1 1 00:00:02,690 --> 00:00:03,840 Hey guys, welcome back. 2 2 00:00:03,840 --> 00:00:08,340 In this video we're going to be talking about metaclasses. 3 3 00:00:08,340 --> 00:00:10,950 A metaclass is a concept in Python 4 4 00:00:10,950 --> 00:00:13,220 that is very frequently overlooked. 5 5 00:00:13,220 --> 00:00:16,550 And what it allows us to do is a few things. 6 6 00:00:16,550 --> 00:00:18,443 But the one we're gonna look at in this video 7 7 00:00:18,443 --> 00:00:23,443 is that it allows us to define a method inside a class 8 8 00:00:24,060 --> 00:00:27,250 that isn't defined in the class, 9 9 00:00:27,250 --> 00:00:31,050 but must be defined in a subclass. 10 10 00:00:31,050 --> 00:00:33,420 So, let's dive right into some code, 11 11 00:00:33,420 --> 00:00:34,670 and that'll be the easiest way 12 12 00:00:34,670 --> 00:00:36,370 for you to learn about this. 13 13 00:00:36,370 --> 00:00:38,610 I'm going to create a new model in here. 14 14 00:00:38,610 --> 00:00:43,333 I'm gonna call it just model.py. 15 15 00:00:44,290 --> 00:00:47,423 And here I'm gonna say, from ABC import, ABCMeta, 16 16 00:00:48,480 --> 00:00:51,240 and abstract method. 17 17 00:00:51,240 --> 00:00:52,173 There we go. 18 18 00:00:53,270 --> 00:00:55,750 And, then we're gonna define our model. 19 19 00:00:55,750 --> 00:00:59,400 And in here, we're going to inherit from ABCMeta. 20 20 00:00:59,400 --> 00:01:02,610 So, we're not going to inherit in the normal way. 21 21 00:01:02,610 --> 00:01:05,520 We're going to be using a metaclass inheritance, 22 22 00:01:05,520 --> 00:01:07,230 or a metaclass property. 23 23 00:01:07,230 --> 00:01:10,053 We're gonna say, metaclass=ABCMeta. 24 24 00:01:11,340 --> 00:01:12,930 Now, there are a couple of ways 25 25 00:01:12,930 --> 00:01:16,260 of using abstract base classes, 26 26 00:01:16,260 --> 00:01:18,280 that's what an ABC is . 27 27 00:01:18,280 --> 00:01:21,990 And this is sort of the most common way. 28 28 00:01:21,990 --> 00:01:25,180 What it means when you define a class 29 29 00:01:25,180 --> 00:01:27,737 as having an ABCMeta metaclass, 30 30 00:01:28,900 --> 00:01:31,780 is that it gives this class the ability 31 31 00:01:31,780 --> 00:01:35,100 to use abstract methods and abstract properties 32 32 00:01:35,100 --> 00:01:36,600 and things like that. 33 33 00:01:36,600 --> 00:01:39,440 What abstract means in Python, 34 34 00:01:39,440 --> 00:01:40,900 and this is what you should take away 35 35 00:01:40,900 --> 00:01:43,313 from this whole abstract class thing, 36 36 00:01:43,313 --> 00:01:48,313 what abstract means, is, this thing that we are using 37 37 00:01:49,140 --> 00:01:53,803 that is abstract, exists but isn't defined yet. 38 38 00:01:54,670 --> 00:01:57,310 So, we're going to say that all models 39 39 00:01:57,310 --> 00:02:00,710 must have a json method. 40 40 00:02:00,710 --> 00:02:02,930 And the reason we're gonna say this is because so far, 41 41 00:02:02,930 --> 00:02:06,720 our alert model has a json method, 42 42 00:02:06,720 --> 00:02:09,410 and our item model has a json method. 43 43 00:02:09,410 --> 00:02:12,850 So, all our models so far, and they will continue to do so, 44 44 00:02:12,850 --> 00:02:15,833 have a json method. 45 45 00:02:16,820 --> 00:02:18,663 So, we want to be able to say, 46 46 00:02:19,800 --> 00:02:23,765 if we have a model, it must have a json method. 47 47 00:02:23,765 --> 00:02:28,180 And the json method must be defined inside the class. 48 48 00:02:28,180 --> 00:02:30,900 And that is what an abstract class allows us to do. 49 49 00:02:30,900 --> 00:02:34,310 The way we do that is, we define the method, 50 50 00:02:34,310 --> 00:02:37,363 and we decorate it with abstract method. 51 51 00:02:38,460 --> 00:02:43,460 And inside the method, we say, raise not implemented error. 52 52 00:02:44,670 --> 00:02:45,730 Okay. 53 53 00:02:45,730 --> 00:02:50,000 So what this is saying is, we have a model class, 54 54 00:02:50,000 --> 00:02:55,000 but this model class isn't an actual thing in our programme. 