1 00:00:05,360 --> 00:00:09,840 Okay, so let's clear up some confusion that you may be feeling. I'm going to work with a copy of 2 00:00:09,840 --> 00:00:15,831 our shallow_copy code. So we're going to right click on that file in our project pane, 3 00:00:15,831 --> 00:00:22,390 and choose Copy, Copy. I'm going to right click the project up here and choose Paste. 4 00:00:22,720 --> 00:00:26,853 I'm going to call the new file shallow_copy_2 5 00:00:27,760 --> 00:00:32,960 Close the project pane. Now when you make a copy of a file like this it's a good idea 6 00:00:32,960 --> 00:00:37,808 to close the original. This makes it less likely that you'll work with the wrong file. 7 00:00:37,808 --> 00:00:42,720 IntelliJ has a neat feature to help with that, so right click on the editor tab for the new file, 8 00:00:42,720 --> 00:00:48,720 and click on Close Others. That closes all files, as you saw, except the one that you're working with. 9 00:00:48,720 --> 00:00:52,320 Alright, so next, we'll have a look at what's going on with our dictionaries, 10 00:00:52,320 --> 00:00:58,400 and what a shallow copy is actually doing. I'm going to rearrange the animals dictionary slightly, 11 00:00:58,400 --> 00:01:04,080 to reflect what Python's doing behind the scenes. Now we've typed it as a literal value, but that's 12 00:01:04,080 --> 00:01:09,611 not really how it's stored. Looking at line two, the value for the key lion, is a list. 13 00:01:09,611 --> 00:01:13,600 The way we've typed it, it looks like the list appears inside the dictionary, 14 00:01:13,600 --> 00:01:18,282 but that's not what's really happening. Instead, the list is created in memory somewhere, 15 00:01:18,282 --> 00:01:22,380 and the value is a reference to it. It really looks more like this: what I'm going to do 16 00:01:22,380 --> 00:01:33,092 is start typing here, lion_list equals, and we're gonna copy that list, paste it, 17 00:01:35,360 --> 00:01:37,767 and replace it in here. 18 00:01:38,640 --> 00:01:43,985 So line one we've got a list, inside the dictionary on line four. 19 00:01:43,985 --> 00:01:50,392 The value for the key lion, is that list. So do the same thing for the other two lists. 20 00:02:16,480 --> 00:02:20,092 Okay. Alright, so that's a more accurate reflection 21 00:02:20,092 --> 00:02:26,652 of how our dictionary's really stored. The values on lines 6, 7 and 8 are references to the lists. 22 00:02:26,652 --> 00:02:33,000 When we perform a shallow copy, like we're doing on line 11, the references are copied into the new dictionary. 23 00:02:33,000 --> 00:02:38,669 Before we have a closer look at what's happening on line 18, I'll run the program again. 24 00:02:39,920 --> 00:02:47,173 The teddy value in both dictionaries now contains the extra item, toy. Because both dictionaries 25 00:02:48,080 --> 00:02:51,999 contain references to the same lists, that really shouldn't be surprising. 26 00:02:51,999 --> 00:02:56,681 Line 18 is effectively doing this, so what I'm going to do is comment out line 18, 27 00:02:57,120 --> 00:03:05,680 and put a new line of code here; teddy_list.append parentheses, toy in double quotes. 28 00:03:05,680 --> 00:03:12,000 And obviously teddy_list was the one that we defined on line three. So when we run this, 29 00:03:12,960 --> 00:03:18,000 you can see we've got the same output again. So hopefully now, that's less surprising. 30 00:03:18,000 --> 00:03:24,164 We appended a new item to the teddy_list. Both dictionaries contain a reference to that list - 31 00:03:24,164 --> 00:03:30,480 that's the value for teddy. Just to labour the point, I'm going to replace line 11. Instead of 32 00:03:30,480 --> 00:03:34,787 calling a copy method, I'm going to do the copy manually. This is what copy is actually doing. 33 00:03:34,787 --> 00:03:40,880 So I'll copy that line out. Line 12, we're going to type things is equal to. We'll put our left and 34 00:03:40,880 --> 00:03:50,018 right curly braces there. I'm going to copy each line, so lion double quotes colon lion list, 35 00:03:51,040 --> 00:03:59,840 elephant, elephant double quotes colon elephant list. And the third and final one, 36 00:04:00,720 --> 00:04:06,144 we've got teddy in double quotes, colon teddy_list. So when we look at it that way, 37 00:04:06,144 --> 00:04:13,113 it's easier to see why animals square brackets teddy in double quotes, on line 26, contained toy. 38 00:04:13,113 --> 00:04:18,000 We've got three different ways to refer to the teddy_list, but they all refer 39 00:04:18,000 --> 00:04:22,460 to the same list, and you can see that more clearly, looking at the dictionary references for animals 40 00:04:22,460 --> 00:04:28,010 and things, as we've defined it on screen. Alright, so what I'm going to do is come down here, add a 41 00:04:28,010 --> 00:04:33,200 line after line 24. I'm going to add some code to update the list, using each of the three references. 42 00:04:33,200 --> 00:04:37,968 We're already using the one on line 24, as you can see, so I'm going to add the other two after that. 43 00:04:37,968 --> 00:04:45,440 I'm going to type animals square brackets, teddy in double quotes, closing square brackets 44 00:04:45,440 --> 00:04:55,694 dot append parentheses and double quotes, added via animals and the back quote. 45 00:04:57,269 --> 00:05:03,518 Then we'll do the same thing on line 25 but for things, so it's things square brackets double quotes, 46 00:05:03,518 --> 00:05:14,427 and teddy in those double quotes, dot append parentheses double quotes, added via. This time we'll put things 47 00:05:15,920 --> 00:05:21,200 And coming out down here, on line 29, we'll print the teddy_list as well. 48 00:05:21,200 --> 00:05:24,987 Let's run the program and check the output, 49 00:05:26,640 --> 00:05:29,680 and there's the code. So whichever reference we use, 50 00:05:29,680 --> 00:05:34,320 the new items are added to the same list. And when we print the list out, using each of 51 00:05:34,320 --> 00:05:40,743 the three references on lines 27 to 29, we get exactly the same list. In the next video, 52 00:05:40,743 --> 00:05:46,400 we'll see some slides explaining what goes on with a shallow copy. So I'll see you in the next video.