1 1 00:00:00,650 --> 00:00:02,120 Hi guys and welcome back. 2 2 00:00:02,120 --> 00:00:05,710 In this video we're going to learn about class composition. 3 3 00:00:05,710 --> 00:00:08,250 Composition is a counterpart to inheritance 4 4 00:00:08,250 --> 00:00:11,530 so you build out classes that use other classes. 5 5 00:00:11,530 --> 00:00:14,660 You're going to be using inheritance very little in Python, 6 6 00:00:14,660 --> 00:00:15,493 if I'm honest, 7 7 00:00:15,493 --> 00:00:18,240 you're going to be using composition much more. 8 8 00:00:18,240 --> 00:00:21,000 Composition allows your classes to be simpler 9 9 00:00:21,000 --> 00:00:23,550 and reduces the complexity of your code overall, 10 10 00:00:23,550 --> 00:00:27,100 so it is recommended that you use that most of the time. 11 11 00:00:27,100 --> 00:00:29,820 In addition, composition means something different 12 12 00:00:29,820 --> 00:00:32,580 than inheritance when you're thinking about your classes. 13 13 00:00:32,580 --> 00:00:35,800 For example, we've got here a bookshelf class 14 14 00:00:35,800 --> 00:00:38,140 that takes in a quantity when you create it 15 15 00:00:38,140 --> 00:00:39,800 and then when you print it out it tells you 16 16 00:00:39,800 --> 00:00:41,990 that it's a bookshelf with x books. 17 17 00:00:41,990 --> 00:00:44,540 For example, here we're creating a bookshelf with 300 books, 18 18 00:00:44,540 --> 00:00:45,510 when you print it out, 19 19 00:00:45,510 --> 00:00:48,450 you can see that it says bookshelf with 300 books. 20 20 00:00:48,450 --> 00:00:51,570 If you wanted to create a book class now, 21 21 00:00:51,570 --> 00:00:53,630 some new programme has maybe attempted 22 22 00:00:53,630 --> 00:00:55,440 to make it inherit from bookshelf. 23 23 00:00:55,440 --> 00:00:57,440 After all, they are somehow related. 24 24 00:00:57,440 --> 00:00:58,273 Correct? 25 25 00:00:58,273 --> 00:00:59,533 Books live in bookshelves so. 26 26 00:01:00,520 --> 00:01:02,880 And then you would do something like this in it 27 27 00:01:02,880 --> 00:01:06,040 and then you would say the name of the book. 28 28 00:01:06,040 --> 00:01:10,860 But now if you want this book to call the superclasses 29 29 00:01:10,860 --> 00:01:14,520 in each method, you also need the quantity of books. 30 30 00:01:14,520 --> 00:01:17,100 So this is where this is trying to break down a little bit, 31 31 00:01:17,100 --> 00:01:21,630 a book object doesn't have a quantity of anything. 32 32 00:01:21,630 --> 00:01:23,810 So anyway, we are going to continue with that example 33 33 00:01:23,810 --> 00:01:26,080 because we firmly believe that 34 34 00:01:26,080 --> 00:01:28,270 a book should inherit from bookshelf. 35 35 00:01:28,270 --> 00:01:30,020 So you'll put your quantity in here 36 36 00:01:31,270 --> 00:01:33,180 and then you'll say self.name equal name. 37 37 00:01:33,180 --> 00:01:34,630 Okay, perfect. 38 38 00:01:34,630 --> 00:01:38,320 Now, this I'm sure you'll realise makes little sense 39 39 00:01:38,320 --> 00:01:42,600 because you are passing the quantity to a book. 40 40 00:01:42,600 --> 00:01:44,700 So when you create a book object, 41 41 00:01:44,700 --> 00:01:47,100 you're going to have to say Harry Potter 42 42 00:01:47,100 --> 00:01:50,780 and then you're going to have to say something like 120. 43 43 00:01:50,780 --> 00:01:52,830 Then if you print book, 44 44 00:01:52,830 --> 00:01:55,890 you'll get something out that's not all that good. 45 45 00:01:55,890 --> 00:02:00,630 You get bookshelf with 120 books when you print the book. 46 46 00:02:00,630 --> 00:02:05,630 So clearly you're going to need an str method here 47 47 00:02:05,820 --> 00:02:07,390 that prints something completely different. 48 48 00:02:07,390 --> 00:02:11,880 So for example, book self.name. 49 49 00:02:11,880 --> 00:02:13,720 Okay, let's press play again 50 50 00:02:13,720 --> 00:02:15,370 and now you get book Harry Potter. 51 51 00:02:15,370 --> 00:02:19,610 So why is this a bad approach? 52 52 00:02:19,610 --> 00:02:21,080 Well, there are two reasons. 53 53 00:02:21,080 --> 00:02:24,500 One of them is conceptual, the way of thinking about it. 54 54 00:02:24,500 --> 00:02:26,470 The other one is technical. 55 55 00:02:26,470 --> 00:02:30,610 The conceptual reason is that when you do inheritance, 56 56 00:02:30,610 --> 00:02:32,130 you are essentially treating it 57 57 00:02:32,130 --> 00:02:34,270 like evolutionary inheritance. 58 58 00:02:34,270 --> 00:02:39,270 You're saying that a book is a bookshelf and something more. 59 59 00:02:41,170 --> 00:02:45,880 For example, in the same way that all tigers are mammals, 60 60 00:02:45,880 --> 00:02:48,010 but not all mammals are tigers. 61 61 00:02:48,010 --> 00:02:50,180 So all books are bookshelves, 62 62 00:02:50,180 --> 00:02:52,120 but not all bookshelves are books. 