1 00:00:03,740 --> 00:00:08,360 I finished the last video by saying that strings are immutable, 2 00:00:08,360 --> 00:00:14,780 whereas lists are mutable. When an object is described as immutable, that means it 3 00:00:14,780 --> 00:00:18,050 can't be changed. The following immutable types are built 4 00:00:18,050 --> 00:00:24,619 into Python: int, float, bool - which is True or False, a subclass of int, string or str, 5 00:00:24,619 --> 00:00:32,029 which is a string, tuple, frozenset and bytes. So it should be obvious that an 6 00:00:32,029 --> 00:00:37,550 int like 5 can't be changed. 5 is always 5. You can add another value to it - for 7 00:00:37,550 --> 00:00:44,480 example, we can add 1 to 5 and we get a new number 6: However, we can't change 5. 8 00:00:44,480 --> 00:00:51,980 The same's true of floats. 3.14159 will always be 3.14159. Once again, we can 9 00:00:51,980 --> 00:00:56,690 perform arithmetic with a float, and get a new float value, but we can't change 10 00:00:56,690 --> 00:01:01,610 the value of 3.14159. So stay with me on this, because there might be an obvious 11 00:01:01,610 --> 00:01:05,330 question that you're asking. I'll explain that next, but at the moment, 12 00:01:05,330 --> 00:01:13,900 trust me that these values are immutable. bool values are also immutable, and so are strings. 13 00:01:13,900 --> 00:01:17,300 The question you're asking is probably becoming disbelief now. 14 00:01:17,310 --> 00:01:20,350 So let's type in some code now, that proves this. 15 00:01:20,350 --> 00:01:26,860 So I'm going to create a new Python file here, called immutable, for these examples. 16 00:01:34,740 --> 00:01:39,860 So let's start by typing result equals True - True with a capital T, 17 00:01:39,860 --> 00:01:48,380 another_result is equal to result, print on the next line, left parenthesis id 18 00:01:48,380 --> 00:01:53,200 left parenthesis result then two right parantheses. Then on line 4, print left 19 00:01:53,200 --> 00:01:57,130 parenthesis id, a left parenthesis another_result and we've 20 00:01:57,130 --> 00:02:01,420 got our two right parentheses to end the line. Alright, so here, we've got two 21 00:02:01,420 --> 00:02:07,090 variables; result and another_result. They're both bound to the value 22 00:02:07,090 --> 00:02:12,680 True. On lines three and four, are printed out the ID of those variables. 23 00:02:12,680 --> 00:02:18,800 Now it's worth checking the documentation for the ID function, so I'll just briefly do that. 24 00:02:21,420 --> 00:02:27,360 So as you can see there, id returns the identity of an object. 25 00:02:27,379 --> 00:02:31,639 Now the documentation doesn't state that it should be the object's address, just 26 00:02:31,639 --> 00:02:36,260 that it must be "guaranteed to be unique and constant for this object during its 27 00:02:36,260 --> 00:02:41,689 lifetime". The CPython implementation does return the object's memory address, but 28 00:02:41,689 --> 00:02:46,639 not all Pythons will do that. The ID for an object may be different each time you 29 00:02:46,640 --> 00:02:51,280 run the program, but while your program is running, the object will have the same id. 30 00:02:51,280 --> 00:02:53,299 Of course, if python has to destroy the 31 00:02:53,299 --> 00:02:58,189 object and re-create it, then it's ID will also change. So that gives us a good way 32 00:02:58,189 --> 00:03:04,080 to tell if an object is changed, or if python has to create a new object. 