1 1 00:00:01,170 --> 00:00:02,470 Hey guys, welcome back. 2 2 00:00:02,470 --> 00:00:06,540 And in this video, we're going to talk about dataclasses, 3 3 00:00:06,540 --> 00:00:09,153 Dataclasses are a new feature in python 3.7. 4 4 00:00:10,480 --> 00:00:14,300 That make defining our classes a little bit easier, 5 5 00:00:14,300 --> 00:00:16,460 and also a little bit better. 6 6 00:00:16,460 --> 00:00:19,810 The dataclasses are essentially a code generator. 7 7 00:00:19,810 --> 00:00:21,480 So we're going to use them. 8 8 00:00:21,480 --> 00:00:23,790 And it's going to generate some code for us 9 9 00:00:23,790 --> 00:00:26,560 so that we don't have to write it ourselves. 10 10 00:00:26,560 --> 00:00:29,423 Let's begin by defining a user class. 11 11 00:00:30,780 --> 00:00:33,540 And in this class, I'm going to have an init method. 12 12 00:00:33,540 --> 00:00:36,990 And this method is going to take three parameters, 13 13 00:00:36,990 --> 00:00:40,780 the id, the username and the password of the user. 14 14 00:00:40,780 --> 00:00:44,020 And it's going to do the usual self Id is Id, 15 15 00:00:44,020 --> 00:00:47,373 self username is username, and so on. 16 16 00:00:49,730 --> 00:00:51,880 Then we're also going to have a json method 17 17 00:00:53,310 --> 00:00:55,960 that is going to return the Id 18 18 00:00:58,890 --> 00:00:59,853 and the username. 19 19 00:01:02,320 --> 00:01:04,960 Very simple, very standard until now, 20 20 00:01:04,960 --> 00:01:06,550 nothing different than the models 21 21 00:01:06,550 --> 00:01:11,360 that we've been using so far in our application. 22 22 00:01:11,360 --> 00:01:12,850 But of course, this is a sample app 23 23 00:01:12,850 --> 00:01:14,660 it doesn't have anything to do with our application 24 24 00:01:14,660 --> 00:01:16,753 just to show you what dataclasses are. 25 25 00:01:18,850 --> 00:01:22,610 This is a very common pattern when we're defining classes, 26 26 00:01:22,610 --> 00:01:24,980 which is to define an init method. 27 27 00:01:24,980 --> 00:01:28,530 And to make it take some parameters. 28 28 00:01:28,530 --> 00:01:30,610 And to make those parameters affect 29 29 00:01:30,610 --> 00:01:35,000 some property inside the class or the object. 30 30 00:01:35,000 --> 00:01:37,430 So self Id is the Id parameter, 31 31 00:01:37,430 --> 00:01:39,224 username is the username parameter, 32 32 00:01:39,224 --> 00:01:41,160 password is password parameter. 33 33 00:01:41,160 --> 00:01:43,890 This is common, we do it all the time. 34 34 00:01:43,890 --> 00:01:45,580 And it's a little bit of duplication 35 35 00:01:45,580 --> 00:01:46,413 when you think about it, 36 36 00:01:46,413 --> 00:01:50,260 because all our models in our app have something like this, 37 37 00:01:50,260 --> 00:01:51,670 this class has something like this 38 38 00:01:51,670 --> 00:01:54,320 most classes we write, will have something like this. 39 39 00:01:55,570 --> 00:02:00,313 So this is one of the things that dataclasses helps us do. 40 40 00:02:01,410 --> 00:02:02,840 For now, I'm going to keep it as this, 41 41 00:02:02,840 --> 00:02:06,003 I'm gonna go over to app.py and I'm gonna import it. 42 42 00:02:07,781 --> 00:02:09,381 Then I'm going to define a user. 43 43 00:02:13,610 --> 00:02:15,540 And I'm gonna print it out. 44 44 00:02:15,540 --> 00:02:17,220 Just to show you a few of the things 45 45 00:02:17,220 --> 00:02:19,070 that dataclasses can do. 46 46 00:02:19,070 --> 00:02:22,110 Now I'm gonna run app.py and what's gonna happen here is 47 47 00:02:22,110 --> 00:02:25,490 you're gonna see the user object printed out. 48 48 00:02:25,490 --> 00:02:26,960 So what's happened? 