WEBVTT 1 00:00:00.650 --> 00:00:02.120 Hi guys and welcome back. 2 00:00:02.120 --> 00:00:05.710 In this video we're going to learn about class composition. 3 00:00:05.710 --> 00:00:08.250 Composition is a counterpart to inheritance 4 00:00:08.250 --> 00:00:11.530 so you build out classes that use other classes. 5 00:00:11.530 --> 00:00:14.660 You're going to be using inheritance very little in Python, 6 00:00:14.660 --> 00:00:15.493 if I'm honest, 7 00:00:15.493 --> 00:00:18.240 you're going to be using composition much more. 8 00:00:18.240 --> 00:00:21.000 Composition allows your classes to be simpler 9 00:00:21.000 --> 00:00:23.550 and reduces the complexity of your code overall, 10 00:00:23.550 --> 00:00:27.100 so it is recommended that you use that most of the time. 11 00:00:27.100 --> 00:00:29.820 In addition, composition means something different 12 00:00:29.820 --> 00:00:32.580 than inheritance when you're thinking about your classes. 13 00:00:32.580 --> 00:00:35.800 For example, we've got here a bookshelf class 14 00:00:35.800 --> 00:00:38.140 that takes in a quantity when you create it 15 00:00:38.140 --> 00:00:39.800 and then when you print it out it tells you 16 00:00:39.800 --> 00:00:41.990 that it's a bookshelf with x books. 17 00:00:41.990 --> 00:00:44.540 For example, here we're creating a bookshelf with 300 books, 18 00:00:44.540 --> 00:00:45.510 when you print it out, 19 00:00:45.510 --> 00:00:48.450 you can see that it says bookshelf with 300 books. 20 00:00:48.450 --> 00:00:51.570 If you wanted to create a book class now, 21 00:00:51.570 --> 00:00:53.630 some new programme has maybe attempted 22 00:00:53.630 --> 00:00:55.440 to make it inherit from bookshelf. 23 00:00:55.440 --> 00:00:57.440 After all, they are somehow related. 24 00:00:57.440 --> 00:00:58.273 Correct? 25 00:00:58.273 --> 00:00:59.533 Books live in bookshelves so. 26 00:01:00.520 --> 00:01:02.880 And then you would do something like this in it 27 00:01:02.880 --> 00:01:06.040 and then you would say the name of the book. 28 00:01:06.040 --> 00:01:10.860 But now if you want this book to call the superclasses 29 00:01:10.860 --> 00:01:14.520 in each method, you also need the quantity of books. 30 00:01:14.520 --> 00:01:17.100 So this is where this is trying to break down a little bit, 31 00:01:17.100 --> 00:01:21.630 a book object doesn't have a quantity of anything. 32 00:01:21.630 --> 00:01:23.810 So anyway, we are going to continue with that example 33 00:01:23.810 --> 00:01:26.080 because we firmly believe that 34 00:01:26.080 --> 00:01:28.270 a book should inherit from bookshelf. 35 00:01:28.270 --> 00:01:30.020 So you'll put your quantity in here 36 00:01:31.270 --> 00:01:33.180 and then you'll say self.name equal name. 37 00:01:33.180 --> 00:01:34.630 Okay, perfect. 38 00:01:34.630 --> 00:01:38.320 Now, this I'm sure you'll realise makes little sense 39 00:01:38.320 --> 00:01:42.600 because you are passing the quantity to a book. 40 00:01:42.600 --> 00:01:44.700 So when you create a book object, 41 00:01:44.700 --> 00:01:47.100 you're going to have to say Harry Potter 42 00:01:47.100 --> 00:01:50.780 and then you're going to have to say something like 120. 43 00:01:50.780 --> 00:01:52.830 Then if you print book, 44 00:01:52.830 --> 00:01:55.890 you'll get something out that's not all that good. 45 00:01:55.890 --> 00:02:00.630 You get bookshelf with 120 books when you print the book. 46 00:02:00.630 --> 00:02:05.630 So clearly you're going to need an str method here 47 00:02:05.820 --> 00:02:07.390 that prints something completely different. 48 00:02:07.390 --> 00:02:11.880 So for example, book self.name. 49 00:02:11.880 --> 00:02:13.720 Okay, let's press play again 50 00:02:13.720 --> 00:02:15.370 and now you get book Harry Potter. 51 00:02:15.370 --> 00:02:19.610 So why is this a bad approach? 52 00:02:19.610 --> 00:02:21.080 Well, there are two reasons. 53 00:02:21.080 --> 00:02:24.500 One of them is conceptual, the way of thinking about it. 54 00:02:24.500 --> 00:02:26.470 The other one is technical. 55 00:02:26.470 --> 00:02:30.610 The conceptual reason is that when you do inheritance, 56 00:02:30.610 --> 00:02:32.130 you are essentially treating it 57 00:02:32.130 --> 00:02:34.270 like evolutionary inheritance. 58 00:02:34.270 --> 00:02:39.270 You're saying that a book is a bookshelf and something more. 59 00:02:41.170 --> 00:02:45.880 For example, in the same way that all tigers are mammals, 60 00:02:45.880 --> 00:02:48.010 but not all mammals are tigers. 61 00:02:48.010 --> 00:02:50.180 So all books are bookshelves, 62 00:02:50.180 --> 00:02:52.120 but not all bookshelves are books. 