WEBVTT 1 00:00:01.020 --> 00:00:02.550 Hi guys and welcome back. 2 00:00:02.550 --> 00:00:03.670 In this video and the next, 3 00:00:03.670 --> 00:00:06.860 I wanted to talk with you about Mutability in Python 4 00:00:06.860 --> 00:00:09.000 and how it can affect your programmes. 5 00:00:09.000 --> 00:00:11.710 It's a very important part of Python to know about, 6 00:00:11.710 --> 00:00:12.860 so let's dive right in. 7 00:00:14.230 --> 00:00:18.370 Here we've got a list, and we have given it the name a. 8 00:00:18.370 --> 00:00:20.510 Then we have created the name b 9 00:00:20.510 --> 00:00:24.000 and made it equal to the value of name a. 10 00:00:24.000 --> 00:00:26.840 This way of saying things that I'm saying right now 11 00:00:26.840 --> 00:00:29.870 is very important because you'll see quite often, 12 00:00:29.870 --> 00:00:32.897 people say, "You've created a list called a, 13 00:00:32.897 --> 00:00:34.507 "and then you created a list called b. 14 00:00:34.507 --> 00:00:37.080 "That's not correct in Python." 15 00:00:37.080 --> 00:00:39.510 a and b are names. 16 00:00:39.510 --> 00:00:42.490 The value is this list. 17 00:00:42.490 --> 00:00:46.570 So if you print the ID of a, 18 00:00:46.570 --> 00:00:49.660 and then you print the ID of b, 19 00:00:49.660 --> 00:00:52.190 and in CPython, the Python version that I'm using 20 00:00:52.190 --> 00:00:53.760 and that most of you will be using, 21 00:00:53.760 --> 00:00:57.350 the ID function, when you give it any object 22 00:00:57.350 --> 00:00:58.510 such as this list, 23 00:00:58.510 --> 00:01:02.740 will give you back the location in memory of them, 24 00:01:02.740 --> 00:01:04.110 so by running this code here, 25 00:01:04.110 --> 00:01:07.300 you can see that the output is exactly the same. 26 00:01:07.300 --> 00:01:11.930 That means that a and b are names for the same object, 27 00:01:11.930 --> 00:01:12.823 the same thing. 28 00:01:14.020 --> 00:01:16.600 Indeed you can see that being the case 29 00:01:16.600 --> 00:01:21.600 if you just do a.append 35, 30 00:01:21.920 --> 00:01:24.723 and then you print a and print b. 31 00:01:26.000 --> 00:01:29.050 If you do that, you'll see that both a and b change 32 00:01:29.050 --> 00:01:30.590 when you modified a. 33 00:01:30.590 --> 00:01:34.893 You appended 35 to a, and both a and b ended up with 35. 34 00:01:36.140 --> 00:01:39.010 That means that both a and b are references 35 00:01:39.010 --> 00:01:42.960 to the same object, this one list. 36 00:01:42.960 --> 00:01:46.330 However, if you did b equals another list, 37 00:01:46.330 --> 00:01:48.259 then this would be different now. 38 00:01:48.259 --> 00:01:51.080 You have one list that has a 35 in it, 39 00:01:51.080 --> 00:01:53.120 and the other one that does not. 40 00:01:53.120 --> 00:01:56.050 In Python, this is the syntax for creating an empty list, 41 00:01:56.050 --> 00:01:58.560 so both of these are created separately, 42 00:01:58.560 --> 00:02:00.370 and they are stored separately. 43 00:02:00.370 --> 00:02:02.540 Once again, going to the ID, 44 00:02:02.540 --> 00:02:06.510 you'll see that these are indeed now different numbers. 45 00:02:06.510 --> 00:02:09.400 So let me lay that, and you can see that you get 46 00:02:09.400 --> 00:02:10.590 two different numbers. 47 00:02:10.590 --> 00:02:12.830 You can compare the last couple digits there. 48 00:02:12.830 --> 00:02:15.650 So the fact that you can change a list 49 00:02:15.650 --> 00:02:17.820 after you have created it means 50 00:02:17.820 --> 00:02:19.770 that a list is mutable. 51 00:02:19.770 --> 00:02:23.160 You can mutate it; that's the same thing as changing. 52 00:02:23.160 --> 00:02:27.830 Some values can't be changed, and they are immutable values. 53 00:02:27.830 --> 00:02:30.750 In Python, all things are mutable 54 00:02:30.750 --> 00:02:35.750 because everything's an object unless there are specifically 55 00:02:35.760 --> 00:02:40.143 no ways of changing the properties of the object itself. 56 00:02:40.980 --> 00:02:43.160 Let's go into an example. 57 00:02:43.160 --> 00:02:45.463 For example, if you define these to be tuples, 58 00:02:46.840 --> 00:02:50.230 there are actually no ways of adding or removing 59 00:02:50.230 --> 00:02:51.770 an element from a tuple. 60 00:02:51.770 --> 00:02:53.740 If you do a.append, you're gonna get an error 61 00:02:53.740 --> 00:02:56.710 saying that tuples don't have an append method in them. 62 00:02:56.710 --> 00:03:01.080 If you do a equal a plus something, 63 00:03:01.080 --> 00:03:03.630 you're going to create a new tuple. 64 00:03:03.630 --> 00:03:05.890 You're not going to be modifying that one. 65 00:03:05.890 --> 00:03:08.070 So tuples are immutable. 