49 49 00:02:26,960 --> 00:02:30,130 Well, we've passed in the Id, username 50 50 00:02:30,130 --> 00:02:32,820 and password to our user constructor. 51 51 00:02:32,820 --> 00:02:36,880 Those have been set as the properties inside self. 52 52 00:02:36,880 --> 00:02:39,880 And then we've printed out the user object. 53 53 00:02:39,880 --> 00:02:43,230 Note that we have not called u.json, 54 54 00:02:43,230 --> 00:02:45,220 we just printed the user object. 55 55 00:02:45,220 --> 00:02:49,090 And we've got this weird user.User object 56 56 00:02:49,090 --> 00:02:51,150 at this thing. 57 57 00:02:51,150 --> 00:02:53,380 This is the memory address of the object 58 58 00:02:53,380 --> 00:02:56,700 and this is the class and then it tells you is an object. 59 59 00:02:56,700 --> 00:02:58,190 So this is not terribly useful 60 60 00:02:58,190 --> 00:03:00,750 because when you print an object out 61 61 00:03:00,750 --> 00:03:02,670 It doesn't tell you anything interesting 62 62 00:03:02,670 --> 00:03:05,400 about it other than the class. 63 63 00:03:05,400 --> 00:03:09,690 So, dataclasses are also going to help us fix that, 64 64 00:03:09,690 --> 00:03:11,770 or at least improve it. 65 65 00:03:11,770 --> 00:03:15,410 Let's begin by importing dataclasses. 66 66 00:03:15,410 --> 00:03:18,083 So from dataclasses, import dataclass. 67 67 00:03:19,370 --> 00:03:23,793 And then we're gonna say @dataclass, up here. 68 68 00:03:24,740 --> 00:03:26,690 Now we're gonna print it out. 69 69 00:03:26,690 --> 00:03:30,530 And now you can see that we get user empty brackets. 70 70 00:03:30,530 --> 00:03:31,780 So this has changed a little bit 71 71 00:03:31,780 --> 00:03:33,950 when you print out a dataclass, 72 72 00:03:33,950 --> 00:03:36,570 you no longer get the memory address. 73 73 00:03:36,570 --> 00:03:39,910 And the user.User object, blah, blah, blah. 74 74 00:03:39,910 --> 00:03:44,450 Now you get the class and then you get a pair of brackets. 75 75 00:03:44,450 --> 00:03:47,660 Inside these brackets, the dataclass is going 76 76 00:03:47,660 --> 00:03:51,873 to tell you the object properties that it knows about. 77 77 00:03:52,930 --> 00:03:55,020 So what we're gonna do next is we're gonna say, 78 78 00:03:55,020 --> 00:03:59,980 _id is a string, username is a string 79 79 00:03:59,980 --> 00:04:01,993 and password is a string. 80 80 00:04:03,490 --> 00:04:06,970 When you do this inside a normal class, 81 81 00:04:06,970 --> 00:04:09,510 what this says is, there aren't gonna be three properties 82 82 00:04:09,510 --> 00:04:10,530 and they're gonna be strings. 83 83 00:04:10,530 --> 00:04:12,460 And we've used this in the past. 84 84 00:04:12,460 --> 00:04:15,040 When you do this inside a dataclass, 85 85 00:04:15,040 --> 00:04:17,910 it knows how to use this information 86 86 00:04:17,910 --> 00:04:20,770 to define the init method for you. 87 87 00:04:20,770 --> 00:04:24,890 So you no longer need an init method. 88 88 00:04:24,890 --> 00:04:29,010 Notice that when I run the app, now, it's all going to work. 89 89 00:04:29,010 --> 00:04:33,140 But the dataclass now knows about these three properties. 90 90 00:04:33,140 --> 00:04:37,350 And it tells you in this thing, when it's prints it out. 91 91 00:04:37,350 --> 00:04:38,980 It tells you that the Id is 123, 92 92 00:04:38,980 --> 00:04:41,393 the username is bob and the password is password. 