WEBVTT 1 00:00:00.810 --> 00:00:02.330 Hi guys, and welcome back. 2 00:00:02.330 --> 00:00:03.640 In this video, we're going to learn 3 00:00:03.640 --> 00:00:05.980 about dictionary comprehensions. 4 00:00:05.980 --> 00:00:07.830 A dictionary comprehension is very much 5 00:00:07.830 --> 00:00:09.570 like a list comprehension, 6 00:00:09.570 --> 00:00:11.980 but we get a dictionary at the end of it, 7 00:00:11.980 --> 00:00:14.510 so we need to be assigning key value pairs 8 00:00:14.510 --> 00:00:16.240 instead of only values. 9 00:00:16.240 --> 00:00:18.500 Let's say we've got a list of users, 10 00:00:18.500 --> 00:00:21.940 where each user information is stored in a tuple. 11 00:00:21.940 --> 00:00:25.130 So we have a list with four user tuples. 12 00:00:25.130 --> 00:00:29.010 Inside it, they have an ID, a unique identifying number 13 00:00:29.010 --> 00:00:30.250 for each user, 14 00:00:30.250 --> 00:00:33.810 a username, and a password. 15 00:00:33.810 --> 00:00:36.520 So, we want to create a mapping of usernames 16 00:00:36.520 --> 00:00:38.130 to user information. 17 00:00:38.130 --> 00:00:39.920 This is something that you'll be doing very often, 18 00:00:39.920 --> 00:00:42.930 especially if you're doing something like web applications 19 00:00:42.930 --> 00:00:44.060 and things like that. 20 00:00:44.060 --> 00:00:46.810 So we can do a username_mapping variable. 21 00:00:46.810 --> 00:00:48.790 Remember, this name here doesn't matter. 22 00:00:48.790 --> 00:00:49.803 You can pick whatever you want. 23 00:00:49.803 --> 00:00:53.370 And then, we're going to do a dictionary comprehension. 24 00:00:53.370 --> 00:00:57.480 So we'll do user[1]:user. 25 00:00:57.480 --> 00:01:02.430 This is our key and our value for user in users. 26 00:01:02.430 --> 00:01:04.140 And that is our for loop. 27 00:01:04.140 --> 00:01:06.980 So in a dictionary comprehension, the value that you're 28 00:01:06.980 --> 00:01:08.670 going to put into your new dictionary 29 00:01:08.670 --> 00:01:10.880 is actually a key value pair. 30 00:01:10.880 --> 00:01:13.030 And then you've got your for loop, just like you had 31 00:01:13.030 --> 00:01:15.030 in your list comprehensions. 32 00:01:15.030 --> 00:01:19.130 What this ends up doing is getting the username, 33 00:01:19.130 --> 00:01:22.820 and associating with it the entire user tuple 34 00:01:22.820 --> 00:01:24.560 for each of your users. 35 00:01:24.560 --> 00:01:27.087 So if we print(username_mapping) out, 36 00:01:27.087 --> 00:01:28.503 you'll see what I mean. 37 00:01:30.100 --> 00:01:33.868 line:15% You end up with a rather long dictionary, where the keys 38 00:01:33.868 --> 00:01:36.163 line:15% are the usernames. 39 00:01:37.360 --> 00:01:40.410 line:15% And the values are the entire tuple, 40 00:01:40.410 --> 00:01:44.493 line:15% the whole piece of information that we have about a user. 41 00:01:45.370 --> 00:01:47.300 Why can this be helpful? 42 00:01:47.300 --> 00:01:50.240 Well, imagine you know a user's username, 43 00:01:50.240 --> 00:01:51.950 and you want to get their information out. 44 00:01:51.950 --> 00:01:56.537 You just access, let's say, "Bob," in your username_mapping, 45 00:01:56.537 --> 00:01:59.040 and you've got the information out. 46 00:01:59.040 --> 00:02:03.430 line:15% So here, you would get back the entire of Bob's tuple. 47 00:02:03.430 --> 00:02:05.700 If you didn't have a mapping, you would 48 00:02:05.700 --> 00:02:07.770 have to do something like this. 49 00:02:07.770 --> 00:02:10.600 For user in users: 50 00:02:10.600 --> 00:02:15.190 Then you would have to check if user[1] = "Bob": 51 00:02:15.190 --> 00:02:17.580 and then you would print(user) out. 52 00:02:17.580 --> 00:02:20.140 So as you can see, this is rather complicated 53 00:02:20.140 --> 00:02:21.230 in comparison. 54 00:02:21.230 --> 00:02:23.850 Once you know what this means, of course. 55 00:02:23.850 --> 00:02:25.770 Let's say that you've got something like this 56 00:02:25.770 --> 00:02:29.900 in your application, and you want users to log in. 57 00:02:29.900 --> 00:02:32.030 What you would do is you would ask the user 58 00:02:32.030 --> 00:02:33.333 for their username, 59 00:02:34.630 --> 00:02:36.950 and then you would ask them for their password. 60 00:02:36.950 --> 00:02:39.193 So, you can do password_input. 61 00:02:41.340 --> 00:02:43.210 Now that you've got the username and password, 62 00:02:43.210 --> 00:02:47.130 all you have to do is go into the mapping and get the user 63 00:02:47.130 --> 00:02:48.920 for their username. 64 00:02:48.920 --> 00:02:50.409 So we can say something like, 65 00:02:50.409 --> 00:02:54.237 username, password = username_mapping[username_input], 66 00:02:55.829 --> 00:03:00.190 and this is going to get the user information tuple, 67 00:03:00.190 --> 00:03:03.960 such as this one, and it's going to unpack it, 68 00:03:03.960 --> 00:03:06.550 or de-structure it into these three variables. 69 00:03:06.550 --> 00:03:09.380 The underscore variable, because we don't care about the ID. 70 00:03:09.380 --> 00:03:12.050 The username variable, that will be "Bob." 71 00:03:12.050 --> 00:03:15.350 And the password variable, that will be "password." 72 00:03:15.350 --> 00:03:19.250 Then, if the password_input = password that we found 73 00:03:19.250 --> 00:03:22.990 in our sort of, "database," on our list, 74 00:03:22.990 --> 00:03:25.773 then we can say that your details are correct. 75 00:03:26.850 --> 00:03:29.163 And otherwise, we can say something else. 76 00:03:32.430 --> 00:03:34.250 So this is an example of performing 77 00:03:34.250 --> 00:03:38.030 some sort of log-in using this structure here, 78 00:03:38.030 --> 00:03:39.240 this dictionary comprehension. 79 00:03:39.240 --> 00:03:41.420 This is really helpful because it saves you 80 00:03:41.420 --> 00:03:44.180 from having to do another for loop here, to make sure 81 00:03:44.180 --> 00:03:48.013 that you are using the right username for their input. 82 00:03:48.850 --> 00:03:50.928 I just wanted to show you how dictionary comprehensions 83 00:03:50.928 --> 00:03:54.450 work in Python, and I thought that an example like this one 84 00:03:54.450 --> 00:03:55.740 could be quite nice as well. 85 00:03:55.740 --> 00:03:57.800 So now that we know about dictionary comprehensions, 86 00:03:57.800 --> 00:03:59.740 we're ready to move on to the next video. 87 00:03:59.740 --> 00:04:00.690 I'll see you there!