55 55 00:02:55,170 --> 00:02:58,630 It is the definition of what a thing should be, 56 56 00:02:58,630 --> 00:03:00,480 or should contain. 57 57 00:03:00,480 --> 00:03:03,330 And things, or models, in our application, 58 58 00:03:03,330 --> 00:03:05,873 should contain json methods. 59 59 00:03:05,873 --> 00:03:08,130 And they are abstract methods. 60 60 00:03:08,130 --> 00:03:09,530 So what does this mean? 61 61 00:03:09,530 --> 00:03:12,640 Well, it means that now you can go into any of your models 62 62 00:03:12,640 --> 00:03:17,150 and create, you know, another model like item or alert, 63 63 00:03:17,150 --> 00:03:18,050 or whatever. 64 64 00:03:18,050 --> 00:03:23,043 And it can inherit from this model abstract class. 65 65 00:03:24,470 --> 00:03:25,303 Okay. 66 66 00:03:26,360 --> 00:03:30,010 So, if we do this, if we create a model class, 67 67 00:03:30,010 --> 00:03:32,830 such as item or alert, any of these, 68 68 00:03:32,830 --> 00:03:37,240 if we make these inherit from the model abstract class, 69 69 00:03:37,240 --> 00:03:40,600 Python will immediately tell you that your class 70 70 00:03:40,600 --> 00:03:44,130 must implement all abstract methods. 71 71 00:03:44,130 --> 00:03:45,800 So what Python is telling you is, 72 72 00:03:45,800 --> 00:03:47,780 hey you've defined this class and it inherits 73 73 00:03:47,780 --> 00:03:52,780 from this abstract class, but you have not implemented 74 74 00:03:52,940 --> 00:03:56,500 the abstract method json, which you need to implement 75 75 00:03:56,500 --> 00:03:58,343 in order for this code to be valid. 76 76 00:03:59,360 --> 00:04:01,500 Now what I'm going to do in order to show you what happens, 77 77 00:04:01,500 --> 00:04:04,640 is I'm going to right-click model.py, which is this file. 78 78 00:04:04,640 --> 00:04:07,960 I'm gonna right-click it and run it using pycharm. 79 79 00:04:07,960 --> 00:04:09,500 And then you can see nothing happens. 80 80 00:04:09,500 --> 00:04:14,500 And if I create on object of MyModel and run it again, 81 81 00:04:15,100 --> 00:04:17,390 you can see that you get an error. 82 82 00:04:17,390 --> 00:04:19,200 It says type error, can't instantiate 83 83 00:04:19,200 --> 00:04:22,053 abstract class MyModel with abstract methods json. 84 84 00:04:22,950 --> 00:04:25,120 So what is going on in the background? 85 85 00:04:25,120 --> 00:04:27,990 Why does Python accept the definition, 86 86 00:04:27,990 --> 00:04:29,400 accept it if we delete this line, 87 87 00:04:29,400 --> 00:04:32,092 but it doesn't accept it when we create it, 88 88 00:04:32,092 --> 00:04:33,600 when we're here in object? 89 89 00:04:33,600 --> 00:04:35,550 Well, what's going on behind the scenes is, 90 90 00:04:35,550 --> 00:04:39,950 Python is realising that the MyModel class 91 91 00:04:39,950 --> 00:04:43,840 does not have a method called json, 92 92 00:04:43,840 --> 00:04:46,183 that is not an abstract method. 93 93 00:04:46,183 --> 00:04:49,480 So MyModel class is inheriting from model, 94 94 00:04:49,480 --> 00:04:52,090 and that means that you're essentially copy, pasting 95 95 00:04:52,090 --> 00:04:55,710 everything inside that class, down to the child. 96 96 00:04:55,710 --> 00:04:58,650 That's essentially what inheritance means in Python. 97 97 00:04:58,650 --> 00:05:01,453 So, that means that the MyModel class 98 98 00:05:01,453 --> 00:05:04,060 also has an abstract method called json. 99 99 00:05:04,060 --> 00:05:05,630 It's essentially the same method. 100 100 00:05:05,630 --> 00:05:07,960 But it is an abstract method. 101 101 00:05:07,960 --> 00:05:12,180 And you can not instantiate, you can not create objects 102 102 00:05:12,180 --> 00:05:15,740 of classes that have abstract methods in them. 103 103 00:05:15,740 --> 00:05:20,070 Similarly, you can not create an instance of model 104 104 00:05:20,070 --> 00:05:21,010 for the same reason. 