93 93 00:04:43,260 --> 00:04:45,660 This is one of the things that dataclasses does. 94 94 00:04:46,900 --> 00:04:51,120 It allows us to simplify our init method 95 95 00:04:51,120 --> 00:04:52,990 by simply telling it the properties 96 96 00:04:52,990 --> 00:04:55,850 and what their values are or the type of values are 97 97 00:04:55,850 --> 00:04:58,430 and then it will define the init method for us, 98 98 00:04:58,430 --> 00:05:03,020 as well as allowing us to print it out a bit more simply. 99 99 00:05:03,020 --> 00:05:05,330 So how do we print something out? 100 100 00:05:05,330 --> 00:05:08,863 Well, that is the dunder repr method. 101 101 00:05:10,690 --> 00:05:13,620 If we define a dunder repr method inside our user class, 102 102 00:05:13,620 --> 00:05:16,190 and we make it return hi, when we run the app, 103 103 00:05:16,190 --> 00:05:18,550 you'll see that hi is printed out. 104 104 00:05:18,550 --> 00:05:19,600 What does that mean? 105 105 00:05:19,600 --> 00:05:23,150 It means that dataclass has defined a repr method for us, 106 106 00:05:23,150 --> 00:05:25,330 which returns all of this stuff. 107 107 00:05:25,330 --> 00:05:27,470 So it does that for us without us having 108 108 00:05:27,470 --> 00:05:29,253 to define our own repr method. 109 109 00:05:31,170 --> 00:05:32,050 Here's a couple more things 110 110 00:05:32,050 --> 00:05:33,773 that we can do using dataclasses. 111 111 00:05:34,950 --> 00:05:38,020 From the dataclasses package, we can import field, 112 112 00:05:38,020 --> 00:05:41,870 and then we can make one of these equal to field. 113 113 00:05:41,870 --> 00:05:45,010 And then we can pass in a bunch of configuration properties 114 114 00:05:45,010 --> 00:05:46,890 that allow us to further customise 115 115 00:05:46,890 --> 00:05:49,867 how this field is going to behave in this dataclass. 116 116 00:05:50,720 --> 00:05:52,690 I don't want my password to be printed out 117 117 00:05:52,690 --> 00:05:54,110 when I print the user object. 118 118 00:05:54,110 --> 00:05:57,593 So what I'm going to do is say, repr equal False. 119 119 00:05:58,530 --> 00:06:02,130 And now when I run the app, password is not printed there. 120 120 00:06:02,130 --> 00:06:04,680 But it's still accepted as a parameter, 121 121 00:06:04,680 --> 00:06:09,193 because it is not changed when you define repr equal False. 122 122 00:06:10,180 --> 00:06:12,330 If you say init equal False, 123 123 00:06:12,330 --> 00:06:14,990 then it won't be accepted as a parameter. 124 124 00:06:14,990 --> 00:06:17,650 And now you get an error, because it doesn't expect you 125 125 00:06:17,650 --> 00:06:20,610 to pass in a password in here, because we've said 126 126 00:06:20,610 --> 00:06:23,543 that it should not be included in the init method. 127 127 00:06:26,810 --> 00:06:29,930 Now, you can also compare user objects, 128 128 00:06:29,930 --> 00:06:33,253 you can say, is this user object greater than that one? 129 129 00:06:34,140 --> 00:06:36,490 And then in the context of user objects, 130 130 00:06:36,490 --> 00:06:39,030 it doesn't really make sense to compare them. 131 131 00:06:39,030 --> 00:06:41,930 But sometimes it makes sense to say, 132 132 00:06:41,930 --> 00:06:46,060 is this user exactly equal to this other user? 133 133 00:06:46,060 --> 00:06:50,610 And dataclasses will allow that to work with these fields. 