1 00:00:00,000 --> 00:00:03,990 When you're creating objects and classes in Python, you're going to want to get 2 00:00:04,000 --> 00:00:05,990 them some functionality. 3 00:00:06,000 --> 00:00:08,990 That functionality is usually created with methods. 4 00:00:09,000 --> 00:00:12,990 So let's go ahead and make a working copy of classes.py. 5 00:00:13,000 --> 00:00:17,990 We'll call this classes-working.py. 6 00:00:18,000 --> 00:00:19,990 We'll open up that working copy. 7 00:00:20,000 --> 00:00:23,990 You can see that we have a class called Duck and it already has couple of 8 00:00:24,000 --> 00:00:25,990 methods in it, quack and walk. 9 00:00:26,000 --> 00:00:27,990 Those get called down here. 10 00:00:28,000 --> 00:00:29,990 So donald gets created as class Duck. 11 00:00:30,000 --> 00:00:33,990 So this is an object with that class and the object donald. 12 00:00:34,000 --> 00:00:37,990 We're calling the quack method on the object donald and we're calling the walk 13 00:00:38,000 --> 00:00:39,990 method on the object donald. 14 00:00:40,000 --> 00:00:45,990 The calls are done using this dot operator, which is used to reference an 15 00:00:46,000 --> 00:00:48,990 attribute of the object. 16 00:00:49,000 --> 00:00:50,990 In this case, the attribute is a method. 17 00:00:51,000 --> 00:00:56,990 So it's called with the parenthesis and that calls it as you would call a function. 18 00:00:57,000 --> 00:00:57,990 These are actually functions. 19 00:00:58,000 --> 00:01:01,990 It's just that these are functions that are attributes of an object. 20 00:01:02,000 --> 00:01:03,990 That's what makes the methods. 21 00:01:04,000 --> 00:01:07,990 So we'll go ahead and we'll run this, and you'll see that the object gets 22 00:01:08,000 --> 00:01:09,990 created and the methods get called. 23 00:01:10,000 --> 00:01:10,990 It prints out Quaaack! 24 00:01:11,000 --> 00:01:12,990 and it prints out Walks like a duck. 25 00:01:13,000 --> 00:01:14,990 Everything is working like we expected it to. 26 00:01:15,000 --> 00:01:17,990 So methods are actually functions. 27 00:01:18,000 --> 00:01:20,990 You'll see that they're defined just like you define functions in Python. 28 00:01:21,000 --> 00:01:25,990 The difference is these are indented under the class Duck. 29 00:01:26,000 --> 00:01:28,990 So that makes them methods of the class Duck. 30 00:01:29,000 --> 00:01:33,990 You'll also notice that the first argument to these functions is this word self. 31 00:01:34,000 --> 00:01:38,990 That is a reference to the object. Not the class, but the object. 32 00:01:39,000 --> 00:01:43,990 In other words, when quack gets called on the donald object, then the donald 33 00:01:44,000 --> 00:01:47,990 object gets passed to the quack method. 34 00:01:48,000 --> 00:01:50,990 So the quack has a way of referring to object attributes. 35 00:01:51,000 --> 00:01:55,990 You'll also notice that nothing is getting passed inside of these parentheses. 36 00:01:56,000 --> 00:02:00,990 It's as if donald was passed inside those parentheses, but you don't 37 00:02:01,000 --> 00:02:01,990 actually put that there. 38 00:02:02,000 --> 00:02:05,990 That happens automatically by virtue of the dot operator. 39 00:02:06,000 --> 00:02:12,990 When quack gets called as a method of donald, then donald gets passed magically 40 00:02:13,000 --> 00:02:18,990 inside those parentheses, so that it's the first argument to the method. 41 00:02:19,000 --> 00:02:22,990 There's a special type of a method that I want to talk about which is called a constructor. 42 00:02:23,000 --> 00:02:27,990 This gets called every time you create an object based on this class. 43 00:02:28,000 --> 00:02:32,990 The constructor in Python is created by naming a method with two underscores and 44 00:02:33,000 --> 00:02:35,990 the word init and two more underscores at the end. 45 00:02:36,000 --> 00:02:37,990 This creates a constructor method. 46 00:02:38,000 --> 00:02:41,990 So I'm just going to print('constructor') here. 47 00:02:42,000 --> 00:02:46,990 And we'll go ahead and we'll save this and we'll run it. 48 00:02:47,000 --> 00:02:49,990 You'll see that constructor gets printed at the beginning. 49 00:02:50,000 --> 00:02:54,990 So when we run this, the first thing we do is we create the donald object and 50 00:02:55,000 --> 00:02:57,990 that's where the constructor gets called. 