63 00:02:52.120 --> 00:02:54.970 The bookshelf can still be used on its own. 64 00:02:54.970 --> 00:02:58.160 That breaks down when you talk about books and bookshelves, 65 00:02:58.160 --> 00:03:00.340 because a book is something completely different 66 00:03:00.340 --> 00:03:01.220 from a bookshelf. 67 00:03:01.220 --> 00:03:06.160 A bookshelf can contain books, but one isn't the other. 68 00:03:06.160 --> 00:03:08.850 The technical reason why this breaks down 69 00:03:08.850 --> 00:03:12.500 is because you've got this book class that inherits 70 00:03:12.500 --> 00:03:17.500 from bookshelf, but actually you are not using inside it 71 00:03:18.760 --> 00:03:20.230 anything about the bookshelf. 72 00:03:20.230 --> 00:03:22.830 So you are completely overriding the str method 73 00:03:22.830 --> 00:03:24.810 because you don't want anything to do with bookshelves 74 00:03:24.810 --> 00:03:27.900 in there and you actually don't need these at all 75 00:03:27.900 --> 00:03:31.930 because I mean, what's the point of setting the quantity 76 00:03:31.930 --> 00:03:34.620 if you're not going to use it in the methods? 77 00:03:34.620 --> 00:03:38.700 So the conceptual reason is a book is not a bookshelf. 78 00:03:38.700 --> 00:03:40.788 The technical reason is there is no reason 79 00:03:40.788 --> 00:03:42.880 to inherit if you're not gonna use 80 00:03:42.880 --> 00:03:44.460 that inheritance anywhere. 81 00:03:44.460 --> 00:03:48.060 So this is where composition comes in. 82 00:03:48.060 --> 00:03:52.740 Composition is for when you wanna say something like, 83 00:03:52.740 --> 00:03:55.120 a bookshelf has many books. 84 00:03:55.120 --> 00:04:00.120 A bookshelf is composed of a bunch of things and books. 85 00:04:00.710 --> 00:04:05.580 So instead of defining our bookshelf and our book like this, 86 00:04:05.580 --> 00:04:08.150 we're going to define it slightly differently. 87 00:04:08.150 --> 00:04:11.550 Instead of setting the quantity of books in the bookshelf, 88 00:04:11.550 --> 00:04:14.600 we're actually going to allow the constructor 89 00:04:14.600 --> 00:04:16.603 to take in a number of books. 90 00:04:17.800 --> 00:04:19.430 So we will do star books 91 00:04:19.430 --> 00:04:22.113 and then we'll say self.books, equal books. 92 00:04:22.990 --> 00:04:27.990 And then here we will say bookshelf with len of self.books. 93 00:04:28.480 --> 00:04:33.480 So what this will take in is a bunch of book objects. 94 00:04:35.400 --> 00:04:38.970 Next up, the book class doesn't actually need, 95 00:04:38.970 --> 00:04:40.660 I'm going to delete this as well for now, 96 00:04:40.660 --> 00:04:43.380 the book class doesn't need to inherit from bookshelf 97 00:04:43.380 --> 00:04:45.240 and it doesn't need the quantity 98 00:04:45.240 --> 00:04:48.270 and it doesn't need to call the superclass of anything. 99 00:04:48.270 --> 00:04:49.200 Okay. 100 00:04:49.200 --> 00:04:52.440 So now we have two much simpler classes, 101 00:04:52.440 --> 00:04:54.270 but how do you use them? 102 00:04:54.270 --> 00:04:56.320 Well, we can create our book 103 00:04:56.320 --> 00:04:58.120 and then we can create another book. 104 00:05:01.780 --> 00:05:02.613 Just like that. 105 00:05:02.613 --> 00:05:03.780 So we have our two books here. 106 00:05:03.780 --> 00:05:08.780 Then we'll create our bookshelf and give it the two books. 107 00:05:11.810 --> 00:05:13.963 Then we can print the shelf. 108 00:05:15.110 --> 00:05:17.180 So I'll press play now and you can see 109 00:05:17.180 --> 00:05:20.193 that you get bookshelf with two books printed out. 110 00:05:21.280 --> 00:05:24.480 This is composition and this is very common, 111 00:05:24.480 --> 00:05:26.250 much more common than inheritance. 112 00:05:26.250 --> 00:05:30.170 It is when you have a class that contains 113 00:05:30.170 --> 00:05:32.220 a bunch of other classes or a class 114 00:05:32.220 --> 00:05:34.640 that has as many of that. 115 00:05:34.640 --> 00:05:37.290 And the here we've got our bookshelf that has many books. 116 00:05:37.290 --> 00:05:39.330 And when you use it, when you create it, 117 00:05:39.330 --> 00:05:43.013 you can pass to it a bunch of your other book objects. 118 00:05:44.320 --> 00:05:45.960 So to recap. 119 00:05:45.960 --> 00:05:50.080 Inheritance means that a book is a bookshelf, 120 00:05:50.080 --> 00:05:53.830 composition means that a bookshelf has many books. 121 00:05:53.830 --> 00:05:55.670 So take your pick from those two, 122 00:05:55.670 --> 00:05:58.960 that's an important decision to make, but as I said earlier, 123 00:05:58.960 --> 00:06:02.190 you'll most frequently be using composition. 124 00:06:02.190 --> 00:06:03.540 Thanks for joining me in this video 125 00:06:03.540 --> 00:06:05.930 to learn about composition in Python, 126 00:06:05.930 --> 00:06:07.580 and I'll see you in the next one.