Questions about cats? 00:43:17.400 --> 00:43:22.190 align:middle line:90% 00:43:22.190 --> 00:43:30.630 align:middle line:84% OK, so I want to briefly touch upon overriding methods 00:43:30.630 --> 00:43:32.920 align:middle line:84% because it can get a little bit confusing. 00:43:32.920 --> 00:43:36.180 align:middle line:84% So you notice, the str method was implemented 00:43:36.180 --> 00:43:37.500 align:middle line:90% in both of these objects. 00:43:37.500 --> 00:43:41.820 align:middle line:84% The str method is in cat, which overrides the animal's method 00:43:41.820 --> 00:43:44.250 align:middle line:84% to print cat, colon, name, colon, age. 00:43:44.250 --> 00:43:46.380 align:middle line:84% And the animal method, str method, 00:43:46.380 --> 00:43:48.450 align:middle line:84% prints animal, colon, name, colon, age. 00:43:48.450 --> 00:43:53.280 align:middle line:84% So the rule is, when you're running a method that you know 00:43:53.280 --> 00:43:58.350 align:middle line:84% exists in a whole bunch of these inherited objects, 00:43:58.350 --> 00:44:00.480 align:middle line:90% you look at-- 00:44:00.480 --> 00:44:01.440 align:middle line:90% which one is it? 00:44:01.440 --> 00:44:02.250 align:middle line:90% It's str, right? 00:44:02.250 --> 00:44:04.500 align:middle line:84% So it would be the print method, or any method. 00:44:04.500 --> 00:44:06.030 align:middle line:90% It doesn't matter what it is. 00:44:06.030 --> 00:44:08.580 align:middle line:84% You look at the object you're calling the method on, right? 00:44:08.580 --> 00:44:11.010 align:middle line:84% So if it's a dot notation, you look at the thing 00:44:11.010 --> 00:44:12.180 align:middle line:90% before the dot. 00:44:12.180 --> 00:44:14.160 align:middle line:84% If it's one of these special methods, 00:44:14.160 --> 00:44:16.720 align:middle line:84% what's the object you're running this method on? 00:44:16.720 --> 00:44:20.970 align:middle line:84% So here, I've got the print method on object c. 00:44:20.970 --> 00:44:23.250 align:middle line:90% Python asks, what is your type? 00:44:23.250 --> 00:44:24.520 align:middle line:90% Oh, you're a cat? 00:44:24.520 --> 00:44:26.650 align:middle line:84% Do you have an str method defined? 00:44:26.650 --> 00:44:27.580 align:middle line:90% Yes, you do. 00:44:27.580 --> 00:44:30.970 align:middle line:84% So then, it uses the one that it finds right away. 00:44:30.970 --> 00:44:35.820 align:middle line:84% But if for some reason, the current object 00:44:35.820 --> 00:44:37.860 align:middle line:84% doesn't have that method, so an example of that 00:44:37.860 --> 00:44:40.350 align:middle line:90% is set_name, right? 00:44:40.350 --> 00:44:43.880 align:middle line:84% set_name is not a method defined in cat. 00:44:43.880 --> 00:44:45.083 align:middle line:90% c is an object of type cat. 00:44:45.083 --> 00:44:46.250 align:middle line:90% It doesn't have that method. 00:44:46.250 --> 00:44:48.410 align:middle line:84% Python says, oh, you don't have that method. 00:44:48.410 --> 00:44:49.850 align:middle line:90% Let me look at your parent. 00:44:49.850 --> 00:44:51.780 align:middle line:84% Does your parent have that method? 00:44:51.780 --> 00:44:53.870 align:middle line:84% And then it looks through in here. 00:44:53.870 --> 00:44:54.860 align:middle line:90% And it finds it. 00:44:54.860 --> 00:44:56.750 align:middle line:84% Good, if it finds it, it uses that one. 00:44:56.750 --> 00:44:59.638 align:middle line:84% If it doesn't find it, it looks at your parent's parent. 00:44:59.638 --> 00:45:01.680 align:middle line:84% If your parent's parent has it, it uses that one. 00:45:01.680 --> 00:45:04.222 align:middle line:84% And if it doesn't, it looks at your parent's parent's parent. 00:45:04.222 --> 00:45:06.680 align:middle line:84% Until it gets to the generic Python object, 00:45:06.680 --> 00:45:10.890 align:middle line:84% this one right here, if they have it, it uses that one. 00:45:10.890 --> 00:45:13.080 align:middle line:84% And if it doesn't, then it throws an error. 00:45:13.080 --> 00:45:16.280 align:middle line:84% So an example of something that the generic Python object has 00:45:16.280 --> 00:45:17.660 align:middle line:90% is the str method, right? 00:45:17.660 --> 00:45:19.370 align:middle line:84% It just prints the memory location. 00:45:19.370 --> 00:45:21.890 align:middle line:84% And that's why, when we don't implement our str 00:45:21.890 --> 00:45:25.650 align:middle line:84% method in our class, Python defaults to the generic Python 00:45:25.650 --> 00:45:26.150 align:middle line:90% object. 00:45:26.150 --> 00:45:35.070 align:middle line:90% 00:45:35.070 --> 00:45:35.850 align:middle line:90% Questions? 00:45:35.850 --> 00:45:39.890 align:middle line:90% 00:45:39.890 --> 00:45:42.460 align:middle line:90% OK, let's look at a person. 00:45:42.460 --> 00:45:47.550 align:middle line:90% So let's create a person object. 00:45:47.550 --> 00:45:51.120 align:middle line:84% This person object, again, will inherit from animal 00:45:51.120 --> 00:45:55.650 align:middle line:84% because the only things we set an animal 00:45:55.650 --> 00:45:59.310 align:middle line:84% as defined as is being alive for some period of time. 00:45:59.310 --> 00:46:01.362 align:middle line:90% And it has no name. 00:46:01.362 --> 00:46:02.070 align:middle line:90% The name is None. 00:46:02.070 --> 00:46:04.150 align:middle line:90% So we don't even pass that in. 00:46:04.150 --> 00:46:08.580 align:middle line:84% So let's say the parent class, the person is animal. 00:46:08.580 --> 00:46:11.420 align:middle line:84% But this is my design choice, also 00:46:11.420 --> 00:46:12.670 align:middle line:90% to highlight a bunch of stuff. 00:46:12.670 --> 00:46:16.590 align:middle line:84% But let's say that this person class, when 00:46:16.590 --> 00:46:19.140 align:middle line:84% I create a new person object, I would like 00:46:19.140 --> 00:46:22.283 align:middle line:90% to pass in an age and a name. 00:46:22.283 --> 00:46:24.450 align:middle line:84% So I don't just want to create a person with an age. 00:46:24.450 --> 00:46:27.870 align:middle line:84% I want to actually create it using a name in that parameter 00:46:27.870 --> 00:46:28.900 align:middle line:90% list. 00:46:28.900 --> 00:46:33.360 align:middle line:84% So as an example, in my code here, when I create a person, 00:46:33.360 --> 00:46:37.020 align:middle line:84% I would like to pass in their name, comma, and the age, two 00:46:37.020 --> 00:46:40.660 align:middle line:90% parameters to make a person. 00:46:40.660 --> 00:46:45.660 align:middle line:84% Well, I can't use the animal's init method, right? 00:46:45.660 --> 00:46:48.180 align:middle line:84% I could for cat because cat was happy to just 00:46:48.180 --> 00:46:50.070 align:middle line:90% be created using an age. 00:46:50.070 --> 00:46:52.530 align:middle line:84% But I can't do that for a person because I 00:46:52.530 --> 00:46:54.240 align:middle line:84% would like to create a person by passing 00:46:54.240 --> 00:46:58.090 align:middle line:84% in two parameters in the creation of the person. 00:46:58.090 --> 00:47:00.960 align:middle line:84% So what I have to do is effectively 00:47:00.960 --> 00:47:03.420 align:middle line:84% override the init method of animal 00:47:03.420 --> 00:47:09.070 align:middle line:84% by implementing it in my class definition, right? 00:47:09.070 --> 00:47:12.310 align:middle line:84% So here, I have to define my own init method. 00:47:12.310 --> 00:47:15.730 align:middle line:84% And I do it because now, I'm not just passing in an age. 00:47:15.730 --> 00:47:20.340 align:middle line:84% I want to pass in a name and an age in the parameter list. 00:47:20.340 --> 00:47:23.650 align:middle line:84% And then, beyond that, what do I do inside the init method? 