1 1 00:00:00,810 --> 00:00:02,330 Hi guys, and welcome back. 2 2 00:00:02,330 --> 00:00:03,640 In this video, we're going to learn 3 3 00:00:03,640 --> 00:00:05,980 about dictionary comprehensions. 4 4 00:00:05,980 --> 00:00:07,830 A dictionary comprehension is very much 5 5 00:00:07,830 --> 00:00:09,570 like a list comprehension, 6 6 00:00:09,570 --> 00:00:11,980 but we get a dictionary at the end of it, 7 7 00:00:11,980 --> 00:00:14,510 so we need to be assigning key value pairs 8 8 00:00:14,510 --> 00:00:16,240 instead of only values. 9 9 00:00:16,240 --> 00:00:18,500 Let's say we've got a list of users, 10 10 00:00:18,500 --> 00:00:21,940 where each user information is stored in a tuple. 11 11 00:00:21,940 --> 00:00:25,130 So we have a list with four user tuples. 12 12 00:00:25,130 --> 00:00:29,010 Inside it, they have an ID, a unique identifying number 13 13 00:00:29,010 --> 00:00:30,250 for each user, 14 14 00:00:30,250 --> 00:00:33,810 a username, and a password. 15 15 00:00:33,810 --> 00:00:36,520 So, we want to create a mapping of usernames 16 16 00:00:36,520 --> 00:00:38,130 to user information. 17 17 00:00:38,130 --> 00:00:39,920 This is something that you'll be doing very often, 18 18 00:00:39,920 --> 00:00:42,930 especially if you're doing something like web applications 19 19 00:00:42,930 --> 00:00:44,060 and things like that. 20 20 00:00:44,060 --> 00:00:46,810 So we can do a username_mapping variable. 21 21 00:00:46,810 --> 00:00:48,790 Remember, this name here doesn't matter. 22 22 00:00:48,790 --> 00:00:49,803 You can pick whatever you want. 23 23 00:00:49,803 --> 00:00:53,370 And then, we're going to do a dictionary comprehension. 24 24 00:00:53,370 --> 00:00:57,480 So we'll do user[1]:user. 25 25 00:00:57,480 --> 00:01:02,430 This is our key and our value for user in users. 26 26 00:01:02,430 --> 00:01:04,140 And that is our for loop. 27 27 00:01:04,140 --> 00:01:06,980 So in a dictionary comprehension, the value that you're 28 28 00:01:06,980 --> 00:01:08,670 going to put into your new dictionary 29 29 00:01:08,670 --> 00:01:10,880 is actually a key value pair. 30 30 00:01:10,880 --> 00:01:13,030 And then you've got your for loop, just like you had 31 31 00:01:13,030 --> 00:01:15,030 in your list comprehensions. 32 32 00:01:15,030 --> 00:01:19,130 What this ends up doing is getting the username, 33 33 00:01:19,130 --> 00:01:22,820 and associating with it the entire user tuple 34 34 00:01:22,820 --> 00:01:24,560 for each of your users. 35 35 00:01:24,560 --> 00:01:27,087 So if we print(username_mapping) out, 36 36 00:01:27,087 --> 00:01:28,503 you'll see what I mean. 37 37 00:01:30,100 --> 00:01:33,868 line:15% You end up with a rather long dictionary, where the keys 38 38 00:01:33,868 --> 00:01:36,163 line:15% are the usernames. 39 39 00:01:37,360 --> 00:01:40,410 line:15% And the values are the entire tuple, 40 40 00:01:40,410 --> 00:01:44,493 line:15% the whole piece of information that we have about a user. 41 41 00:01:45,370 --> 00:01:47,300 Why can this be helpful? 42 42 00:01:47,300 --> 00:01:50,240 Well, imagine you know a user's username, 43 43 00:01:50,240 --> 00:01:51,950 and you want to get their information out. 44 44 00:01:51,950 --> 00:01:56,537 You just access, let's say, "Bob," in your username_mapping, 45 45 00:01:56,537 --> 00:01:59,040 and you've got the information out. 46 46 00:01:59,040 --> 00:02:03,430 line:15% So here, you would get back the entire of Bob's tuple. 