1 00:00:04,660 --> 00:00:09,860 Let's talk about mutable objects now. So a mutable object is one whose value can 2 00:00:09,860 --> 00:00:15,349 be changed. Python has the following mutable objects built in: list, dict, 3 00:00:15,349 --> 00:00:20,150 set and byte array. We'll be looking at dictionaries and sets in the next 4 00:00:20,150 --> 00:00:24,230 section in the course. So we can change the value of mutable objects, without the 5 00:00:24,230 --> 00:00:29,610 object being destroyed and re-created. Alright, so let's see how our previous 6 00:00:29,610 --> 00:00:32,580 example works when we're dealing with a mutable object. I'm going to be using the 7 00:00:32,580 --> 00:00:38,400 shopping list that we saw in a previous section, and create a new Python file. 8 00:00:38,400 --> 00:00:44,610 This time I'm going to call the file mutable.py. I'm going to paste in the 9 00:00:44,610 --> 00:00:47,910 definition for shopping list so grab that from the previous section. You don't 10 00:00:47,910 --> 00:00:52,260 want to do our typing in here. Now this is future Tim putting an edit in this 11 00:00:52,260 --> 00:00:55,860 program. I wondered whether you just noticed the mistake I made when I created 12 00:00:55,860 --> 00:01:01,380 mutable.py. So let's open the project pane again. Notice the file mutable.py 13 00:01:01,380 --> 00:01:05,900 doesn't exist in there, and that's because I had the .idea folder selected, 14 00:01:05,900 --> 00:01:09,240 and the file actually went in there when I open it. So that happens - it's actually 15 00:01:09,240 --> 00:01:12,020 the wrong place to put it in. Now obviously, it'll still work - 16 00:01:12,020 --> 00:01:15,180 you can see that we haven't got any errors. To move it back, though, just 17 00:01:15,180 --> 00:01:18,750 select the file, drag it back up to the top of the project - Sequences, in this 18 00:01:18,750 --> 00:01:22,740 case. You'll get a warning here telling you, basically, are you sure you want to 19 00:01:22,740 --> 00:01:25,830 move it, in which folder. We're going to go to the folder that I selected, click 20 00:01:25,830 --> 00:01:29,520 on OK, and you can see mutable.py now exists in the right place and it's 21 00:01:29,520 --> 00:01:34,940 been taken out of the .idea folder. Alright, so let's go back to coding now. 22 00:01:34,940 --> 00:01:39,510 We're going to add some code to this and start by typing another, or under that, 23 00:01:39,510 --> 00:01:46,110 another_list is equal to shopping_list, and we'll do the 24 00:01:46,110 --> 00:01:49,200 same thing - we're gonna print the IDs. So print id left 25 00:01:49,200 --> 00:01:52,950 parenthesis and shopping_list, and the next line we're going to do the 26 00:01:52,950 --> 00:01:56,520 same, this time for another_list - very similar to what we did in the 27 00:01:56,520 --> 00:02:02,790 last video. Run the program - checking the IDs, you can see not surprisingly, 28 00:02:02,790 --> 00:02:07,440 we get the same id for both variables. Alright, so with that in mind, let's now 29 00:02:07,440 --> 00:02:11,280 attempt to mutate our list. Now I'm going to be going through all the things you 30 00:02:11,280 --> 00:02:16,019 can do with lists, shortly. For now, though, I'll use augmented assignment to concatenate 31 00:02:16,019 --> 00:02:20,190 another list to the end of this one. That's very similar to concatenating one 32 00:02:20,190 --> 00:02:28,880 string onto another. So I'm going to type shopping_list plus equals 33 00:02:28,880 --> 00:02:34,800 and left square brackets, double quotes cookies. Just the one entry in the list there. 34 00:02:34,800 --> 00:02:38,600 Then we're going to print the shopping list. 35 00:02:40,090 --> 00:02:45,440 Run this, and I've printed out the shopping list there because we haven't seen lists 36 00:02:45,440 --> 00:02:51,110 being concatenated before. So notice that I put a list, after the plus equals on 37 00:02:51,110 --> 00:02:54,800 line 12. We have to enclose the string cookies in square brackets, to make it a 38 00:02:54,800 --> 00:02:59,120 list, as we've done there. And as you can see by output, cookies is now in the list. 39 00:02:59,120 --> 00:03:03,230 Alright, so we've successfully mutated our list - we've added cookies to it. 40 00:03:03,230 --> 00:03:09,290 Let's check the ID to see what's happened. So I'm going to take a copy of 41 00:03:09,290 --> 00:03:16,370 line 9 and paste that below line 13, so on line 14, in other words. So before we 42 00:03:16,370 --> 00:03:21,020 actually run this, what do you think will happen? Will the ID remain the same, or 43 00:03:21,020 --> 00:03:27,820 will we get a new ID after changing the list? Well let's see. So I'm going to run the program. 44 00:03:27,820 --> 00:03:29,750 Notice here now, that the ID remains 45 00:03:29,750 --> 00:03:35,210 unchanged, and that's because lists are mutable and Python was able to add a new 46 00:03:35,210 --> 00:03:40,520 item to the end of the list, without creating a new list. 47 00:03:40,520 --> 00:03:44,319 So now that you've seen the same thing done with a mutable object, and have seen the 48 00:03:44,319 --> 00:03:48,610 different behavior, the previous video on immutable objects, should hopefully make 49 00:03:48,610 --> 00:03:54,099 more sense. Strings are immutable. When we tried to change a string, Python created 50 00:03:54,099 --> 00:03:59,739 a new object - a new string - and re-attached the name to it. Lists are mutable - they 51 00:03:59,739 --> 00:04:03,970 can be changed. When we appended a new item in this code, Python was able to 52 00:04:03,970 --> 00:04:09,320 change the contents of the list, without creating a new one. 53 00:04:09,320 --> 00:04:13,940 So that's the difference between mutable and immutable objects. An immutable 54 00:04:13,940 --> 00:04:18,260 object can't be changed. You can create a new object, and use the same variable 55 00:04:18,260 --> 00:04:21,410 name for it, but you can't change the value of an 56 00:04:21,410 --> 00:04:25,790 immutable object. Mutable objects, such as lists, can be changed, and I'm showing you an 57 00:04:25,790 --> 00:04:29,840 example of that in this video. So walk through the code from this video, and 58 00:04:29,840 --> 00:04:33,110 perhaps the last one again, to check the different behavior. When you're happy 59 00:04:33,110 --> 00:04:37,040 with both videos, we'll have a look at one implication of this difference. 60 00:04:37,040 --> 00:04:40,000 See you in the next video.