51 00:02:58,000 --> 00:03:04,990 In fact, one of the most common purposes for a constructor is to initialize some data. 52 00:03:05,000 --> 00:03:09,990 So if I put in the number 47 here and then I pass in that value in the 53 00:03:10,000 --> 00:03:15,990 constructor, I'll say value here, instead of printing constructor here, I can 54 00:03:16,000 --> 00:03:21,990 save this value, saying self._v = value. 55 00:03:22,000 --> 00:03:27,990 What this does is this creates a local variable that is an attribute of the object. 56 00:03:28,000 --> 00:03:31,990 In this case, the donald object, but if I had different objects with different names, 57 00:03:32,000 --> 00:03:34,990 it would be attributes of those objects. 58 00:03:35,000 --> 00:03:37,990 We'll see how valuable this is in a moment. 59 00:03:38,000 --> 00:03:41,990 Then I can use that value in the methods. 60 00:03:42,000 --> 00:03:44,990 For example, I can print it out here. 61 00:03:45,000 --> 00:03:48,990 I can print it out here. 62 00:03:49,000 --> 00:03:53,990 So now I'm initializing this variable to the number 47. 63 00:03:54,000 --> 00:03:57,990 Each time I use it, I'm going to go ahead and print it out. 64 00:03:58,000 --> 00:03:59,990 So we'll save this and run it. 65 00:04:00,000 --> 00:04:03,990 Now we see that we have a 47 here and a 47 there. 66 00:04:04,000 --> 00:04:07,990 If I were to change this to something else, save it and run it. 67 00:04:08,000 --> 00:04:09,990 We have 52 here and 52 there. 68 00:04:10,000 --> 00:04:20,990 If I create a different object with a different number in it and go ahead and 69 00:04:21,000 --> 00:04:29,990 call it, now when I save that and run it, then we have the donald ducks and we 70 00:04:30,000 --> 00:04:30,990 have the frank ducks. 71 00:04:31,000 --> 00:04:38,990 So the value here is that this self._v is actually attached to the object, not to the class. 72 00:04:39,000 --> 00:04:44,990 This is what's called encapsulation in object oriented programming, because that 73 00:04:45,000 --> 00:04:47,990 value is a part of the object. 74 00:04:48,000 --> 00:04:51,990 So I can have different objects with different values. 75 00:04:52,000 --> 00:04:54,990 I could actually have all kinds of different data associated with those objects. 76 00:04:55,000 --> 00:04:58,990 They could be opening different databases, they could be accessing different 77 00:04:59,000 --> 00:05:02,990 files, they could be keeping their place in these databases and in these files. 78 00:05:03,000 --> 00:05:06,990 This is the power of encapsulation in object oriented programming. 79 00:05:07,000 --> 00:05:09,990 We'll see a lot of examples of that as we move forward, but for now what's 80 00:05:10,000 --> 00:05:15,990 important to know is that all this happens to this self variable right there and right there. 81 00:05:16,000 --> 00:05:20,990 When the object calls a method that self variable gets passed, and that's a 82 00:05:21,000 --> 00:05:22,990 reference to the object. 83 00:05:23,000 --> 00:05:25,990 All the things that are attached to the object, its other methods, its 84 00:05:26,000 --> 00:05:28,990 attributes, its data, is all carried there. 85 00:05:29,000 --> 00:05:31,990 So here we've defined methods in our object. 86 00:05:32,000 --> 00:05:36,990 These methods are defined in exactly the same way that we define a function in 87 00:05:37,000 --> 00:05:40,990 Python except that its first argument is always self and that argument doesn't 88 00:05:41,000 --> 00:05:42,990 get passed explicitly. 89 00:05:43,000 --> 00:05:45,990 It's passed implicitly through the dot operator. 90 00:05:46,000 --> 00:05:49,990 Then we have a special method called init which is used as a constructor. 91 00:05:50,000 --> 00:05:51,990 We can use that to set up data. 92 00:05:52,000 --> 00:05:53,990 We can use it to open databases. 93 00:05:54,000 --> 00:05:56,990 We can use it for all kinds of purposes. 94 00:05:57,000 --> 00:06:00,990 The point of it is to initialize whatever needs to get initialized when you 95 00:06:01,000 --> 00:06:03,990 create the object based on this class. 96 00:06:04,000 --> 00:06:06,990 So that's how you can create a constructor and that's how you create methods in 97 00:06:07,000 --> 00:06:17,000 classes that will be used in objects in Python.