33 00:03:04,080 --> 00:03:11,620 Let's go back to our code. Let's run it. We can see here that both variables have the 34 00:03:11,629 --> 00:03:17,060 same ID. They're both bound to the same value - True - so that makes sense that it 35 00:03:17,060 --> 00:03:21,580 should have the same ID. Remember that we're printing the ID of True here. 36 00:03:21,580 --> 00:03:26,580 The variables - result and another_result - are just names that we've bound 37 00:03:26,580 --> 00:03:31,099 to that value. Alright, so if bool values could be changed, then changing 38 00:03:31,099 --> 00:03:34,909 the values should mean that the ID doesn't change. So let's see if that's 39 00:03:34,909 --> 00:03:43,099 the case, so I'm going to add some new code down here, line 6, result equals 40 00:03:43,099 --> 00:03:47,569 False with a capital F. Then we're going to print left parenthesis id left 41 00:03:47,569 --> 00:03:52,129 parenthesis result, then two right parentheses to end the line. Let's run 42 00:03:52,129 --> 00:03:58,099 that code, and you can see here, we've got a different ID for a result with the 43 00:03:58,099 --> 00:04:03,079 code that we've just added. Because bools are immutable, we weren't able to change 44 00:04:03,080 --> 00:04:10,080 the value of True. What Python's done instead, is rebound result to a new value - False. 45 00:04:10,080 --> 00:04:13,099 When we were looking at the slides, when I said that bools were immutable, 46 00:04:13,099 --> 00:04:16,820 you may have been thinking, but hang on - I can assign a new value to a boolean 47 00:04:16,820 --> 00:04:22,220 variable. Now that would be true in other languages. In C or Java, for example, 48 00:04:22,220 --> 00:04:26,419 you can assign new values to boolean variables. In Python, though, that's not 49 00:04:26,419 --> 00:04:29,840 the case. Instead, we've attached the name result, 50 00:04:29,840 --> 00:04:39,300 to a new value. So let's comment out all that code, and here's another example here. 51 00:04:39,300 --> 00:04:41,920 I'm going to type result equals double 52 00:04:41,920 --> 00:04:46,560 quotes Correct with a capital C, and that's in double quotes, as I mentioned, 53 00:04:46,560 --> 00:04:53,820 and then on line 10, I'm going to type another_result is equal to result, 54 00:04:53,820 --> 00:04:56,100 and I probably could have pasted those lines before I commented it out but 55 00:04:56,100 --> 00:05:00,240 I'll just type it again: print and left parentheses, id left parenthesis 56 00:05:00,240 --> 00:05:06,520 result, and the same thing, this time for another_result. So I'm printing 57 00:05:06,520 --> 00:05:13,280 the ID for both variables, again, as we did earlier. Alright, so let's run the program, 58 00:05:13,280 --> 00:05:17,880 and you can see here that the variables, again, have the same id. 59 00:05:17,890 --> 00:05:22,210 Alright, so moving on, let's attempt to mutate result. Now we can use augmented 60 00:05:22,210 --> 00:05:25,540 assignment to concatenate more characters to the end. Let's have a go at 61 00:05:25,540 --> 00:05:37,660 doing that: result plus equals double quotes ish. I'll print the id again. 62 00:05:37,660 --> 00:05:41,340 So if we could mutate the string, then the value that result is bound to would 63 00:05:41,350 --> 00:05:45,910 change. The ID should remain the same if that happened. Let's run the program and 64 00:05:45,910 --> 00:05:52,480 see what happens - and it's not. As you can see there, result now has a different id. 65 00:05:52,480 --> 00:05:55,480 Now just in case you're not convinced, I'm going to print the id for 66 00:05:55,480 --> 00:06:00,760 another_result as well, so let's do that. I'll just take a copy of that from 67 00:06:00,760 --> 00:06:09,130 line 12 and paste it in. Run that. So notice that the id of result has 68 00:06:09,130 --> 00:06:12,790 changed, but another_result still has the same id that it had to 69 00:06:12,790 --> 00:06:17,080 start with. Because strings are immutable, Python couldn't change the string 70 00:06:17,080 --> 00:06:21,700 correct. What it did instead was create a new string, and bind the name result to 71 00:06:21,700 --> 00:06:24,910 this new string. Now don't worry if you're still a bit confused about this - 72 00:06:24,910 --> 00:06:28,660 it will make sense once you've seen the same thing with a mutable object, 73 00:06:28,660 --> 00:06:32,580 and we'll look at doing that in the next video.