00:47:23.650 --> 00:47:27.070 align:middle line:84% Well, I know that this person is an animal. 00:47:27.070 --> 00:47:30.660 align:middle line:84% So what I'm going to do to make my life simpler 00:47:30.660 --> 00:47:33.100 align:middle line:90% is to call animals init method. 00:47:33.100 --> 00:47:35.040 align:middle line:84% So here, we use this dot notation 00:47:35.040 --> 00:47:36.660 align:middle line:84% on the name of the class, sort of 00:47:36.660 --> 00:47:40.590 align:middle line:84% similar to how I showed you that long way of calling methods. 00:47:40.590 --> 00:47:42.600 align:middle line:84% Well, here's the name of the class, dot 00:47:42.600 --> 00:47:44.040 align:middle line:90% the name of the method, init. 00:47:44.040 --> 00:47:46.785 align:middle line:84% And now, I pass in all the parameters, self and age. 00:47:46.785 --> 00:47:49.610 align:middle line:90% 00:47:49.610 --> 00:47:51.860 align:middle line:84% So I'm going to call animal's init method, which 00:47:51.860 --> 00:47:55.280 align:middle line:84% will create that self.age, set it to age, 00:47:55.280 --> 00:47:57.920 align:middle line:84% and create that self.name and set it to none. 00:47:57.920 --> 00:47:59.480 align:middle line:84% So I'm taking advantage of the fact 00:47:59.480 --> 00:48:02.300 align:middle line:84% that that init method already does those two lines for me, 00:48:02.300 --> 00:48:02.840 align:middle line:90% right? 00:48:02.840 --> 00:48:06.290 align:middle line:84% So I've turned those two lines into one line here. 00:48:06.290 --> 00:48:08.450 align:middle line:84% And then I'm going to say, well, I'd 00:48:08.450 --> 00:48:10.310 align:middle line:84% like to set the name of my person. 00:48:10.310 --> 00:48:13.520 align:middle line:84% So I'm going to call the method set_name with the parameter 00:48:13.520 --> 00:48:14.960 align:middle line:90% that's passed in. 00:48:14.960 --> 00:48:17.180 align:middle line:84% And then I'm also going to initialize 00:48:17.180 --> 00:48:19.610 align:middle line:84% another data attribute for a person, which 00:48:19.610 --> 00:48:22.900 align:middle line:84% is a list of friends, initially empty. 00:48:22.900 --> 00:48:25.420 align:middle line:84% So what's nice about this, and when we implement the student 00:48:25.420 --> 00:48:28.150 align:middle line:84% class, it'll look even nicer, what's 00:48:28.150 --> 00:48:30.580 align:middle line:84% nice about this is we're taking advantage of the fact 00:48:30.580 --> 00:48:33.500 align:middle line:84% that the init method of animal already does some work for us. 00:48:33.500 --> 00:48:35.890 align:middle line:84% But at the same time, we can clearly 00:48:35.890 --> 00:48:40.150 align:middle line:84% see, in this subclass, what the person 00:48:40.150 --> 00:48:44.620 align:middle line:84% object brings in addition to the animal object, right? 00:48:44.620 --> 00:48:50.050 align:middle line:84% So in addition to just being an animal, we give a name 00:48:50.050 --> 00:48:52.060 align:middle line:84% and get a list of friends, right? 00:48:52.060 --> 00:48:55.270 align:middle line:84% So it's very nice to see the extra data attributes 00:48:55.270 --> 00:48:59.140 align:middle line:84% or what you need to change with respect to the animal 00:48:59.140 --> 00:49:00.760 align:middle line:90% to make a person. 00:49:00.760 --> 00:49:02.570 align:middle line:90% And then, beyond that-- 00:49:02.570 --> 00:49:04.390 align:middle line:90% so I think that's what I said. 00:49:04.390 --> 00:49:06.620 align:middle line:84% Sorry, I didn't go through that as I said it. 00:49:06.620 --> 00:49:09.370 align:middle line:84% And then beyond that, I've got some-- 00:49:09.370 --> 00:49:10.870 align:middle line:84% we can add some getters and setters. 00:49:10.870 --> 00:49:12.290 align:middle line:90% I just did a select few. 00:49:12.290 --> 00:49:13.970 align:middle line:84% But you should add them for all of them. 00:49:13.970 --> 00:49:17.050 align:middle line:84% So the get_friends just returns a copy of my list 00:49:17.050 --> 00:49:19.422 align:middle line:84% because maybe I want to keep my original order 00:49:19.422 --> 00:49:20.380 align:middle line:90% or something like that. 00:49:20.380 --> 00:49:24.080 align:middle line:84% So it's just good style to return a copy of a list. 00:49:24.080 --> 00:49:26.630 align:middle line:84% The ability to add a friend to my list 00:49:26.630 --> 00:49:30.320 align:middle line:84% basically just adds a friend's name as a string, 00:49:30.320 --> 00:49:32.040 align:middle line:90% if it's not already in the list. 00:49:32.040 --> 00:49:35.030 align:middle line:84% So I can't have two Ana's in my list. 00:49:35.030 --> 00:49:37.220 align:middle line:90% I consider them the same. 00:49:37.220 --> 00:49:39.830 align:middle line:84% Ability to speak just prints hello to the screen. 00:49:39.830 --> 00:49:42.020 align:middle line:84% And then I added this cute little function 00:49:42.020 --> 00:49:45.560 align:middle line:84% to tell me the age difference between this object that I'm 00:49:45.560 --> 00:49:49.190 align:middle line:84% calling age difference on and some other person, right? 00:49:49.190 --> 00:49:51.710 align:middle line:84% And all it does is grab the two ages, 00:49:51.710 --> 00:49:53.660 align:middle line:84% take the absolute value of the difference, 00:49:53.660 --> 00:49:55.990 align:middle line:90% and print that to the screen. 00:49:55.990 --> 00:49:59.980 align:middle line:84% And then lastly, we're going to override the str 00:49:59.980 --> 00:50:02.890 align:middle line:84% method of animal, instead of saying animal, colon, name, 00:50:02.890 --> 00:50:05.650 align:middle line:84% colon, age, to say person, colon, name, colon, age. 00:50:05.650 --> 00:50:09.470 align:middle line:84% So this way, it helps me figure out the type as well. 00:50:09.470 --> 00:50:15.560 align:middle line:84% So in my code here, I've got two people, p1, p2. 00:50:15.560 --> 00:50:16.940 align:middle line:90% Here's Jack, age 30. 00:50:16.940 --> 00:50:18.365 align:middle line:90% Here's Jill, age 25. 00:50:18.365 --> 00:50:25.880 align:middle line:84% If I run the get_name, get_age on both of these, 00:50:25.880 --> 00:50:28.970 align:middle line:84% this will run animal's get_age get_name. 00:50:28.970 --> 00:50:32.390 align:middle line:84% I've not defined these in here, which is fine. 00:50:32.390 --> 00:50:33.413 align:middle line:90% We inherit from animal. 00:50:33.413 --> 00:50:35.330 align:middle line:84% And animal knows how to grab the age and name. 00:50:35.330 --> 00:50:36.515 align:middle line:90% So there they are. 00:50:36.515 --> 00:50:39.710 align:middle line:90% 00:50:39.710 --> 00:50:44.350 align:middle line:84% If I print P1, it'll print person, colon, name, age. 00:50:44.350 --> 00:50:48.150 align:middle line:84% If I ask p1 to speak, it just prints, hello. 00:50:48.150 --> 00:50:51.475 align:middle line:84% If I ask the age difference between p1 and p2, no matter 00:50:51.475 --> 00:50:53.100 align:middle line:84% what, it just takes the absolute value, 00:50:53.100 --> 00:50:54.930 align:middle line:90% prints five year difference. 00:50:54.930 --> 00:50:58.590 align:middle line:84% And then let's add some friends to p1. 00:50:58.590 --> 00:51:00.060 align:middle line:90% So here, I've got two Bob's. 00:51:00.060 --> 00:51:02.565 align:middle line:84% But it's just the list, keeping unique names. 00:51:02.565 --> 00:51:08.940 align:middle line:90% 00:51:08.940 --> 00:51:11.550 align:middle line:84% OK, so let's have you try this for a little bit. 00:51:11.550 --> 00:51:14.335 align:middle line:90% 00:51:14.335 --> 00:51:15.710 align:middle line:84% It's a little bit, again, working 00:51:15.710 --> 00:51:17.270 align:middle line:90% with objects of this type. 00:51:17.270 --> 00:51:21.270 align:middle line:84% So it's a function that takes in a dictionary. 