1 1 00:00:01,020 --> 00:00:02,550 Hi guys and welcome back. 2 2 00:00:02,550 --> 00:00:03,670 In this video and the next, 3 3 00:00:03,670 --> 00:00:06,860 I wanted to talk with you about Mutability in Python 4 4 00:00:06,860 --> 00:00:09,000 and how it can affect your programmes. 5 5 00:00:09,000 --> 00:00:11,710 It's a very important part of Python to know about, 6 6 00:00:11,710 --> 00:00:12,860 so let's dive right in. 7 7 00:00:14,230 --> 00:00:18,370 Here we've got a list, and we have given it the name a. 8 8 00:00:18,370 --> 00:00:20,510 Then we have created the name b 9 9 00:00:20,510 --> 00:00:24,000 and made it equal to the value of name a. 10 10 00:00:24,000 --> 00:00:26,840 This way of saying things that I'm saying right now 11 11 00:00:26,840 --> 00:00:29,870 is very important because you'll see quite often, 12 12 00:00:29,870 --> 00:00:32,897 people say, "You've created a list called a, 13 13 00:00:32,897 --> 00:00:34,507 "and then you created a list called b. 14 14 00:00:34,507 --> 00:00:37,080 "That's not correct in Python." 15 15 00:00:37,080 --> 00:00:39,510 a and b are names. 16 16 00:00:39,510 --> 00:00:42,490 The value is this list. 17 17 00:00:42,490 --> 00:00:46,570 So if you print the ID of a, 18 18 00:00:46,570 --> 00:00:49,660 and then you print the ID of b, 19 19 00:00:49,660 --> 00:00:52,190 and in CPython, the Python version that I'm using 20 20 00:00:52,190 --> 00:00:53,760 and that most of you will be using, 21 21 00:00:53,760 --> 00:00:57,350 the ID function, when you give it any object 22 22 00:00:57,350 --> 00:00:58,510 such as this list, 23 23 00:00:58,510 --> 00:01:02,740 will give you back the location in memory of them, 24 24 00:01:02,740 --> 00:01:04,110 so by running this code here, 25 25 00:01:04,110 --> 00:01:07,300 you can see that the output is exactly the same. 26 26 00:01:07,300 --> 00:01:11,930 That means that a and b are names for the same object, 27 27 00:01:11,930 --> 00:01:12,823 the same thing. 28 28 00:01:14,020 --> 00:01:16,600 Indeed you can see that being the case 29 29 00:01:16,600 --> 00:01:21,600 if you just do a.append 35, 30 30 00:01:21,920 --> 00:01:24,723 and then you print a and print b. 31 31 00:01:26,000 --> 00:01:29,050 If you do that, you'll see that both a and b change 32 32 00:01:29,050 --> 00:01:30,590 when you modified a. 33 33 00:01:30,590 --> 00:01:34,893 You appended 35 to a, and both a and b ended up with 35. 34 34 00:01:36,140 --> 00:01:39,010 That means that both a and b are references 35 35 00:01:39,010 --> 00:01:42,960 to the same object, this one list. 36 36 00:01:42,960 --> 00:01:46,330 However, if you did b equals another list, 37 37 00:01:46,330 --> 00:01:48,259 then this would be different now. 38 38 00:01:48,259 --> 00:01:51,080 You have one list that has a 35 in it, 39 39 00:01:51,080 --> 00:01:53,120 and the other one that does not. 40 40 00:01:53,120 --> 00:01:56,050 In Python, this is the syntax for creating an empty list, 41 41 00:01:56,050 --> 00:01:58,560 so both of these are created separately, 42 42 00:01:58,560 --> 00:02:00,370 and they are stored separately. 43 43 00:02:00,370 --> 00:02:02,540 Once again, going to the ID, 44 44 00:02:02,540 --> 00:02:06,510 you'll see that these are indeed now different numbers. 45 45 00:02:06,510 --> 00:02:09,400 So let me lay that, and you can see that you get 46 46 00:02:09,400 --> 00:02:10,590 two different numbers. 47 47 00:02:10,590 --> 00:02:12,830 You can compare the last couple digits there. 48 48 00:02:12,830 --> 00:02:15,650 So the fact that you can change a list 49 49 00:02:15,650 --> 00:02:17,820 after you have created it means 50 50 00:02:17,820 --> 00:02:19,770 that a list is mutable. 51 51 00:02:19,770 --> 00:02:23,160 You can mutate it; that's the same thing as changing. 52 52 00:02:23,160 --> 00:02:27,830 Some values can't be changed, and they are immutable values. 53 53 00:02:27,830 --> 00:02:30,750 In Python, all things are mutable 54 54 00:02:30,750 --> 00:02:35,750 because everything's an object unless there are specifically 55 55 00:02:35,760 --> 00:02:40,143 no ways of changing the properties of the object itself. 56 56 00:02:40,980 --> 00:02:43,160 Let's go into an example. 57 57 00:02:43,160 --> 00:02:45,463 For example, if you define these to be tuples, 58 58 00:02:46,840 --> 00:02:50,230 there are actually no ways of adding or removing 59 59 00:02:50,230 --> 00:02:51,770 an element from a tuple. 60 60 00:02:51,770 --> 00:02:53,740 If you do a.append, you're gonna get an error 61 61 00:02:53,740 --> 00:02:56,710 saying that tuples don't have an append method in them. 62 62 00:02:56,710 --> 00:03:01,080 If you do a equal a plus something, 63 63 00:03:01,080 --> 00:03:03,630 you're going to create a new tuple. 64 64 00:03:03,630 --> 00:03:05,890 You're not going to be modifying that one. 65 65 00:03:05,890 --> 00:03:08,070 So tuples are immutable. 