1 00:00:04,260 --> 00:00:09,690 Let's talk about creating lists now, in Python. So it may seem strange to have 2 00:00:09,690 --> 00:00:13,590 the video on creating lists, so far into the section, but some of this wouldn't 3 00:00:13,590 --> 00:00:17,100 have made sense earlier. Now we've already seen a couple of ways of 4 00:00:17,100 --> 00:00:21,600 creating a list. Firstly, let's open up our number_lists.py 5 00:00:21,600 --> 00:00:25,940 program that we wrote, and let's review those methods. So I'm going to delete all but 6 00:00:25,949 --> 00:00:29,910 the first two lines and then add an empty list at the top, so let's go ahead 7 00:00:29,910 --> 00:00:34,580 and do that. Okay, then back on line 1 what we're going to do, is type empty 8 00:00:34,580 --> 00:00:39,980 underscore list equals left and right square brackets - so it's an empty list, in 9 00:00:39,989 --> 00:00:44,670 other words, that we've created. Alright, so we can create a list by using a 10 00:00:44,670 --> 00:00:48,989 list literal. Line one creates an empty list - there's nothing between the square 11 00:00:48,989 --> 00:00:54,660 brackets. We've also got two other lists, as you can see on lines 2 & 3. Each list 12 00:00:54,660 --> 00:01:00,179 is a comma separated list of values enclosed in square brackets. You can also 13 00:01:00,180 --> 00:01:05,920 create a list by concatenating existing lists. Let's have a go at doing that. 14 00:01:05,920 --> 00:01:13,180 So on line 5 I'm going to type numbers is equal to even plus odd. And let's just print 15 00:01:13,180 --> 00:01:17,540 out the result, so print and then numbers in parentheses on the next line. 16 00:01:17,540 --> 00:01:21,610 So the new list here - numbers - contains all the items from the two lists that 17 00:01:21,610 --> 00:01:28,150 we've concatenated. So let's just run this to confirm. So notice after I ran it 18 00:01:28,150 --> 00:01:32,780 there, that the two items are in the same order as they were in the original list. 19 00:01:32,780 --> 00:01:38,260 The odd numbers appear after the even ones because we added odd to even. 20 00:01:38,260 --> 00:01:42,850 A third way to create a list is to use a function that returns one. We've seen a 21 00:01:42,850 --> 00:01:47,400 function that does that - the sorted function. Let's put some code in for that. 22 00:01:47,400 --> 00:01:52,990 So line eight I'm going to type sorted underscore numbers is equal to sorted, 23 00:01:52,990 --> 00:01:57,759 and in parentheses, numbers. Let's print out the result of sorted underscore 24 00:01:57,760 --> 00:02:03,580 numbers, or the value of sorted underscore numbers. Let's run this. 25 00:02:03,580 --> 00:02:07,500 We've now got another list there - sorted underscore numbers - that contains all the 26 00:02:07,509 --> 00:02:12,190 items for numbers but now in order. And we've seen that before. I mention it 27 00:02:12,190 --> 00:02:16,100 again because it may not have been obvious that a new list is created. 28 00:02:16,100 --> 00:02:20,760 The original numbers list is still in the same order as it was, and we can check that 29 00:02:20,769 --> 00:02:33,700 out by printing it out: print numbers, run it again. If you want to sort a list in 30 00:02:33,700 --> 00:02:38,350 place without creating a new list, then you'd call the .sort method. If you 31 00:02:38,350 --> 00:02:42,310 want a sorted list without changing the original, as we've done here, you can use 32 00:02:42,310 --> 00:02:47,530 the sorted function. sorted() will take any iterable object. We can give it a 33 00:02:47,530 --> 00:02:53,010 string, and a new list will be created from the characters in the string. 34 00:02:54,360 --> 00:03:01,000 So let's try that: line twelve, digits equals sorted parentheses double quotes, 35 00:03:01,000 --> 00:03:08,210 432985617. Then close off the string and 36 00:03:08,210 --> 00:03:12,620 parentheses, and print digits on the next line with print, parentheses and digits 37 00:03:12,620 --> 00:03:16,850 in the middle of that. Now before I run the program, have a think about what it's 38 00:03:16,850 --> 00:03:21,290 going to produce. What will the digits list contain? Pause the video and start 39 00:03:21,290 --> 00:03:24,240 it again when you've worked out your answer. 