134 134 00:06:50,610 --> 00:06:54,251 So if we define u and u2, 135 135 00:06:54,251 --> 00:06:59,251 and we say u equal u2, you'll see that it comes out as True. 136 136 00:06:59,990 --> 00:07:01,440 Why is that? 137 137 00:07:01,440 --> 00:07:04,630 Because the assumption is that all 138 138 00:07:04,630 --> 00:07:06,330 of these fields are the same 139 139 00:07:06,330 --> 00:07:09,260 these fields that the dataclass knows about. 140 140 00:07:09,260 --> 00:07:11,740 So because they are the same across both users, 141 141 00:07:11,740 --> 00:07:13,770 when you compare two dataclasses, 142 142 00:07:13,770 --> 00:07:15,070 what happens in the background is 143 143 00:07:15,070 --> 00:07:17,083 the fields themselves are compared. 144 144 00:07:18,860 --> 00:07:20,290 If you want one of these fields 145 145 00:07:20,290 --> 00:07:23,120 to not be included in the comparison, 146 146 00:07:23,120 --> 00:07:26,160 you can say compare equal False. 147 147 00:07:26,160 --> 00:07:29,100 What happens then, if they have different passwords, 148 148 00:07:29,100 --> 00:07:32,683 such as password and password2 it will still return True. 149 149 00:07:35,040 --> 00:07:39,049 Note that if one of them is bob, and the other one is bob2, 150 150 00:07:39,049 --> 00:07:40,810 this will return False because the fields 151 151 00:07:40,810 --> 00:07:43,010 that we're comparing are no longer the same. 152 152 00:07:46,320 --> 00:07:49,540 In our MongoDB, we've been looking at 153 153 00:07:49,540 --> 00:07:52,610 how to make this _id property, 154 154 00:07:52,610 --> 00:07:55,870 something that isn't passed necessarily by the user. 155 155 00:07:55,870 --> 00:08:00,810 i.e, given it a uuid value by default. 156 156 00:08:00,810 --> 00:08:03,030 Because we no longer have an init method, 157 157 00:08:03,030 --> 00:08:07,240 you may think that you cannot set this value dynamically. 158 158 00:08:07,240 --> 00:08:08,590 But that's not true. 159 159 00:08:08,590 --> 00:08:11,150 You can always define a function 160 160 00:08:13,110 --> 00:08:14,160 such as generate_hex, 161 161 00:08:15,260 --> 00:08:19,110 make it return uuid.uuid4().hex, 162 162 00:08:19,110 --> 00:08:20,423 make sure to import that. 163 163 00:08:22,260 --> 00:08:24,150 And then in this id property, 164 164 00:08:24,150 --> 00:08:26,440 you can say, this is equal to a field. 165 165 00:08:26,440 --> 00:08:30,910 And the default factory is going to be this function here, 166 166 00:08:30,910 --> 00:08:35,070 make sure to pass in the function and not the function call. 167 167 00:08:35,070 --> 00:08:37,540 Pass in the function name, because this function is going 168 168 00:08:37,540 --> 00:08:41,400 to be evaluated when the object is created, 169 169 00:08:41,400 --> 00:08:44,320 if the id property is not given, 170 170 00:08:44,320 --> 00:08:46,480 and the return value of this function is going 171 171 00:08:46,480 --> 00:08:50,410 to be used as a value for that property at runtime, 172 172 00:08:50,410 --> 00:08:53,400 so it will calculate the new uuid value 173 173 00:08:53,400 --> 00:08:55,450 and it will assign it to the id property. 174 174 00:08:56,680 --> 00:08:59,510 What this does in the background is essentially 175 175 00:09:00,420 --> 00:09:03,473 this self.username = username. 