47 47 00:02:03,430 --> 00:02:05,700 If you didn't have a mapping, you would 48 48 00:02:05,700 --> 00:02:07,770 have to do something like this. 49 49 00:02:07,770 --> 00:02:10,600 For user in users: 50 50 00:02:10,600 --> 00:02:15,190 Then you would have to check if user[1] = "Bob": 51 51 00:02:15,190 --> 00:02:17,580 and then you would print(user) out. 52 52 00:02:17,580 --> 00:02:20,140 So as you can see, this is rather complicated 53 53 00:02:20,140 --> 00:02:21,230 in comparison. 54 54 00:02:21,230 --> 00:02:23,850 Once you know what this means, of course. 55 55 00:02:23,850 --> 00:02:25,770 Let's say that you've got something like this 56 56 00:02:25,770 --> 00:02:29,900 in your application, and you want users to log in. 57 57 00:02:29,900 --> 00:02:32,030 What you would do is you would ask the user 58 58 00:02:32,030 --> 00:02:33,333 for their username, 59 59 00:02:34,630 --> 00:02:36,950 and then you would ask them for their password. 60 60 00:02:36,950 --> 00:02:39,193 So, you can do password_input. 61 61 00:02:41,340 --> 00:02:43,210 Now that you've got the username and password, 62 62 00:02:43,210 --> 00:02:47,130 all you have to do is go into the mapping and get the user 63 63 00:02:47,130 --> 00:02:48,920 for their username. 64 64 00:02:48,920 --> 00:02:50,409 So we can say something like, 65 65 00:02:50,409 --> 00:02:54,237 username, password = username_mapping[username_input], 66 66 00:02:55,829 --> 00:03:00,190 and this is going to get the user information tuple, 67 67 00:03:00,190 --> 00:03:03,960 such as this one, and it's going to unpack it, 68 68 00:03:03,960 --> 00:03:06,550 or de-structure it into these three variables. 69 69 00:03:06,550 --> 00:03:09,380 The underscore variable, because we don't care about the ID. 70 70 00:03:09,380 --> 00:03:12,050 The username variable, that will be "Bob." 71 71 00:03:12,050 --> 00:03:15,350 And the password variable, that will be "password." 72 72 00:03:15,350 --> 00:03:19,250 Then, if the password_input = password that we found 73 73 00:03:19,250 --> 00:03:22,990 in our sort of, "database," on our list, 74 74 00:03:22,990 --> 00:03:25,773 then we can say that your details are correct. 75 75 00:03:26,850 --> 00:03:29,163 And otherwise, we can say something else. 76 76 00:03:32,430 --> 00:03:34,250 So this is an example of performing 77 77 00:03:34,250 --> 00:03:38,030 some sort of log-in using this structure here, 78 78 00:03:38,030 --> 00:03:39,240 this dictionary comprehension. 79 79 00:03:39,240 --> 00:03:41,420 This is really helpful because it saves you 80 80 00:03:41,420 --> 00:03:44,180 from having to do another for loop here, to make sure 81 81 00:03:44,180 --> 00:03:48,013 that you are using the right username for their input. 82 82 00:03:48,850 --> 00:03:50,928 I just wanted to show you how dictionary comprehensions 83 83 00:03:50,928 --> 00:03:54,450 work in Python, and I thought that an example like this one 84 84 00:03:54,450 --> 00:03:55,740 could be quite nice as well. 85 85 00:03:55,740 --> 00:03:57,800 So now that we know about dictionary comprehensions, 86 86 00:03:57,800 --> 00:03:59,740 we're ready to move on to the next video. 87 87 00:03:59,740 --> 00:04:00,690 I'll see you there!