40 00:03:25,380 --> 00:03:29,560 Okay, so run the program now to see if you're right and I'll do that. We get a 41 00:03:29,560 --> 00:03:35,409 list of strings. So each string is a single character - a digit. When you create 42 00:03:35,409 --> 00:03:39,159 a list from another object, the contents of the list will normally be the same 43 00:03:39,160 --> 00:03:45,960 type as the items in that object. On line eight we create a list from a list of ints. 44 00:03:45,960 --> 00:03:50,319 The list that was produced - sorted_numbers - is a list of ints. 45 00:03:50,319 --> 00:03:55,060 On line 12, we create a sequence of characters, and the digits list that's 46 00:03:55,060 --> 00:03:58,150 produced is a list of strings. So in Python a character is just a string of 47 00:03:58,150 --> 00:04:02,460 length 1. Let's just run that again to confirm. So we've got integers first, 48 00:04:02,460 --> 00:04:08,040 and we've got a list of strings, or characters, on the last line of output. 49 00:04:08,040 --> 00:04:11,909 What if you didn't want to sort the items? You might have noticed the list 50 00:04:11,909 --> 00:04:16,040 function when we looked at the built-in functions earlier, in a previous video. 51 00:04:16,040 --> 00:04:21,120 Now technically, list isn't a function - it's a class initializer. But don't worry 52 00:04:21,120 --> 00:04:24,750 about that - you can just call it, just as if it is a function. And you will be 53 00:04:24,750 --> 00:04:28,020 learning about classes in the Object-Oriented Programming section, 54 00:04:28,020 --> 00:04:31,410 and then you'll see exactly what list is doing. Until then we're going to pretend 55 00:04:31,410 --> 00:04:35,070 it's a function, and we can use that instead of sorted on line 12. So if I go 56 00:04:35,070 --> 00:04:44,160 back and change sorted here, to list, and I run, this time we still get a list, 57 00:04:44,160 --> 00:04:48,900 but the items are in the same order as they appeared in the string. You can use 58 00:04:48,900 --> 00:04:53,009 the list function to create a list from any iterable, and we can use another list, 59 00:04:53,009 --> 00:04:58,650 for example. So let's have a go at doing that. So line 15 I'm going to type 60 00:04:58,650 --> 00:05:05,460 more_numbers is equal to list and in parentheses, numbers. 61 00:05:05,460 --> 00:05:10,160 Let's then print that out, so print on the next line, parentheses more_numbers. 62 00:05:10,160 --> 00:05:15,080 Run the program, and you can see we get the same list of numbers 63 00:05:15,080 --> 00:05:19,760 that line 10 produced, now in the output on the last line of output. So why might you 64 00:05:19,770 --> 00:05:24,300 want to do that. Well it's one way to copy a list. We've now got two lists with 65 00:05:24,300 --> 00:05:29,310 the same items; numbers and more_numbers. They're not the same 66 00:05:29,310 --> 00:05:32,909 list, though. We can check that in a couple of ways. I've already shown you 67 00:05:32,909 --> 00:05:38,010 how to print out the IDs, so you could do that. We can also use is to see if 68 00:05:38,010 --> 00:05:40,860 they're the same, so let's have a go at doing that. 69 00:05:40,860 --> 00:05:48,020 So line 17 I'm going to type print, and in parentheses, numbers is more 70 00:05:48,029 --> 00:05:54,299 underscore numbers, then the closing parenthesis. We'll run this, and you can see 71 00:05:54,300 --> 00:05:58,700 that the output is False, meaning they're not the same list. They are equal though, 72 00:05:58,700 --> 00:06:03,960 because they contain the same items in the same order. Let's verify that. 73 00:06:03,960 --> 00:06:11,480 print, this is on line 18, parenthesis numbers is equal to, so two equal signs, more 74 00:06:11,489 --> 00:06:17,129 underscore numbers, closing parenthesis. Run the program again now. This time you 75 00:06:17,129 --> 00:06:21,569 can see line 18 has produced the value True, meaning the lists are equal because 76 00:06:21,569 --> 00:06:25,979 they both contain the same items. However, the previous line of output from line 17 77 00:06:25,979 --> 00:06:30,269 showed us that they're not the same list. There's another way to copy lists and 78 00:06:30,269 --> 00:06:33,980 that's to use a slice, and that's something else we've seen before. 