176 176 00:09:05,170 --> 00:09:07,940 self.password = password, 177 177 00:09:07,940 --> 00:09:12,450 self._id = _id 178 178 00:09:12,450 --> 00:09:14,383 or generate_hex. 179 179 00:09:17,257 --> 00:09:20,563 And of course you do need username, password, _id. 180 180 00:09:23,503 --> 00:09:25,660 It essentially does something like this. 181 181 00:09:25,660 --> 00:09:28,760 But due to the way that these properties work, 182 182 00:09:28,760 --> 00:09:32,200 you cannot have this one at the top. 183 183 00:09:32,200 --> 00:09:34,510 Fields with a default value must come 184 184 00:09:34,510 --> 00:09:36,520 after fields without a default. 185 185 00:09:36,520 --> 00:09:41,123 So this one has to come down here underneath the password. 186 186 00:09:43,050 --> 00:09:45,610 And of course, when you're working with your app.py, 187 187 00:09:45,610 --> 00:09:49,633 the id property must be at the end if you wanna pass it in. 188 188 00:09:51,810 --> 00:09:53,443 Then when we run this, you see 189 189 00:09:53,443 --> 00:09:56,530 that you get username and bob printed out 190 190 00:09:56,530 --> 00:09:59,970 and username bob and the id printed out there. 191 191 00:09:59,970 --> 00:10:02,053 If you don't pass in the id, 192 192 00:10:03,440 --> 00:10:07,523 then a uuid will be generated for us. 193 193 00:10:09,780 --> 00:10:12,850 To make things simpler, you can remove this function here 194 194 00:10:12,850 --> 00:10:15,113 and instead, use a lambda function. 195 195 00:10:18,760 --> 00:10:20,250 This is exactly the same thing 196 196 00:10:20,250 --> 00:10:23,810 you are passing on to the default factory function. 197 197 00:10:23,810 --> 00:10:27,583 And when you run this function, you get a new uuid value. 198 198 00:10:28,470 --> 00:10:30,510 So this one is no longer needed. 199 199 00:10:30,510 --> 00:10:32,730 And I actually prefer this notation a little bit better 200 200 00:10:32,730 --> 00:10:35,400 because this lambda function is short enough 201 201 00:10:35,400 --> 00:10:40,170 that it is readable in a single sweep of your eyes 202 202 00:10:40,170 --> 00:10:43,020 to see what it's gonna do when it runs. 203 203 00:10:43,020 --> 00:10:44,530 What this default factory's gonna do is 204 204 00:10:44,530 --> 00:10:48,143 is gonna give you a new uuid value when it runs. 205 205 00:10:49,160 --> 00:10:52,070 Notice that you cannot do this. 206 206 00:10:52,070 --> 00:10:56,480 You cannot say, default_factory=uuid.uuid4.hex 207 207 00:10:56,480 --> 00:11:00,060 Because this is not a function. 208 208 00:11:00,060 --> 00:11:03,890 So if you run this, you will get an error saying 209 209 00:11:03,890 --> 00:11:08,470 that the string that this returns is not callable, 210 210 00:11:08,470 --> 00:11:10,360 because what it's trying to do in there 211 211 00:11:10,360 --> 00:11:14,113 again, is essentially this. 212 212 00:11:15,390 --> 00:11:18,940 Where this fun is this factory that we passed in. 213 213 00:11:18,940 --> 00:11:21,930 So the username and password will be passed in as strings, 214 214 00:11:21,930 --> 00:11:24,990 but this function will be called. 215 215 00:11:24,990 --> 00:11:27,290 And because this is not a function is a string, 216 216 00:11:27,290 --> 00:11:29,410 it cannot be called so you get an error saying 217 217 00:11:29,410 --> 00:11:31,743 you cannot call the string object. 218 218 00:11:35,550 --> 00:11:39,980 However, you can pass in defaults that are not functions. 219 219 00:11:39,980 --> 00:11:41,410 For example, let's say that we wanted 220 220 00:11:41,410 --> 00:11:44,560 to define a country for our users. 221 221 00:11:44,560 --> 00:11:46,190 And we're gonna say that this has 222 222 00:11:46,190 --> 00:11:49,800 a default value of United Kingdom. 