00:51:21.270 --> 00:51:23.270 align:middle line:84% So I'll tell you what the dictionary looks like. 00:51:23.270 --> 00:51:29.900 align:middle line:84% It maps a person object to a cat object, all right? 00:51:29.900 --> 00:51:30.900 align:middle line:90% So that's my dictionary. 00:51:30.900 --> 00:51:31.940 align:middle line:90% So this is the key. 00:51:31.940 --> 00:51:32.820 align:middle line:90% This is the value. 00:51:32.820 --> 00:51:35.750 align:middle line:84% So I've got all these person objects 00:51:35.750 --> 00:51:37.235 align:middle line:90% being mapped to cat objects. 00:51:37.235 --> 00:51:39.860 align:middle line:90% 00:51:39.860 --> 00:51:42.760 align:middle line:84% So as an example, here's an input dictionary. 00:51:42.760 --> 00:51:47.200 align:middle line:90% p1 is this person here. 00:51:47.200 --> 00:51:50.890 align:middle line:84% And p2 is this person here, right? 00:51:50.890 --> 00:51:54.640 align:middle line:84% So my two keys, p1, p2, are person objects, right? 00:51:54.640 --> 00:51:56.290 align:middle line:84% They're not integers, floats, strings. 00:51:56.290 --> 00:51:57.850 align:middle line:90% They're person objects. 00:51:57.850 --> 00:52:01.250 align:middle line:84% And then the values associated with those are cat objects. 00:52:01.250 --> 00:52:03.880 align:middle line:84% So here's an object of type cat with its name. 00:52:03.880 --> 00:52:08.170 align:middle line:84% I just ran set_name on that cat after I created it, same here. 00:52:08.170 --> 00:52:11.980 align:middle line:84% Here's the name set to this new cat object. 00:52:11.980 --> 00:52:15.160 align:middle line:84% So I've mapped p1 to c1, p2 to c2. 00:52:15.160 --> 00:52:17.155 align:middle line:84% So if I run this function, what I'd like to do 00:52:17.155 --> 00:52:18.910 align:middle line:84% is not return anything this function. 00:52:18.910 --> 00:52:20.530 align:middle line:90% It just prints something. 00:52:20.530 --> 00:52:23.230 align:middle line:84% On each line, as you're going through all the items 00:52:23.230 --> 00:52:25.240 align:middle line:84% in the dictionary, it just prints 00:52:25.240 --> 00:52:33.302 align:middle line:84% the name of that key, colon, the name of the value. 00:52:33.302 --> 00:52:34.760 align:middle line:84% So all I'd like to do is write code 00:52:34.760 --> 00:52:37.410 align:middle line:84% that extracts the name from my person object 00:52:37.410 --> 00:52:44.353 align:middle line:90% and from the cat object, OK? 00:52:44.353 --> 00:52:45.520 align:middle line:90% I know what you're thinking. 00:52:45.520 --> 00:52:48.850 align:middle line:90% I look really young for 86. 00:52:48.850 --> 00:52:51.790 align:middle line:84% But it's diet, exercise, and hanging out with you 00:52:51.790 --> 00:52:58.286 align:middle line:90% guys, and candy, for sure. 00:52:58.286 --> 00:53:04.200 align:middle line:84% So here, let's write this code on 178. 00:53:04.200 --> 00:53:05.700 align:middle line:84% All right, does anyone have a start? 00:53:05.700 --> 00:53:10.700 align:middle line:90% 00:53:10.700 --> 00:53:11.200 align:middle line:90% Yeah. 00:53:11.200 --> 00:53:16.586 align:middle line:90% 00:53:16.586 --> 00:53:18.810 align:middle line:90% d.items, yep. 00:53:18.810 --> 00:53:23.970 align:middle line:84% Let's write a note for ourselves. k is person. 00:53:23.970 --> 00:53:27.056 align:middle line:90% v is cat, yep. 00:53:27.056 --> 00:53:29.864 align:middle line:84% AUDIENCE: [INAUDIBLE] k.get_name. 00:53:29.864 --> 00:53:33.150 align:middle line:90% 00:53:33.150 --> 00:53:35.940 align:middle line:84% ANA BELL: Yep, so k.get_name, you want to save it 00:53:35.940 --> 00:53:37.816 align:middle line:90% as a variable? 00:53:37.816 --> 00:53:40.480 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:53:40.480 --> 00:53:41.585 align:middle line:90% ANA BELL: Or no? 00:53:41.585 --> 00:53:42.950 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:53:42.950 --> 00:53:44.992 align:middle line:84% ANA BELL: Oh, you want to put it on the one line. 00:53:44.992 --> 00:53:46.470 align:middle line:90% That's fine, yep. 00:53:46.470 --> 00:53:49.091 align:middle line:90% Print, k.get_name. 00:53:49.091 --> 00:53:51.550 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:53:51.550 --> 00:53:59.160 align:middle line:84% ANA BELL: Yup, v.get_name, exactly, yep, perfect. 00:53:59.160 --> 00:54:01.420 align:middle line:90% And yeah, nothing to return. 00:54:01.420 --> 00:54:05.760 align:middle line:90% So let's run that, cool. 00:54:05.760 --> 00:54:08.450 align:middle line:90% 00:54:08.450 --> 00:54:10.440 align:middle line:84% Does anyone have questions about that? 00:54:10.440 --> 00:54:15.740 align:middle line:90% 00:54:15.740 --> 00:54:17.960 align:middle line:84% All right, so we're just manipulating these object 00:54:17.960 --> 00:54:18.460 align:middle line:90% types. 00:54:18.460 --> 00:54:20.880 align:middle line:84% And again, if it's confusing, I highly 00:54:20.880 --> 00:54:23.010 align:middle line:84% recommend, quiz situations and things 00:54:23.010 --> 00:54:25.620 align:middle line:84% like that, now that we're working with object types, 00:54:25.620 --> 00:54:27.987 align:middle line:90% just make little notes, right? 00:54:27.987 --> 00:54:29.820 align:middle line:84% I know we're iterating through a dictionary. 00:54:29.820 --> 00:54:31.440 align:middle line:84% And it's kind of convention, right? 00:54:31.440 --> 00:54:33.220 align:middle line:84% Keys are integers, things like that. 00:54:33.220 --> 00:54:36.630 align:middle line:84% But this particular case, just a little note that k is a person 00:54:36.630 --> 00:54:41.190 align:middle line:84% will help you remember that you need to run a method on this k 00:54:41.190 --> 00:54:45.570 align:middle line:84% variable, like we did here, k.get_name and then v.get_name. 00:54:45.570 --> 00:54:53.940 align:middle line:90% 00:54:53.940 --> 00:54:58.120 align:middle line:90% OK, yeah. 00:54:58.120 --> 00:55:02.070 align:middle line:84% AUDIENCE: How do we ensure that the get_name, 00:55:02.070 --> 00:55:06.060 align:middle line:90% that the [INAUDIBLE]? 00:55:06.060 --> 00:55:06.660 align:middle line:90% And-- 00:55:06.660 --> 00:55:08.868 align:middle line:84% ANA BELL: How do you ensure that the keys are person? 00:55:08.868 --> 00:55:11.490 align:middle line:90% 00:55:11.490 --> 00:55:14.440 align:middle line:84% You can't ensure it in this particular case. 00:55:14.440 --> 00:55:19.200 align:middle line:84% I mean, you could say, if type of k equal, equal, 00:55:19.200 --> 00:55:22.860 align:middle line:84% person, capital P, person, then do the code, 00:55:22.860 --> 00:55:25.125 align:middle line:84% and else probably just skip it or raise 00:55:25.125 --> 00:55:26.250 align:middle line:90% a value error or something. 00:55:26.250 --> 00:55:27.750 align:middle line:84% Like, you could enforce it that way. 00:55:27.750 --> 00:55:29.190 align:middle line:84% But in this particular case, we're 00:55:29.190 --> 00:55:34.200 align:middle line:84% just assuming that the tester will make person objects mapped 00:55:34.200 --> 00:55:37.350 align:middle line:90% to cat objects, yeah. 00:55:37.350 --> 00:55:41.010 align:middle line:84% But yes, certainly, if you're making a software for something 00:55:41.010 --> 00:55:43.350 align:middle line:84% more complex, you should probably make sure of that, 00:55:43.350 --> 00:55:45.360 align:middle line:90% enforce that. 00:55:45.360 --> 00:55:47.310 align:middle line:84% OK, so the big idea with inheritance 00:55:47.310 --> 00:55:50.400 align:middle line:84% is that now that we have sub-classes, also 00:55:50.400 --> 00:55:53.160 align:middle line:84% known as child classes, those sub-classes 00:55:53.160 --> 00:55:54.910 align:middle line:90% use a parent's attributes. 