79 00:06:33,980 --> 00:06:38,020 So what I'm going to do is comment out line 15. 80 00:06:39,169 --> 00:06:44,759 We're going to add a new line 16, which will copy the list using a slice. So on 81 00:06:44,759 --> 00:06:51,169 line 16 I'm going to type more_numbers = numbers, 82 00:06:51,169 --> 00:06:56,980 in square brackets, and then just with a colon in those square brackets. 83 00:06:56,980 --> 00:07:04,139 And that should be numbers, not number. So if we run this now, this does the same thing. 84 00:07:04,139 --> 00:07:07,889 We've now got two lists again with the same items. So another way to create 85 00:07:07,889 --> 00:07:11,999 a list, as you saw there, was to slice an existing list. There's actually at least 86 00:07:11,999 --> 00:07:16,919 12 different ways to copy a list in Python. I don't suggest you use most of 87 00:07:16,920 --> 00:07:20,820 them, because they're not very readable, and some are also not very efficient. 88 00:07:20,820 --> 00:07:25,100 The most readable way is to use the copy method that was added to lists in 89 00:07:25,109 --> 00:07:32,429 pythons 3.3. So let's comment out line 16 I'm going to replace that, this time on 90 00:07:32,429 --> 00:07:42,809 line 17, with more_numbers is equal to numbers.copy, this time with 91 00:07:42,809 --> 00:07:45,999 parentheses because we do want to call the method. 92 00:07:45,999 --> 00:07:51,999 If you run this you can see we still got the same output. A lot of code you'll 93 00:07:51,999 --> 00:07:56,079 come across will have been converted from Python 2. That means you'll see the 94 00:07:56,079 --> 00:08:00,999 slice I've now commented out, on line 16, being commonly used to copy a 95 00:08:00,999 --> 00:08:06,009 sequence. In Python 3, the slice still performs quite well. The fastest way, 96 00:08:06,009 --> 00:08:10,700 though, involves using star to unpack the list, but we haven't covered that yet. 97 00:08:10,700 --> 00:08:13,899 We'll look at that in the section on functions. If you want to check out the 98 00:08:13,899 --> 00:08:17,829 various ways of copying lists and their relative performance, there's an 99 00:08:17,829 --> 00:08:22,020 interesting Stack Overflow post here. I'm just going to bring up a browser. 100 00:08:22,020 --> 00:08:27,819 The list is in the resources section as always. So this is interesting for a 101 00:08:27,820 --> 00:08:32,120 number of reasons, not the least of which is that it benchmarks the various approaches. 102 00:08:32,120 --> 00:08:35,419 So check that out at your leisure. There's some good information in there 103 00:08:35,419 --> 00:08:42,280 if you want to find out more. Alright, so back to the code. 104 00:08:42,280 --> 00:08:47,040 So this video didn't contain much that was new but hopefully, you found the revision helpful. 105 00:08:47,040 --> 00:08:50,300 We've seen a few different ways to create lists and a few ways to copy an 106 00:08:50,320 --> 00:08:54,840 existing list. It may not have been obvious that you can test lists for equality. 107 00:08:54,840 --> 00:08:58,899 == will return True if the lists contain the same items, in 108 00:08:58,899 --> 00:09:03,430 the same order. The is operator will return True if two variables are 109 00:09:03,430 --> 00:09:07,629 referring to the same list. So moving on now, in the next video we'll go back to 110 00:09:07,629 --> 00:09:11,230 that table on operations on mutable sequence types, and we'll see how to 111 00:09:11,230 --> 00:09:16,620 replace slices in a list. See you in the next video.