1 00:00:05,360 --> 00:00:09,870 In this video, we'll look at what the documentation means when it talks about a shallow copy. 2 00:00:09,870 --> 00:00:17,848 I'm going to create a new Python file for these examples. I'm going to call mine shallow copy underscore py. 3 00:00:22,960 --> 00:00:27,571 Alright, so we need a dictionary to work with. Let's create just a simple one; 4 00:00:27,571 --> 00:00:37,071 animals equals, left and right curly braces and we'll start with a lion as the key. Value is scary. 5 00:00:38,480 --> 00:00:50,354 elephant, value, big. Last one teddy, is the key, colon, cuddly as the value. 6 00:00:50,880 --> 00:00:56,960 Alright. Before we start working with copies, let's review references to mutable objects; things 7 00:00:56,960 --> 00:01:04,655 equals animals. On the next line, animals square brackets double quotes, teddy, 8 00:01:05,120 --> 00:01:09,200 closing right square brackets, equals toy in double quotes, 9 00:01:09,200 --> 00:01:16,080 and print parentheses things square brackets, teddy in double quotes, 10 00:01:16,080 --> 00:01:22,080 and it's closing square brackets and closing parentheses. Alright, so pause the video, 11 00:01:22,080 --> 00:01:26,320 before running this next bit of code, and work out what the output will be. And if you're not sure what 12 00:01:26,320 --> 00:01:31,040 will be printed, review the lecture Binding multiple names to a list in the Lists and 13 00:01:31,040 --> 00:01:36,657 Tuples section of this course. So pause the video now and come back when you've got your answer. 14 00:01:37,680 --> 00:01:39,840 Alright, so welcome back - let's run this. 15 00:01:40,960 --> 00:01:46,400 Okay, so there's a result - both animals and things are referring to the same dictionary. If we change 16 00:01:46,400 --> 00:01:50,480 a value in the dictionary, we get the new value printed. It doesn't matter which reference we 17 00:01:50,480 --> 00:01:56,240 use to change the dictionary - there is only one dictionary, and it gets updated on line eight. 18 00:01:56,240 --> 00:02:02,400 And we saw that when I ran the program. Now if we want two separate dictionaries, we should create a copy. 19 00:02:02,400 --> 00:02:10,479 I'm going to change line seven to do that. Instead of things equals animals, put animals.copy. 20 00:02:10,479 --> 00:02:15,440 We now have two separate dictionaries. We modify the animals dictionary on line eight, 21 00:02:15,440 --> 00:02:22,240 but things isn't affected. So I run the program - let's confirm that is the case. 22 00:02:22,240 --> 00:02:28,400 We can see now the entry for teddy in the things dictionary is still cuddly. And continuing on to 23 00:02:28,400 --> 00:02:36,975 that on the next line, we can do a print parentheses animals square brackets, in double quotes, teddy, 24 00:02:37,760 --> 00:02:42,344 then square brackets and closing off the parentheses. If we run that, 25 00:02:44,320 --> 00:02:48,349 we can see that the teddy entry from animals, now has the value toy. 26 00:02:48,349 --> 00:02:53,200 so we use the copy method, on line 7, to create a copy of the original dictionary. 27 00:02:53,200 --> 00:02:58,480 Any changes we make to animals isn't reflected in the things dictionary, and that's because now, 28 00:02:58,480 --> 00:03:04,560 things is a new dictionary created by copying animals. That's fairly straightforward but what's 29 00:03:04,560 --> 00:03:09,840 all this shallow copy about? With immutable values, it makes no difference whether a copy 30 00:03:09,840 --> 00:03:15,840 is shallow or deep. Where it becomes important is when you have mutable values in the dictionary. 31 00:03:15,840 --> 00:03:20,869 So let's change our values to be lists instead of strings. So coming back up to our dictionary, 32 00:03:20,869 --> 00:03:28,050 for lion, we're going to enclose that in square brackets. I'm going to add a few entries here. 33 00:03:31,040 --> 00:03:36,000 Alright, and do the same for the next two lines as well; 34 00:03:37,280 --> 00:03:39,520 grey, wrinkled, 35 00:03:41,600 --> 00:03:48,327 three entries. And for the last one, the teddy, we'll do the same; cuddly 36 00:03:49,760 --> 00:03:56,579 stuffed. Okay. Each of our values in our dictionary are now lists - 37 00:03:57,600 --> 00:04:02,003 I'm going to delete this entry here, line 8. 38 00:04:03,200 --> 00:04:05,565 When we run this now, 39 00:04:06,480 --> 00:04:12,729 we get the same list printed; the teddy key in each dictionary. Now I emphasize the word same here, 40 00:04:12,729 --> 00:04:18,000 because they really are the same list. We performed a shallow copy and we get two dictionaries, 41 00:04:18,000 --> 00:04:23,456 but there are only three lists, not the six lists that you may have expected. So to demonstrate that, 42 00:04:23,456 --> 00:04:28,733 what I'm going to do is mutate the entry for teddy in the things dictionary. 43 00:04:31,680 --> 00:04:35,090 So we'll do a print parentheses to separate the output; 44 00:04:35,090 --> 00:04:40,425 on line 14, things teddy in square brackets and double quotes, 45 00:04:41,600 --> 00:04:51,000 dot append, and in parentheses, toy in double quotes. Let's go up and copy these two lines 46 00:04:52,947 --> 00:04:57,709 to print those values out again. Alright, so what I've done here is, I've appended another item 47 00:04:57,709 --> 00:05:02,640 to teddy's list in the things directory. We've added toy to the list, on line 14. 48 00:05:02,640 --> 00:05:06,834 Run the program now, and we'll see if the output is what you expected. 49 00:05:07,760 --> 00:05:13,760 As you can see, the new item toy has appeared both times. Now some students will be quite 50 00:05:13,760 --> 00:05:18,800 happy with what happened here. Others - probably most - will be totally confused. If you're totally 51 00:05:18,800 --> 00:05:24,880 confused, don't worry - this is confusing. At some point, you'll have a eureka moment, and it will 52 00:05:24,880 --> 00:05:28,800 all suddenly make perfect sense. It really is worth putting in the effort to understand all 53 00:05:28,800 --> 00:05:33,200 this because it's fundamental to many things that happen, when programming. I'll use the next 54 00:05:33,200 --> 00:05:39,033 couple of videos to explain it in more detail, and in different ways. See you in the next video.