00:55:54.910 --> 00:55:57.030 align:middle line:84% So everything that a parent has and can do, 00:55:57.030 --> 00:55:59.160 align:middle line:90% a child has and can do as well. 00:55:59.160 --> 00:56:03.480 align:middle line:84% But that child can override certain parent's behaviors. 00:56:03.480 --> 00:56:07.680 align:middle line:84% And the child can add new behaviors or new attributes, 00:56:07.680 --> 00:56:09.980 align:middle line:90% in addition to the parent. 00:56:09.980 --> 00:56:12.200 align:middle line:84% Let's look at one more subclass student before we 00:56:12.200 --> 00:56:14.090 align:middle line:90% go on to one last thing. 00:56:14.090 --> 00:56:19.370 align:middle line:84% So a student here from our pictures and diagrams 00:56:19.370 --> 00:56:21.500 align:middle line:84% inherits from person, not from animal, 00:56:21.500 --> 00:56:23.660 align:middle line:84% but indirectly from animal, right? 00:56:23.660 --> 00:56:26.250 align:middle line:90% So a student is a person. 00:56:26.250 --> 00:56:28.400 align:middle line:84% And when I create a person, I would 00:56:28.400 --> 00:56:31.910 align:middle line:84% love to create it using a name an age and a major. 00:56:31.910 --> 00:56:34.520 align:middle line:84% But we can use a default parameter for that major 00:56:34.520 --> 00:56:36.770 align:middle line:84% to be None if we don't actually want to pass it in. 00:56:36.770 --> 00:56:41.460 align:middle line:84% But I would like to create it by setting their major as well. 00:56:41.460 --> 00:56:44.930 align:middle line:84% So now, I can't use the parent's init method 00:56:44.930 --> 00:56:46.880 align:middle line:84% because I've got three parameters I would like 00:56:46.880 --> 00:56:48.740 align:middle line:90% to initiate my student with. 00:56:48.740 --> 00:56:52.740 align:middle line:84% So I would like to create my own init method inside person. 00:56:52.740 --> 00:56:57.010 align:middle line:84% So here I am, defining my own init method. 00:56:57.010 --> 00:57:00.510 align:middle line:84% And now, it becomes apparent why it's 00:57:00.510 --> 00:57:02.820 align:middle line:84% nice to call the init method of your parent 00:57:02.820 --> 00:57:07.020 align:middle line:84% because if I say a student is a person, 00:57:07.020 --> 00:57:10.590 align:middle line:84% all I need to do to initialize a person type, 00:57:10.590 --> 00:57:12.503 align:middle line:84% all the attributes associated with a person 00:57:12.503 --> 00:57:13.920 align:middle line:84% and the init method of the person. 00:57:13.920 --> 00:57:16.260 align:middle line:84% So just call the init method of the person. 00:57:16.260 --> 00:57:20.280 align:middle line:84% That will create my name, my age, set my name, 00:57:20.280 --> 00:57:22.240 align:middle line:84% create my list of friends, all that stuff. 00:57:22.240 --> 00:57:26.320 align:middle line:84% So those five lines get compacted into this one line. 00:57:26.320 --> 00:57:30.250 align:middle line:84% And then it also becomes really easy to see what the student 00:57:30.250 --> 00:57:32.410 align:middle line:90% has, in addition to the person. 00:57:32.410 --> 00:57:37.510 align:middle line:84% Well, it just has a major data attribute. self.major is 00:57:37.510 --> 00:57:39.370 align:middle line:90% set to whatever is passed in. 00:57:39.370 --> 00:57:42.910 align:middle line:84% And then, beyond this, it's just methods 00:57:42.910 --> 00:57:44.420 align:middle line:90% here and there to do stuff. 00:57:44.420 --> 00:57:47.920 align:middle line:84% So here, I've got a change_major method. 00:57:47.920 --> 00:57:49.420 align:middle line:84% It just sets the major to something. 00:57:49.420 --> 00:57:51.560 align:middle line:84% I should probably add a getter in there as well. 00:57:51.560 --> 00:57:52.810 align:middle line:90% But I ran out of room. 00:57:52.810 --> 00:57:55.600 align:middle line:84% And here's a speak method that gets overridden 00:57:55.600 --> 00:57:57.700 align:middle line:90% from the method of person. 00:57:57.700 --> 00:57:59.530 align:middle line:84% So the speak method, for student, 00:57:59.530 --> 00:58:06.500 align:middle line:84% I made it slightly more complex than what the parent has. 00:58:06.500 --> 00:58:10.480 align:middle line:84% So here, I'm using this random library, not a random library 00:58:10.480 --> 00:58:14.150 align:middle line:84% I found, arbitrary library, it's a library called random. 00:58:14.150 --> 00:58:16.750 align:middle line:84% And it has a bunch of functions that allow you 00:58:16.750 --> 00:58:19.010 align:middle line:90% to deal with random numbers. 00:58:19.010 --> 00:58:21.520 align:middle line:84% So one of the functions that this random library has 00:58:21.520 --> 00:58:24.190 align:middle line:84% gives you a number between 0 and 1 00:58:24.190 --> 00:58:27.440 align:middle line:90% at random, so a float at random. 00:58:27.440 --> 00:58:29.980 align:middle line:84% So what I'm doing in the speak method for students 00:58:29.980 --> 00:58:35.230 align:middle line:84% is randomly printing one of four strings, 00:58:35.230 --> 00:58:37.690 align:middle line:84% according to where that random number that's gotten 00:58:37.690 --> 00:58:40.780 align:middle line:90% lines between 0 and 1. 00:58:40.780 --> 00:58:43.730 align:middle line:90% And then-- oops, not yet. 00:58:43.730 --> 00:58:47.060 align:middle line:84% And then, here, I'm overriding my str method. 00:58:47.060 --> 00:58:52.750 align:middle line:84% So we can see, in the student class 00:58:52.750 --> 00:58:54.560 align:middle line:84% here, here, I've created two students. 00:58:54.560 --> 00:58:56.230 align:middle line:84% So this one actually has a major. 00:58:56.230 --> 00:58:59.230 align:middle line:84% This one's major is going to be set to None, just the default 00:58:59.230 --> 00:59:00.370 align:middle line:90% value. 00:59:00.370 --> 00:59:03.370 align:middle line:84% And then, if I run this code, you can see, 00:59:03.370 --> 00:59:06.610 align:middle line:84% every time I run it, the student 1 says something different. 00:59:06.610 --> 00:59:08.240 align:middle line:84% Student two says something different. 00:59:08.240 --> 00:59:11.280 align:middle line:84% So it's just running this random number 00:59:11.280 --> 00:59:15.030 align:middle line:90% and then choosing what to print. 00:59:15.030 --> 00:59:18.190 align:middle line:84% Maybe more often than not, I should bias it 00:59:18.190 --> 00:59:21.470 align:middle line:90% towards something. 00:59:21.470 --> 00:59:24.375 align:middle line:84% All right, so one more class I'd like to talk about, rabbit. 00:59:24.375 --> 00:59:26.000 align:middle line:84% That's the one that we actually haven't 00:59:26.000 --> 00:59:28.310 align:middle line:84% talked about from those little subcategories. 00:59:28.310 --> 00:59:30.260 align:middle line:84% And as we talk about this rabbit class, 00:59:30.260 --> 00:59:33.390 align:middle line:84% I'd like to introduce one more idea of a variable. 00:59:33.390 --> 00:59:35.360 align:middle line:84% So far, we've had just plain old variables 00:59:35.360 --> 00:59:39.290 align:middle line:84% that go away as soon as a environment disappears. 00:59:39.290 --> 00:59:41.540 align:middle line:84% We've talked about instance variables, a.k.a. 00:59:41.540 --> 00:59:45.560 align:middle line:84% data attributes, which are consistent for objects 00:59:45.560 --> 00:59:47.120 align:middle line:84% that you create of a certain type 00:59:47.120 --> 00:59:50.480 align:middle line:84% but have different values for different instances. 00:59:50.480 --> 00:59:54.650 align:middle line:84% The last variable I'd like to talk about is a class variable. 