66 66 00:03:08,070 --> 00:03:10,470 Let's take a look at another example. 67 67 00:03:10,470 --> 00:03:15,190 Imagine you have two numbers such as 8597 and 8597. 68 68 00:03:16,710 --> 00:03:20,713 These are the same value, 8597 in both cases, 69 69 00:03:22,370 --> 00:03:25,250 and you can see that the IDs are the same. 70 70 00:03:25,250 --> 00:03:27,710 So why did this differ in lists 71 71 00:03:27,710 --> 00:03:30,990 when you created two empty lists, but not in integers? 72 72 00:03:30,990 --> 00:03:32,150 Python has this optimization 73 73 00:03:32,150 --> 00:03:33,750 where when an integer is created, 74 74 00:03:33,750 --> 00:03:36,103 then it won't recreate it if another one 75 75 00:03:36,103 --> 00:03:38,220 identical to it is used. 76 76 00:03:38,220 --> 00:03:41,850 So here Python knows that 8597 has already been created, 77 77 00:03:41,850 --> 00:03:44,150 so it doesn't create a new one. 78 78 00:03:44,150 --> 00:03:47,740 Integers are also immutable; you can't change them. 79 79 00:03:47,740 --> 00:03:49,570 New Python developers often believe 80 80 00:03:49,570 --> 00:03:53,000 that if you do something like 8598, 81 81 00:03:53,000 --> 00:03:55,070 now b should change as well 82 82 00:03:55,070 --> 00:03:58,140 because you have changed this number here, 83 83 00:03:58,140 --> 00:04:01,000 but that is not at all what's happened. 84 84 00:04:01,000 --> 00:04:02,730 This value was created first, 85 85 00:04:02,730 --> 00:04:05,050 and a is a name for that value. 86 86 00:04:05,050 --> 00:04:08,610 The name assignment happens with the equals sign, 87 87 00:04:08,610 --> 00:04:11,710 and here, you have created the value 8598, 88 88 00:04:11,710 --> 00:04:14,070 and you have given it the name a. 89 89 00:04:14,070 --> 00:04:17,360 However, b is still a name for 8597. 90 90 00:04:17,360 --> 00:04:19,360 That hasn't changed at all. 91 91 00:04:19,360 --> 00:04:21,690 However, if you do print these IDs, 92 92 00:04:21,690 --> 00:04:23,120 you can see that that's all correct, 93 93 00:04:23,120 --> 00:04:25,270 and they are now different. 94 94 00:04:25,270 --> 00:04:26,873 They are identical at the start, 95 95 00:04:28,290 --> 00:04:30,393 and then they differ afterwards. 96 96 00:04:31,410 --> 00:04:34,350 As I said earlier, most things in Python are mutable 97 97 00:04:34,350 --> 00:04:37,960 except for tuples, strings, integers, floats, and booleans. 98 98 00:04:37,960 --> 00:04:40,470 I believe that's most of the immutable ones. 99 99 00:04:40,470 --> 00:04:42,090 All the rest are mutable, 100 100 00:04:42,090 --> 00:04:42,970 and if you want to create 101 101 00:04:42,970 --> 00:04:46,050 an immutable class and immutable objects, 102 102 00:04:46,050 --> 00:04:48,220 just don't add any methods in them 103 103 00:04:48,220 --> 00:04:51,110 that can change the object's properties. 104 104 00:04:51,110 --> 00:04:52,930 For example, if you have a book object 105 105 00:04:52,930 --> 00:04:54,370 and you want it immutable, 106 106 00:04:54,370 --> 00:04:56,730 don't allow anybody to change 107 107 00:04:56,730 --> 00:04:59,023 the name property of that book. 108 108 00:04:59,920 --> 00:05:02,240 One more example for beginner Python developers 109 109 00:05:02,240 --> 00:05:05,270 is that if you create a string called hello 110 110 00:05:05,270 --> 00:05:06,610 and give it the name a, 111 111 00:05:06,610 --> 00:05:10,190 and then make b and give it the same value as a's value, 112 112 00:05:10,190 --> 00:05:11,960 then these will be the same. 113 113 00:05:11,960 --> 00:05:14,280 If you do plus equal, 114 114 00:05:14,280 --> 00:05:18,060 that is the same thing as doing equal a plus world, 115 115 00:05:18,060 --> 00:05:23,060 so you are reassigning the name a into this new string, 116 116 00:05:23,390 --> 00:05:28,170 but b will still be hello, so this will change the name a. 117 117 00:05:28,170 --> 00:05:30,913 It won't change the string hello. 118 118 00:05:31,836 --> 00:05:33,800 So that's something important to remember. 119 119 00:05:33,800 --> 00:05:36,800 Immutable types like strings, you can't change them. 120 120 00:05:36,800 --> 00:05:39,722 It's physically not possible in Python to change them, 121 121 00:05:39,722 --> 00:05:41,790 so whenever you are assigning, 122 122 00:05:41,790 --> 00:05:45,340 you're always going to be changing the existing name 123 123 00:05:45,340 --> 00:05:46,940 and not the value, 124 124 00:05:46,940 --> 00:05:48,970 and the same thing for tuples, 125 125 00:05:48,970 --> 00:05:50,883 integers, floats, and booleans. 126 126 00:05:52,620 --> 00:05:53,470 That's it for this video. 127 127 00:05:53,470 --> 00:05:56,810 I wanted to tell you a bit about mutability in Python. 128 128 00:05:56,810 --> 00:05:58,380 In the next one, we're going to learn 129 129 00:05:58,380 --> 00:06:00,480 why it's important to know about it. 130 130 00:06:00,480 --> 00:06:03,130 So stick with me, and I'll see you in the next video.