66 00:03:08.070 --> 00:03:10.470 Let's take a look at another example. 67 00:03:10.470 --> 00:03:15.190 Imagine you have two numbers such as 8597 and 8597. 68 00:03:16.710 --> 00:03:20.713 These are the same value, 8597 in both cases, 69 00:03:22.370 --> 00:03:25.250 and you can see that the IDs are the same. 70 00:03:25.250 --> 00:03:27.710 So why did this differ in lists 71 00:03:27.710 --> 00:03:30.990 when you created two empty lists, but not in integers? 72 00:03:30.990 --> 00:03:32.150 Python has this optimization 73 00:03:32.150 --> 00:03:33.750 where when an integer is created, 74 00:03:33.750 --> 00:03:36.103 then it won't recreate it if another one 75 00:03:36.103 --> 00:03:38.220 identical to it is used. 76 00:03:38.220 --> 00:03:41.850 So here Python knows that 8597 has already been created, 77 00:03:41.850 --> 00:03:44.150 so it doesn't create a new one. 78 00:03:44.150 --> 00:03:47.740 Integers are also immutable; you can't change them. 79 00:03:47.740 --> 00:03:49.570 New Python developers often believe 80 00:03:49.570 --> 00:03:53.000 that if you do something like 8598, 81 00:03:53.000 --> 00:03:55.070 now b should change as well 82 00:03:55.070 --> 00:03:58.140 because you have changed this number here, 83 00:03:58.140 --> 00:04:01.000 but that is not at all what's happened. 84 00:04:01.000 --> 00:04:02.730 This value was created first, 85 00:04:02.730 --> 00:04:05.050 and a is a name for that value. 86 00:04:05.050 --> 00:04:08.610 The name assignment happens with the equals sign, 87 00:04:08.610 --> 00:04:11.710 and here, you have created the value 8598, 88 00:04:11.710 --> 00:04:14.070 and you have given it the name a. 89 00:04:14.070 --> 00:04:17.360 However, b is still a name for 8597. 90 00:04:17.360 --> 00:04:19.360 That hasn't changed at all. 91 00:04:19.360 --> 00:04:21.690 However, if you do print these IDs, 92 00:04:21.690 --> 00:04:23.120 you can see that that's all correct, 93 00:04:23.120 --> 00:04:25.270 and they are now different. 94 00:04:25.270 --> 00:04:26.873 They are identical at the start, 95 00:04:28.290 --> 00:04:30.393 and then they differ afterwards. 96 00:04:31.410 --> 00:04:34.350 As I said earlier, most things in Python are mutable 97 00:04:34.350 --> 00:04:37.960 except for tuples, strings, integers, floats, and booleans. 98 00:04:37.960 --> 00:04:40.470 I believe that's most of the immutable ones. 99 00:04:40.470 --> 00:04:42.090 All the rest are mutable, 100 00:04:42.090 --> 00:04:42.970 and if you want to create 101 00:04:42.970 --> 00:04:46.050 an immutable class and immutable objects, 102 00:04:46.050 --> 00:04:48.220 just don't add any methods in them 103 00:04:48.220 --> 00:04:51.110 that can change the object's properties. 104 00:04:51.110 --> 00:04:52.930 For example, if you have a book object 105 00:04:52.930 --> 00:04:54.370 and you want it immutable, 106 00:04:54.370 --> 00:04:56.730 don't allow anybody to change 107 00:04:56.730 --> 00:04:59.023 the name property of that book. 108 00:04:59.920 --> 00:05:02.240 One more example for beginner Python developers 109 00:05:02.240 --> 00:05:05.270 is that if you create a string called hello 110 00:05:05.270 --> 00:05:06.610 and give it the name a, 111 00:05:06.610 --> 00:05:10.190 and then make b and give it the same value as a's value, 112 00:05:10.190 --> 00:05:11.960 then these will be the same. 113 00:05:11.960 --> 00:05:14.280 If you do plus equal, 114 00:05:14.280 --> 00:05:18.060 that is the same thing as doing equal a plus world, 115 00:05:18.060 --> 00:05:23.060 so you are reassigning the name a into this new string, 116 00:05:23.390 --> 00:05:28.170 but b will still be hello, so this will change the name a. 117 00:05:28.170 --> 00:05:30.913 It won't change the string hello. 118 00:05:31.836 --> 00:05:33.800 So that's something important to remember. 119 00:05:33.800 --> 00:05:36.800 Immutable types like strings, you can't change them. 120 00:05:36.800 --> 00:05:39.722 It's physically not possible in Python to change them, 121 00:05:39.722 --> 00:05:41.790 so whenever you are assigning, 122 00:05:41.790 --> 00:05:45.340 you're always going to be changing the existing name 123 00:05:45.340 --> 00:05:46.940 and not the value, 124 00:05:46.940 --> 00:05:48.970 and the same thing for tuples, 125 00:05:48.970 --> 00:05:50.883 integers, floats, and booleans. 126 00:05:52.620 --> 00:05:53.470 That's it for this video. 127 00:05:53.470 --> 00:05:56.810 I wanted to tell you a bit about mutability in Python. 128 00:05:56.810 --> 00:05:58.380 In the next one, we're going to learn 129 00:05:58.380 --> 00:06:00.480 why it's important to know about it. 130 00:06:00.480 --> 00:06:03.130 So stick with me, and I'll see you in the next video.