00:59:54.650 --> 00:59:57.830 align:middle line:84% What's cool about a class variable is that, 00:59:57.830 --> 01:00:00.450 align:middle line:84% think of it like a shared resource. 01:00:00.450 --> 01:00:04.700 align:middle line:84% So it's a variable that any instance 01:00:04.700 --> 01:00:09.980 align:middle line:84% of this particular type can access and modify. 01:00:09.980 --> 01:00:13.130 align:middle line:84% And if it's modified, all the other instances 01:00:13.130 --> 01:00:16.250 align:middle line:84% will see this modified value, right? 01:00:16.250 --> 01:00:19.880 align:middle line:84% So it's just shared across all the instances of type rabbit, 01:00:19.880 --> 01:00:21.760 align:middle line:90% in this particular case. 01:00:21.760 --> 01:00:26.070 align:middle line:84% And so there's many different ways to use class variables. 01:00:26.070 --> 01:00:29.140 align:middle line:84% In object-oriented programming, they're pretty useful. 01:00:29.140 --> 01:00:32.010 align:middle line:84% The way I'm going to use it here is 01:00:32.010 --> 01:00:35.190 align:middle line:84% to give me the ability to basically count 01:00:35.190 --> 01:00:38.640 align:middle line:84% how many instances of this type rabbit 01:00:38.640 --> 01:00:40.440 align:middle line:90% I've created in my program. 01:00:40.440 --> 01:00:41.940 align:middle line:84% So when I run the program, remember, 01:00:41.940 --> 01:00:43.890 align:middle line:84% I can create a whole bunch of instances. 01:00:43.890 --> 01:00:46.830 align:middle line:84% I'm going to try to use this class variable as a way for me 01:00:46.830 --> 01:00:49.980 align:middle line:84% to basically keep a counter of how many of these instances 01:00:49.980 --> 01:00:51.960 align:middle line:90% I've created. 01:00:51.960 --> 01:00:55.900 align:middle line:84% All right, so let's look at the code. 01:00:55.900 --> 01:00:58.590 align:middle line:84% So the first thing I'm going to do is just inherit from animal. 01:00:58.590 --> 01:01:00.820 align:middle line:90% It gets a name and an age. 01:01:00.820 --> 01:01:04.480 align:middle line:84% And that's about it, all those getters and setters and the str 01:01:04.480 --> 01:01:05.710 align:middle line:90% method. 01:01:05.710 --> 01:01:08.500 align:middle line:84% Now, to create my class variable, 01:01:08.500 --> 01:01:11.070 align:middle line:84% notice I'm defining this variable, just plain 01:01:11.070 --> 01:01:15.120 align:middle line:84% old variable, outside of any methods within the class 01:01:15.120 --> 01:01:16.770 align:middle line:90% definition, right? 01:01:16.770 --> 01:01:18.720 align:middle line:90% So here's tag is equal to 1. 01:01:18.720 --> 01:01:21.390 align:middle line:84% The very first variable-- the very first instance 01:01:21.390 --> 01:01:25.620 align:middle line:84% of a rabbit I create will grab the value of whatever it 01:01:25.620 --> 01:01:27.290 align:middle line:90% says here. 01:01:27.290 --> 01:01:32.720 align:middle line:84% But then, if any instance changes this value, 01:01:32.720 --> 01:01:36.320 align:middle line:84% other instances will see that changed value, OK? 01:01:36.320 --> 01:01:38.870 align:middle line:84% So what we're going to do is, we're 01:01:38.870 --> 01:01:43.790 align:middle line:84% going to implement ID numbers for these rabbits, 01:01:43.790 --> 01:01:47.420 align:middle line:84% so sort of tagging them to keep track of how many there are. 01:01:47.420 --> 01:01:50.950 align:middle line:84% So in the init method of animal, or of rabbit, 01:01:50.950 --> 01:01:55.840 align:middle line:84% I'm going to create a new rabbit using an age and two parents. 01:01:55.840 --> 01:01:57.460 align:middle line:84% So, again, different than animal, 01:01:57.460 --> 01:02:00.530 align:middle line:84% so I'm going to have to implement my own init method. 01:02:00.530 --> 01:02:02.600 align:middle line:84% But I'll call animal's init method 01:02:02.600 --> 01:02:04.610 align:middle line:84% because it does some work for me. 01:02:04.610 --> 01:02:08.930 align:middle line:84% Then, I'm going to add two data attributes for the two parents 01:02:08.930 --> 01:02:11.190 align:middle line:90% to be whatever is passed in. 01:02:11.190 --> 01:02:13.530 align:middle line:84% And then, down here is where I'm going 01:02:13.530 --> 01:02:16.560 align:middle line:84% to use this class variable, the shared resource, these two 01:02:16.560 --> 01:02:17.440 align:middle line:90% lines. 01:02:17.440 --> 01:02:19.620 align:middle line:84% So the first thing I'm going to do 01:02:19.620 --> 01:02:22.980 align:middle line:84% is add one last data attribute for my rabbit, 01:02:22.980 --> 01:02:24.520 align:middle line:90% which is the rid value. 01:02:24.520 --> 01:02:26.160 align:middle line:90% So it's the rabbit ID. 01:02:26.160 --> 01:02:28.080 align:middle line:84% And this is going to be a unique value 01:02:28.080 --> 01:02:29.880 align:middle line:90% for every rabbit I create. 01:02:29.880 --> 01:02:31.800 align:middle line:84% First rabbit will have a value of 1 01:02:31.800 --> 01:02:32.980 align:middle line:90% that I create in my program. 01:02:32.980 --> 01:02:36.490 align:middle line:84% Second rabbit I create will have a value of 2, and so on. 01:02:36.490 --> 01:02:37.980 align:middle line:90% So what am I setting it to? 01:02:37.980 --> 01:02:40.140 align:middle line:84% Well, I'm going to set it to whatever the tag is. 01:02:40.140 --> 01:02:45.390 align:middle line:84% So the very first rabbit I create, their rid will be 1. 01:02:45.390 --> 01:02:48.000 align:middle line:84% That's what the tag is initially set to. 01:02:48.000 --> 01:02:51.840 align:middle line:84% But then, before I finish the init method, 01:02:51.840 --> 01:02:57.870 align:middle line:84% there's one other line of code, rabbit.tag plus equals 1. 01:02:57.870 --> 01:03:01.950 align:middle line:84% So this instance, right before it finishes creating itself, 01:03:01.950 --> 01:03:05.370 align:middle line:84% is going to take that tag and increment it by 1. 01:03:05.370 --> 01:03:10.460 align:middle line:84% So the next rabbit I create, it's 01:03:10.460 --> 01:03:15.100 align:middle line:84% going to grab the tag value that was just changed, OK? 01:03:15.100 --> 01:03:17.300 align:middle line:90% Let's visualize it. 01:03:17.300 --> 01:03:20.510 align:middle line:84% So we're going to do it with actual rabbits. 01:03:20.510 --> 01:03:24.530 align:middle line:84% OK, so first, I'm going to-- so there's going 01:03:24.530 --> 01:03:25.620 align:middle line:90% to be three lines of code. 01:03:25.620 --> 01:03:27.330 align:middle line:84% And this is the program I'm going to run. 01:03:27.330 --> 01:03:29.000 align:middle line:84% So the first thing I'm going to do 01:03:29.000 --> 01:03:33.770 align:middle line:84% is create my first rabbit, right? 01:03:33.770 --> 01:03:37.340 align:middle line:84% Its rid will be whatever the value of tag 01:03:37.340 --> 01:03:38.510 align:middle line:90% is originally, right? 01:03:38.510 --> 01:03:40.820 align:middle line:84% So, originally, we said the tag is 1. 01:03:40.820 --> 01:03:44.580 align:middle line:84% So behind the scenes, what's going to happen is Python says, 01:03:44.580 --> 01:03:47.040 align:middle line:84% oh, you're the first instance of rabbit class. 01:03:47.040 --> 01:03:49.610 align:middle line:90% So the tag was initialized to 1. 01:03:49.610 --> 01:03:55.640 align:middle line:84% So your rid is going to be whatever the value is, 1. 01:03:55.640 --> 01:03:58.340 align:middle line:84% So I've got, this rabbit, its age is 8. 01:03:58.340 --> 01:03:59.300 align:middle line:90% Two parents are None. 01:03:59.300 --> 01:04:00.590 align:middle line:90% And rid is 1. 