105 105 00:05:21,010 --> 00:05:23,580 You can't instantiate abstract class model 106 106 00:05:23,580 --> 00:05:25,900 with abstract methods json. 107 107 00:05:25,900 --> 00:05:27,540 So, what can you do? 108 108 00:05:27,540 --> 00:05:29,530 We're gonna go back to MyModel, 109 109 00:05:29,530 --> 00:05:31,170 and now what we're gonna say is, 110 110 00:05:31,170 --> 00:05:33,790 I'm going to implement this json method. 111 111 00:05:33,790 --> 00:05:37,310 And it's gonna return a dictionary such as, 112 112 00:05:37,310 --> 00:05:40,190 for example, name is MyModel. 113 113 00:05:42,640 --> 00:05:43,910 What does this mean now? 114 114 00:05:43,910 --> 00:05:46,370 Now it means the MyModel class 115 115 00:05:46,370 --> 00:05:49,159 no longer has an abstract method. 116 116 00:05:49,159 --> 00:05:50,290 Why? 117 117 00:05:50,290 --> 00:05:53,480 Because as soon as you try to call the json method, 118 118 00:05:53,480 --> 00:05:54,930 you can see that it doesn't have 119 119 00:05:54,930 --> 00:05:56,370 the abstract method decorator. 120 120 00:05:56,370 --> 00:05:59,660 And this is the way Python inheritance work. 121 121 00:05:59,660 --> 00:06:02,553 If we try to call model.json, 122 122 00:06:03,430 --> 00:06:05,790 what happens is, it calls the method 123 123 00:06:05,790 --> 00:06:08,660 that is defined inside the MyModel class. 124 124 00:06:08,660 --> 00:06:11,259 And if doesn't exist, then it calls the method 125 125 00:06:11,259 --> 00:06:14,250 that is defined on the parent class. 126 126 00:06:14,250 --> 00:06:16,490 And that is an abstract method. 127 127 00:06:16,490 --> 00:06:19,180 So, as soon as you define the method, 128 128 00:06:19,180 --> 00:06:21,730 not abstract, inside your MyModel class, 129 129 00:06:21,730 --> 00:06:24,900 this all works, and you get your dictionary there. 130 130 00:06:24,900 --> 00:06:29,480 So, this is a perfect way of enforcing the definition 131 131 00:06:29,480 --> 00:06:32,160 of methods in child classes. 132 132 00:06:32,160 --> 00:06:34,950 So, what we're going to do, is we're gonna leave this as is, 133 133 00:06:34,950 --> 00:06:37,570 for just a moment, and then we're gonna go 134 134 00:06:37,570 --> 00:06:39,330 over to our alert. 135 135 00:06:39,330 --> 00:06:43,770 And we're gonna say from models, model import model. 136 136 00:06:43,770 --> 00:06:47,810 And we're gonna make the alert a subclass of model. 137 137 00:06:47,810 --> 00:06:50,410 Remember, all this does in Python, 138 138 00:06:50,410 --> 00:06:53,970 is essentially access this method 139 139 00:06:53,970 --> 00:06:58,750 if the alert model does not have its own json method. 140 140 00:06:58,750 --> 00:07:01,170 The alert model does have its own json method, 141 141 00:07:01,170 --> 00:07:04,000 so whenever we access the json method of alert, 142 142 00:07:04,000 --> 00:07:07,070 we're gonna use this one, not the parent class one. 143 143 00:07:07,070 --> 00:07:10,160 But, if we define another method, sorry, another model, 144 144 00:07:10,160 --> 00:07:13,500 and we don't include the json method there, 145 145 00:07:13,500 --> 00:07:15,740 we're gonna get this warning saying, 146 146 00:07:15,740 --> 00:07:18,800 class alert, must implement all abstract methods. 147 147 00:07:18,800 --> 00:07:21,600 And we're gonna get an error if we try to do that. 148 148 00:07:21,600 --> 00:07:25,170 So, it's going to enforce this method definition. 149 149 00:07:25,170 --> 00:07:27,270 We're gonna do the same in the item model. 150 150 00:07:37,472 --> 00:07:38,420 Alright. 151 151 00:07:38,420 --> 00:07:41,340 So, now that we have this in both our models, 152 152 00:07:41,340 --> 00:07:45,380 we can go and start moving things from our models 153 153 00:07:45,380 --> 00:07:47,407 into this class. 154 154 00:07:47,407 --> 00:07:49,320 Why would you want to do that? 155 155 00:07:49,320 --> 00:07:52,810 Well, abstract classes don't mean 156 156 00:07:52,810 --> 00:07:56,730 that all the methods in them must be abstract methods. 157 157 00:07:56,730 --> 00:08:00,030 You can have methods that are not abstract methods. 158 158 00:08:00,030 --> 00:08:03,070 We're going to learn what that means in the next video. 159 159 00:08:03,070 --> 00:08:04,220 So, I'll see you there.