63 63 00:02:52,120 --> 00:02:54,970 The bookshelf can still be used on its own. 64 64 00:02:54,970 --> 00:02:58,160 That breaks down when you talk about books and bookshelves, 65 65 00:02:58,160 --> 00:03:00,340 because a book is something completely different 66 66 00:03:00,340 --> 00:03:01,220 from a bookshelf. 67 67 00:03:01,220 --> 00:03:06,160 A bookshelf can contain books, but one isn't the other. 68 68 00:03:06,160 --> 00:03:08,850 The technical reason why this breaks down 69 69 00:03:08,850 --> 00:03:12,500 is because you've got this book class that inherits 70 70 00:03:12,500 --> 00:03:17,500 from bookshelf, but actually you are not using inside it 71 71 00:03:18,760 --> 00:03:20,230 anything about the bookshelf. 72 72 00:03:20,230 --> 00:03:22,830 So you are completely overriding the str method 73 73 00:03:22,830 --> 00:03:24,810 because you don't want anything to do with bookshelves 74 74 00:03:24,810 --> 00:03:27,900 in there and you actually don't need these at all 75 75 00:03:27,900 --> 00:03:31,930 because I mean, what's the point of setting the quantity 76 76 00:03:31,930 --> 00:03:34,620 if you're not going to use it in the methods? 77 77 00:03:34,620 --> 00:03:38,700 So the conceptual reason is a book is not a bookshelf. 78 78 00:03:38,700 --> 00:03:40,788 The technical reason is there is no reason 79 79 00:03:40,788 --> 00:03:42,880 to inherit if you're not gonna use 80 80 00:03:42,880 --> 00:03:44,460 that inheritance anywhere. 81 81 00:03:44,460 --> 00:03:48,060 So this is where composition comes in. 82 82 00:03:48,060 --> 00:03:52,740 Composition is for when you wanna say something like, 83 83 00:03:52,740 --> 00:03:55,120 a bookshelf has many books. 84 84 00:03:55,120 --> 00:04:00,120 A bookshelf is composed of a bunch of things and books. 85 85 00:04:00,710 --> 00:04:05,580 So instead of defining our bookshelf and our book like this, 86 86 00:04:05,580 --> 00:04:08,150 we're going to define it slightly differently. 87 87 00:04:08,150 --> 00:04:11,550 Instead of setting the quantity of books in the bookshelf, 88 88 00:04:11,550 --> 00:04:14,600 we're actually going to allow the constructor 89 89 00:04:14,600 --> 00:04:16,603 to take in a number of books. 90 90 00:04:17,800 --> 00:04:19,430 So we will do star books 91 91 00:04:19,430 --> 00:04:22,113 and then we'll say self.books, equal books. 92 92 00:04:22,990 --> 00:04:27,990 And then here we will say bookshelf with len of self.books. 93 93 00:04:28,480 --> 00:04:33,480 So what this will take in is a bunch of book objects. 94 94 00:04:35,400 --> 00:04:38,970 Next up, the book class doesn't actually need, 95 95 00:04:38,970 --> 00:04:40,660 I'm going to delete this as well for now, 96 96 00:04:40,660 --> 00:04:43,380 the book class doesn't need to inherit from bookshelf 97 97 00:04:43,380 --> 00:04:45,240 and it doesn't need the quantity 98 98 00:04:45,240 --> 00:04:48,270 and it doesn't need to call the superclass of anything. 99 99 00:04:48,270 --> 00:04:49,200 Okay. 100 100 00:04:49,200 --> 00:04:52,440 So now we have two much simpler classes, 101 101 00:04:52,440 --> 00:04:54,270 but how do you use them? 102 102 00:04:54,270 --> 00:04:56,320 Well, we can create our book 103 103 00:04:56,320 --> 00:04:58,120 and then we can create another book. 104 104 00:05:01,780 --> 00:05:02,613 Just like that. 105 105 00:05:02,613 --> 00:05:03,780 So we have our two books here. 106 106 00:05:03,780 --> 00:05:08,780 Then we'll create our bookshelf and give it the two books. 107 107 00:05:11,810 --> 00:05:13,963 Then we can print the shelf. 108 108 00:05:15,110 --> 00:05:17,180 So I'll press play now and you can see 109 109 00:05:17,180 --> 00:05:20,193 that you get bookshelf with two books printed out. 110 110 00:05:21,280 --> 00:05:24,480 This is composition and this is very common, 111 111 00:05:24,480 --> 00:05:26,250 much more common than inheritance. 112 112 00:05:26,250 --> 00:05:30,170 It is when you have a class that contains 113 113 00:05:30,170 --> 00:05:32,220 a bunch of other classes or a class 114 114 00:05:32,220 --> 00:05:34,640 that has as many of that. 115 115 00:05:34,640 --> 00:05:37,290 And the here we've got our bookshelf that has many books. 116 116 00:05:37,290 --> 00:05:39,330 And when you use it, when you create it, 117 117 00:05:39,330 --> 00:05:43,013 you can pass to it a bunch of your other book objects. 118 118 00:05:44,320 --> 00:05:45,960 So to recap. 119 119 00:05:45,960 --> 00:05:50,080 Inheritance means that a book is a bookshelf, 120 120 00:05:50,080 --> 00:05:53,830 composition means that a bookshelf has many books. 121 121 00:05:53,830 --> 00:05:55,670 So take your pick from those two, 122 122 00:05:55,670 --> 00:05:58,960 that's an important decision to make, but as I said earlier, 123 123 00:05:58,960 --> 00:06:02,190 you'll most frequently be using composition. 124 124 00:06:02,190 --> 00:06:03,540 Thanks for joining me in this video 125 125 00:06:03,540 --> 00:06:05,930 to learn about composition in Python, 126 126 00:06:05,930 --> 00:06:07,580 and I'll see you in the next one.