01:04:00.590 --> 01:04:03.710 align:middle line:84% But then, before I finish creating this rabbit, 01:04:03.710 --> 01:04:06.530 align:middle line:84% the last line of the init method says, take the tag 01:04:06.530 --> 01:04:07.850 align:middle line:90% and increment it by 1. 01:04:07.850 --> 01:04:10.470 align:middle line:90% 01:04:10.470 --> 01:04:13.020 align:middle line:84% All right, next line in the code says, here, 01:04:13.020 --> 01:04:14.640 align:middle line:90% let me create another rabbit. 01:04:14.640 --> 01:04:17.710 align:middle line:84% This one, I'm going to pass in age 6 as my parameter. 01:04:17.710 --> 01:04:19.080 align:middle line:90% So that's the age 6. 01:04:19.080 --> 01:04:22.710 align:middle line:84% Two parents are None by default. So Python says, 01:04:22.710 --> 01:04:25.020 align:middle line:84% all right, well, here's a new rabbit object. 01:04:25.020 --> 01:04:25.980 align:middle line:90% It's age is 6. 01:04:25.980 --> 01:04:28.100 align:middle line:90% The two parents are None. 01:04:28.100 --> 01:04:32.490 align:middle line:84% A line that says self.rid, so the rid of r2 01:04:32.490 --> 01:04:34.980 align:middle line:84% will be whatever tag is right now. 01:04:34.980 --> 01:04:37.650 align:middle line:84% Well, the previous rabbit incremented it to 2. 01:04:37.650 --> 01:04:43.570 align:middle line:84% So the rid of this next rabbit is 2, right? 01:04:43.570 --> 01:04:46.540 align:middle line:84% OK, the last line of code before this rabbit 01:04:46.540 --> 01:04:51.310 align:middle line:84% finishes it creating itself is to increment the tag to 3. 01:04:51.310 --> 01:04:54.170 align:middle line:84% So now, if I have one more line of code, 01:04:54.170 --> 01:04:55.690 align:middle line:90% I'm creating one more rabbit. 01:04:55.690 --> 01:04:57.670 align:middle line:90% This age is 10, right? 01:04:57.670 --> 01:05:01.900 align:middle line:84% So behind the scenes, Python creates this variable named R3. 01:05:01.900 --> 01:05:05.778 align:middle line:84% It's bound to an object, a rabbit object, whose age is 10. 01:05:05.778 --> 01:05:07.570 align:middle line:84% Two parents are None, of course, because we 01:05:07.570 --> 01:05:08.920 align:middle line:90% didn't pass in any parents. 01:05:08.920 --> 01:05:14.110 align:middle line:84% And the rid is whatever the tag is right now, 3, OK? 01:05:14.110 --> 01:05:18.010 align:middle line:84% Well, here's the one with rid of 3. 01:05:18.010 --> 01:05:20.020 align:middle line:84% And before we finish creating, let's 01:05:20.020 --> 01:05:21.940 align:middle line:84% just increment the tag so that we set it up 01:05:21.940 --> 01:05:23.980 align:middle line:90% for the next rabbit, OK? 01:05:23.980 --> 01:05:26.055 align:middle line:90% Everyone OK so far? 01:05:26.055 --> 01:05:27.430 align:middle line:84% AUDIENCE: [INAUDIBLE] just create 01:05:27.430 --> 01:05:30.610 align:middle line:90% the first line [INAUDIBLE]? 01:05:30.610 --> 01:05:36.420 align:middle line:84% ANA BELL: Yep, yes, yes, it gives you two 01:05:36.420 --> 01:05:39.990 align:middle line:84% because when you run this line, rabbit 8, 01:05:39.990 --> 01:05:43.210 align:middle line:84% it has to run the init to completion. 01:05:43.210 --> 01:05:44.980 align:middle line:84% And the last line of the init always 01:05:44.980 --> 01:05:50.120 align:middle line:84% increments it to be one more than what it started with. 01:05:50.120 --> 01:05:54.800 align:middle line:84% You can't, I guess, pause the function run in the middle 01:05:54.800 --> 01:05:55.340 align:middle line:90% to check. 01:05:55.340 --> 01:05:59.270 align:middle line:90% 01:05:59.270 --> 01:06:03.100 align:middle line:84% OK, so let's look at a couple other methods 01:06:03.100 --> 01:06:07.510 align:middle line:84% that we can implement-- sorry, other questions about that very 01:06:07.510 --> 01:06:10.870 align:middle line:90% cool way of creating rabbits? 01:06:10.870 --> 01:06:12.125 align:middle line:90% Yeah. 01:06:12.125 --> 01:06:13.750 align:middle line:84% AUDIENCE: I guess I just wanted to know 01:06:13.750 --> 01:06:18.430 align:middle line:84% more about the space above the definitions, so 01:06:18.430 --> 01:06:20.070 align:middle line:90% the [INAUDIBLE]. 01:06:20.070 --> 01:06:22.860 align:middle line:84% ANA BELL: Yes, let's go back, here. 01:06:22.860 --> 01:06:26.370 align:middle line:84% AUDIENCE: Yeah, so above the init definition, 01:06:26.370 --> 01:06:27.960 align:middle line:90% that space there, what else-- 01:06:27.960 --> 01:06:30.180 align:middle line:90% like, what goes on there? 01:06:30.180 --> 01:06:31.620 align:middle line:90% ANA BELL: Mostly just this. 01:06:31.620 --> 01:06:34.380 align:middle line:84% Yeah, mostly, you want the object 01:06:34.380 --> 01:06:37.350 align:middle line:84% to have things associated with it. 01:06:37.350 --> 01:06:40.590 align:middle line:84% So, really, shared stuff is nice. 01:06:40.590 --> 01:06:44.280 align:middle line:84% But it's a little tenuous in using it just because-- 01:06:44.280 --> 01:06:46.627 align:middle line:84% you should use it for pretty specific situations, right? 01:06:46.627 --> 01:06:48.960 align:middle line:84% You don't just want to define a whole bunch of variables 01:06:48.960 --> 01:06:51.270 align:middle line:84% that everybody can access here and there, 01:06:51.270 --> 01:06:52.980 align:middle line:90% only specific situations. 01:06:52.980 --> 01:06:56.340 align:middle line:84% Yeah, most of the time, you just have methods in the definition, 01:06:56.340 --> 01:06:58.500 align:middle line:90% yeah. 01:06:58.500 --> 01:06:59.860 align:middle line:90% But maybe there's other stuff. 01:06:59.860 --> 01:07:01.845 align:middle line:84% I just don't know about it right now, yeah. 01:07:01.845 --> 01:07:05.330 align:middle line:90% 01:07:05.330 --> 01:07:09.110 align:middle line:84% OK, let's look at a couple more methods for the rabbit so here, 01:07:09.110 --> 01:07:12.210 align:middle line:84% I've got a getter, just three getters. 01:07:12.210 --> 01:07:13.310 align:middle line:90% I should probably put-- 01:07:13.310 --> 01:07:15.140 align:middle line:84% I don't want to put a setter for the rid 01:07:15.140 --> 01:07:17.840 align:middle line:84% because that would mess up my counting. 01:07:17.840 --> 01:07:20.450 align:middle line:84% And probably, I don't want setters for parents, too. 01:07:20.450 --> 01:07:21.590 align:middle line:90% But maybe we might. 01:07:21.590 --> 01:07:22.407 align:middle line:90% I don't know. 01:07:22.407 --> 01:07:23.990 align:middle line:84% The only thing that looks a little bit 01:07:23.990 --> 01:07:26.660 align:middle line:84% weird for the get_rid is this zfill. 01:07:26.660 --> 01:07:29.960 align:middle line:84% And I added that as a cute little thing to basically make 01:07:29.960 --> 01:07:31.550 align:middle line:90% the ID look like an ID number. 01:07:31.550 --> 01:07:33.860 align:middle line:84% So it pre-fills the front with zeros. 01:07:33.860 --> 01:07:35.570 align:middle line:84% Like, it pads the front with zeros. 01:07:35.570 --> 01:07:38.990 align:middle line:84% So for the ID of 1, you can see, it's 00001. 01:07:38.990 --> 01:07:42.860 align:middle line:84% For an ID of 123, it would be 00123, right? 01:07:42.860 --> 01:07:47.780 align:middle line:84% So it just makes it look nice when we print it out, 01:07:47.780 --> 01:07:49.310 align:middle line:90% when we print out the ID. 01:07:49.310 --> 01:07:52.050 align:middle line:84% And otherwise, the two parents just return the parent objects. 01:07:52.050 --> 01:07:54.890 align:middle line:90% 01:07:54.890 --> 01:07:57.680 align:middle line:84% One interesting method that I would like to add, 01:07:57.680 --> 01:08:00.890 align:middle line:84% and we'll play on the fact that rabbits mate here, 01:08:00.890 --> 01:08:03.450 align:middle line:90% is to add two rabbits together. 01:08:03.450 --> 01:08:06.590 align:middle line:84% So we're implementing the dunder method, 01:08:06.590 --> 01:08:09.230 align:middle line:84% double underscore, add, double underscore, 01:08:09.230 --> 01:08:14.450 align:middle line:84% to have the ability to add two rabbits together in our code, 01:08:14.450 --> 01:08:14.950 align:middle line:90% all right? 01:08:14.950 --> 01:08:18.170 align:middle line:84% So, again, this is a design decision I made. 01:08:18.170 --> 01:08:21.100 align:middle line:84% So when I add two rabbits together, 01:08:21.100 --> 01:08:23.560 align:middle line:84% I'm going to create a new rabbit object. 01:08:23.560 --> 01:08:27.250 align:middle line:84% And that's exactly what the code is doing inside here. 01:08:27.250 --> 01:08:32.140 align:middle line:84% So I'm going to run this add dunder method on self 01:08:32.140 --> 01:08:33.490 align:middle line:90% and other, right? 01:08:33.490 --> 01:08:36.069 align:middle line:84% And then, behind, or in front of the scenes, I guess, 01:08:36.069 --> 01:08:38.029 align:middle line:84% is going to be this plus operator. 01:08:38.029 --> 01:08:40.330 align:middle line:84% So the self will be the thing before the plus. 01:08:40.330 --> 01:08:43.270 align:middle line:84% And the other will be the thing after the plus, right? 01:08:43.270 --> 01:08:46.189 align:middle line:84% Just like what we saw last lecture. 01:08:46.189 --> 01:08:50.710 align:middle line:84% So when we add r1 plus r2, what I would like the result to be 01:08:50.710 --> 01:08:56.319 align:middle line:84% is another rabbit object, who has one parent, r1, 01:08:56.319 --> 01:08:58.390 align:middle line:90% and the other parent, r2. 01:08:58.390 --> 01:09:00.430 align:middle line:84% Those are the things we added together. 01:09:00.430 --> 01:09:04.000 align:middle line:84% And let's say this new rabbit object is age of 0, right? 01:09:04.000 --> 01:09:06.290 align:middle line:90% It's a newborn. 01:09:06.290 --> 01:09:09.649 align:middle line:84% So to implement that, we just have-- 01:09:09.649 --> 01:09:12.229 align:middle line:84% we're returning a new rabbit object here, right? 01:09:12.229 --> 01:09:14.149 align:middle line:84% So we're just creating a new rabbit object 01:09:14.149 --> 01:09:17.120 align:middle line:90% on the fly in this method. 01:09:17.120 --> 01:09:18.560 align:middle line:84% How do we create a rabbit object? 01:09:18.560 --> 01:09:22.880 align:middle line:84% We need to give it an age and the two parents. 01:09:22.880 --> 01:09:28.069 align:middle line:84% Originally, when we created those three r1, r2, r3's, they 01:09:28.069 --> 01:09:29.840 align:middle line:90% didn't have parents, right? 01:09:29.840 --> 01:09:32.310 align:middle line:84% They were just unknown or something like that. 01:09:32.310 --> 01:09:34.340 align:middle line:84% But in this particular case, we do 01:09:34.340 --> 01:09:35.840 align:middle line:84% want to know what their parents are. 01:09:35.840 --> 01:09:38.450 align:middle line:84% Their parents are the thing before the plus and the thing 01:09:38.450 --> 01:09:39.740 align:middle line:90% after the plus. 01:09:39.740 --> 01:09:41.495 align:middle line:90% So one parent will be self. 01:09:41.495 --> 01:09:43.370 align:middle line:84% And the other parent will be other, the thing 01:09:43.370 --> 01:09:45.965 align:middle line:90% that's in the parameter list. 01:09:45.965 --> 01:09:49.970 align:middle line:90% 01:09:49.970 --> 01:09:53.109 align:middle line:84% So let's continue on with our program here. 01:09:53.109 --> 01:09:55.390 align:middle line:84% We had these three lines of code that were run. 01:09:55.390 --> 01:09:57.190 align:middle line:84% And I created these three rabbits 01:09:57.190 --> 01:10:00.790 align:middle line:90% with these IDs, 1, 2, 3. 01:10:00.790 --> 01:10:04.420 align:middle line:84% If I add two rabbits together, r1 plus r2, 01:10:04.420 --> 01:10:07.150 align:middle line:84% to give me a rabbit object variable, 01:10:07.150 --> 01:10:10.660 align:middle line:84% e4, Python says, all right, well, 01:10:10.660 --> 01:10:14.650 align:middle line:84% let me run this dunder method behind the scenes of the plus. 01:10:14.650 --> 01:10:18.400 align:middle line:90% So r4 effectively becomes what? 01:10:18.400 --> 01:10:24.150 align:middle line:84% Well, we replace, in the previous slide right here, 01:10:24.150 --> 01:10:27.180 align:middle line:84% the return is rabbit(0), one parent, comma, 01:10:27.180 --> 01:10:28.450 align:middle line:90% the other parent. 01:10:28.450 --> 01:10:30.570 align:middle line:84% So when we make this addition, we 01:10:30.570 --> 01:10:34.350 align:middle line:84% have rabbit(0), comma, one parent, 01:10:34.350 --> 01:10:37.080 align:middle line:84% the thing before the dot, comma, the other parent, or the thing 01:10:37.080 --> 01:10:39.810 align:middle line:84% before the plus and then the thing after the plus. 01:10:39.810 --> 01:10:45.000 align:middle line:84% So my r4 becomes the result of adding r1 plus r2. 01:10:45.000 --> 01:10:48.330 align:middle line:90% So its parents are these two. 01:10:48.330 --> 01:10:53.050 align:middle line:84% , Now how does this rabbit get created, right? 01:10:53.050 --> 01:10:54.370 align:middle line:90% It's a new rabbit object. 01:10:54.370 --> 01:10:58.450 align:middle line:84% So we run the init method of the rabid object, which 01:10:58.450 --> 01:11:00.820 align:middle line:90% means that, here's a variable. 01:11:00.820 --> 01:11:02.500 align:middle line:90% It's bound to a rabbit object. 01:11:02.500 --> 01:11:03.730 align:middle line:90% Its age is 0. 01:11:03.730 --> 01:11:06.460 align:middle line:84% It has these two parents that our object is 01:11:06.460 --> 01:11:10.270 align:middle line:84% bound to other rabbit objects, up here, r1 and r2. 01:11:10.270 --> 01:11:15.220 align:middle line:84% And the ID, just like before, is whatever the tag is right now. 01:11:15.220 --> 01:11:17.740 align:middle line:84% Well, we already created three rabbit objects 01:11:17.740 --> 01:11:18.520 align:middle line:90% ahead of this one. 01:11:18.520 --> 01:11:21.420 align:middle line:90% So this one's tag will be 4. 01:11:21.420 --> 01:11:23.040 align:middle line:84% And then, right before we finish, 01:11:23.040 --> 01:11:25.770 align:middle line:90% we increment the tag to 5. 01:11:25.770 --> 01:11:30.540 align:middle line:84% So no matter how we're creating these rabbit objects, either 01:11:30.540 --> 01:11:33.420 align:middle line:84% just plain old in our program directly 01:11:33.420 --> 01:11:39.300 align:middle line:84% or through an indirect method, in this case, the plus, 01:11:39.300 --> 01:11:42.450 align:middle line:84% we're still creating rabbit objects in our program, right? 01:11:42.450 --> 01:11:45.600 align:middle line:84% So that counter, that shared variable tag, 01:11:45.600 --> 01:11:47.820 align:middle line:84% is still coming into play, right? 01:11:47.820 --> 01:11:50.955 align:middle line:84% So we're still counting all of these rabbit objects created. 01:11:50.955 --> 01:11:53.920 align:middle line:90% 01:11:53.920 --> 01:11:55.960 align:middle line:90% Does that make sense? 01:11:55.960 --> 01:11:59.030 align:middle line:90% OK, good. 01:11:59.030 --> 01:12:02.950 align:middle line:90% So, yeah, that's fine. 01:12:02.950 --> 01:12:05.620 align:middle line:84% So one last method, so this is a method 01:12:05.620 --> 01:12:08.380 align:middle line:84% that checks for equality between two rabbits. 01:12:08.380 --> 01:12:10.690 align:middle line:84% And again, my design choice is to say 01:12:10.690 --> 01:12:13.100 align:middle line:90% that two rabbits are equal. 01:12:13.100 --> 01:12:16.660 align:middle line:84% So if I say r1 equal equal r2, that 01:12:16.660 --> 01:12:18.190 align:middle line:90% will tell me true or false. 01:12:18.190 --> 01:12:21.010 align:middle line:84% And my design choice is to say that two rabbits are 01:12:21.010 --> 01:12:23.510 align:middle line:84% equal if they have the same parents. 01:12:23.510 --> 01:12:31.330 align:middle line:84% So if I create another rabbit object, 4 was r1 plus r2. 01:12:31.330 --> 01:12:34.660 align:middle line:84% But if 5 is r2 plus r1, I want to say 01:12:34.660 --> 01:12:36.730 align:middle line:84% that 5 and 4 are equal because they 01:12:36.730 --> 01:12:38.800 align:middle line:90% have the same parents, right? 01:12:38.800 --> 01:12:42.190 align:middle line:84% I don't care that it was r1 plus r2 or r2 plus r1. 01:12:42.190 --> 01:12:43.450 align:middle line:90% They have the same parents. 01:12:43.450 --> 01:12:45.570 align:middle line:90% It's just an opposite order. 01:12:45.570 --> 01:12:48.080 align:middle line:84% And so that's what this eq method is doing. 01:12:48.080 --> 01:12:50.000 align:middle line:84% It's a dunder method to implement 01:12:50.000 --> 01:12:52.640 align:middle line:90% equality between two rabbits. 01:12:52.640 --> 01:12:57.140 align:middle line:84% So parents_same is a Boolean here that just checks the rid. 01:12:57.140 --> 01:12:59.300 align:middle line:84% So this Boolean, parents_same, is 01:12:59.300 --> 01:13:01.340 align:middle line:84% going to check that the addition was made, 01:13:01.340 --> 01:13:04.400 align:middle line:90% r1 plus r2, r1 plus r2, right? 01:13:04.400 --> 01:13:07.190 align:middle line:84% And parents_opposite is also going 01:13:07.190 --> 01:13:09.650 align:middle line:84% to be a Boolean, either true or false, that 01:13:09.650 --> 01:13:13.970 align:middle line:84% checks if I made the rabbits, r1 plus r2, and then r2 plus r1, 01:13:13.970 --> 01:13:16.490 align:middle line:90% so backward in the parents. 01:13:16.490 --> 01:13:18.470 align:middle line:84% But they still have the same parents. 01:13:18.470 --> 01:13:22.890 align:middle line:90% 01:13:22.890 --> 01:13:29.010 align:middle line:84% And the reason I'm checking for IDs is because IDs are unique. 01:13:29.010 --> 01:13:31.590 align:middle line:84% So originally, when I wrote this code a long time ago, 01:13:31.590 --> 01:13:33.480 align:middle line:84% I actually ended up, my first iteration, 01:13:33.480 --> 01:13:37.830 align:middle line:84% checking just the straight up parents values, right? 01:13:37.830 --> 01:13:41.320 align:middle line:84% So it was comparing, basically, rabbit objects together. 01:13:41.320 --> 01:13:44.700 align:middle line:84% But the problem with that code is that, at some point, 01:13:44.700 --> 01:13:49.940 align:middle line:84% it tried to compare a none, some rabbits might 01:13:49.940 --> 01:13:53.030 align:middle line:84% have a none as their parent, with an actual rabbit object. 01:13:53.030 --> 01:13:54.650 align:middle line:90% And then the code crashed. 01:13:54.650 --> 01:13:58.370 align:middle line:84% And then I realized, I can just compare the ID values directly 01:13:58.370 --> 01:14:00.950 align:middle line:84% because those are, one, just numbers, so very 01:14:00.950 --> 01:14:01.880 align:middle line:90% easy to compare. 01:14:01.880 --> 01:14:02.990 align:middle line:90% And two, they're unique. 01:14:02.990 --> 01:14:07.530 align:middle line:84% So I know I'm not going to have two rabbits with the same ID. 01:14:07.530 --> 01:14:10.455 align:middle line:84% And so, in this particular case, I've got these two rabbits. 01:14:10.455 --> 01:14:11.700 align:middle line:90% I should say they're equal. 01:14:11.700 --> 01:14:15.330 align:middle line:84% But then, if I add r2 plus r3, r6, 01:14:15.330 --> 01:14:17.730 align:middle line:84% this one is not going to be equal to any 01:14:17.730 --> 01:14:18.690 align:middle line:90% of my other rabbits. 01:14:18.690 --> 01:14:22.500 align:middle line:90% 01:14:22.500 --> 01:14:24.750 align:middle line:90% So here's my code. 01:14:24.750 --> 01:14:29.670 align:middle line:90% 01:14:29.670 --> 01:14:31.490 align:middle line:84% So here, I've got my three rabbits, right? 01:14:31.490 --> 01:14:36.260 align:middle line:84% So I think we've printed this out already. 01:14:36.260 --> 01:14:37.070 align:middle line:90% So here's r1. 01:14:37.070 --> 01:14:39.530 align:middle line:84% It's a rabbit with this ID, rabbit with this ID, 01:14:39.530 --> 01:14:41.850 align:middle line:90% rabbit with this ID. 01:14:41.850 --> 01:14:45.060 align:middle line:84% And then r1's parents, r2's parents, and r3's parents 01:14:45.060 --> 01:14:47.920 align:middle line:90% all have none, are none. 01:14:47.920 --> 01:14:56.965 align:middle line:84% But then, when I add r4 as r1 plus r2, I can print-- 01:14:56.965 --> 01:15:00.820 align:middle line:90% 01:15:00.820 --> 01:15:03.130 align:middle line:90% r4 is a rabbit with ID of 4. 01:15:03.130 --> 01:15:05.950 align:middle line:84% And then R1 and R2 are, as usual, what we just saw. 01:15:05.950 --> 01:15:08.200 align:middle line:84% And then, when we grab the parents of r4, 01:15:08.200 --> 01:15:11.800 align:middle line:84% it's going to be r1, which is this rabbit with this ID, 01:15:11.800 --> 01:15:16.600 align:middle line:84% and r2 with this rabbit with this ID, right? 01:15:16.600 --> 01:15:19.910 align:middle line:84% And then, we can check the equality. 01:15:19.910 --> 01:15:25.000 align:middle line:84% So here, I can create r4, r5, and r6. 01:15:25.000 --> 01:15:28.630 align:middle line:84% So r3 plus r4 and r4 plus r3, they 01:15:28.630 --> 01:15:30.850 align:middle line:90% should be equivalent, right? 01:15:30.850 --> 01:15:35.050 align:middle line:84% So here, I've got r5 and r6 down here. 01:15:35.050 --> 01:15:37.660 align:middle line:84% See, I'm just running the double equal sign 01:15:37.660 --> 01:15:42.043 align:middle line:84% on objects of type rabbit, which is pretty cool. 01:15:42.043 --> 01:15:43.210 align:middle line:90% And they're the same, right? 01:15:43.210 --> 01:15:45.190 align:middle line:84% Because they have the same two parents. 01:15:45.190 --> 01:15:47.450 align:middle line:84% I don't care that they're in opposite order. 01:15:47.450 --> 01:15:50.740 align:middle line:84% But then, r4 and r6 have different parents, 01:15:50.740 --> 01:15:52.030 align:middle line:90% right? r4 had 1 and 2. 01:15:52.030 --> 01:15:54.940 align:middle line:90% And R6 had 3 and 4. 01:15:54.940 --> 01:15:59.630 align:middle line:90% 01:15:59.630 --> 01:16:00.935 align:middle line:90% Questions about this code? 01:16:00.935 --> 01:16:05.510 align:middle line:90% 01:16:05.510 --> 01:16:07.730 align:middle line:84% OK, so class variable is pretty cool. 01:16:07.730 --> 01:16:09.840 align:middle line:84% You share them across all the instances. 01:16:09.840 --> 01:16:11.930 align:middle line:84% So if one instance modifies it, they'll 01:16:11.930 --> 01:16:14.100 align:middle line:84% be modified for all the other instances. 01:16:14.100 --> 01:16:17.280 align:middle line:84% So we have one more example to look at next lecture. 01:16:17.280 --> 01:16:19.760 align:middle line:84% We're actually going to implement our own fitness 01:16:19.760 --> 01:16:21.060 align:middle line:90% tracker class. 01:16:21.060 --> 01:16:23.210 align:middle line:84% So it's going to be a little bit more complex. 01:16:23.210 --> 01:16:25.520 align:middle line:84% But we're going to see a lot of the same ideas 01:16:25.520 --> 01:16:28.400 align:middle line:84% that we saw today, just in this slightly more 01:16:28.400 --> 01:16:32.600 align:middle line:84% complex setting of implementing our own fitness tracker. 01:16:32.600 --> 01:16:36.020 align:middle line:84% So it's still kind of an abstract thing, but more useful 01:16:36.020 --> 01:16:41.110 align:middle line:84% than animals and rabbits and person and student classes. 01:16:41.110 --> 01:16:55.000 align:middle line:90%