223 223 00:11:49,800 --> 00:11:52,030 That's totally fine, just say default 224 224 00:11:52,030 --> 00:11:53,590 and not default factory. 225 225 00:11:53,590 --> 00:11:56,060 And now if you don't pass in a country, 226 226 00:11:56,060 --> 00:11:58,750 then the United Kingdom will be used. 227 227 00:11:58,750 --> 00:11:59,670 By the way, we do have 228 228 00:11:59,670 --> 00:12:01,523 to bring back this lambda there. 229 229 00:12:03,000 --> 00:12:05,410 See, now we've got our user with username bob, 230 230 00:12:05,410 --> 00:12:07,100 the id that we've generated, 231 231 00:12:07,100 --> 00:12:09,170 and the country which is United Kingdom. 232 232 00:12:09,170 --> 00:12:12,230 Do note that because both id 233 233 00:12:12,230 --> 00:12:14,660 and country have default values, 234 234 00:12:14,660 --> 00:12:16,870 no one of them has to be at the end, 235 235 00:12:16,870 --> 00:12:20,260 they both have to be after the non default fields. 236 236 00:12:20,260 --> 00:12:24,230 But the order of the ones that have a default is irrelevant. 237 237 00:12:24,230 --> 00:12:26,633 I do like putting the id at the end, though. 238 238 00:12:31,090 --> 00:12:32,850 I would prefer it if the id was at the start, 239 239 00:12:32,850 --> 00:12:34,110 but you can't do that. 240 240 00:12:34,110 --> 00:12:37,063 So and that's just how it is. 241 241 00:12:39,780 --> 00:12:40,823 So this is a few of the things 242 242 00:12:40,823 --> 00:12:43,390 that dataclasses can do for us, 243 243 00:12:43,390 --> 00:12:47,560 it can help us define an init method more effectively. 244 244 00:12:47,560 --> 00:12:51,750 It helps with the repr method and makes it better. 245 245 00:12:51,750 --> 00:12:54,460 It also greatly helps with comparisons, 246 246 00:12:54,460 --> 00:12:58,860 because now you can compare different objects very easily 247 247 00:12:58,860 --> 00:13:02,860 because it automatically will compare the fields for you. 248 248 00:13:02,860 --> 00:13:06,370 It also does a bunch more things, including hashing, 249 249 00:13:06,370 --> 00:13:11,370 and equality operators, and a whole host of stuff. 250 250 00:13:11,800 --> 00:13:15,070 Think of dataclasses as a code generator. 251 251 00:13:15,070 --> 00:13:17,900 It is writing the code for you 252 252 00:13:17,900 --> 00:13:20,900 and putting it into your class, but you never get to see it. 253 253 00:13:21,830 --> 00:13:23,360 So that's good if you trust 254 254 00:13:23,360 --> 00:13:26,960 that the code it's generating is good for you. 255 255 00:13:26,960 --> 00:13:28,240 That's why you have to understand a bit 256 256 00:13:28,240 --> 00:13:30,990 about dataclasses before you start using them, 257 257 00:13:30,990 --> 00:13:33,483 so that you're comfortable with what it's doing. 258 258 00:13:35,540 --> 00:13:38,120 I'm going to link in the resources section of this lecture, 259 259 00:13:38,120 --> 00:13:41,550 a talk, which talks more about dataclasses, 260 260 00:13:41,550 --> 00:13:43,910 it goes a lot, much more in-depth. 261 261 00:13:43,910 --> 00:13:45,740 I think it's a really great talk 262 262 00:13:45,740 --> 00:13:47,740 and I would recommend that you watch it. 263 263 00:13:48,660 --> 00:13:51,000 But for now, we're going to be using dataclasses 264 264 00:13:51,000 --> 00:13:54,650 in our project, just to make things a little bit simpler. 265 265 00:13:54,650 --> 00:13:56,030 So join me there. 266 266 00:13:56,030 --> 00:13:57,580 I'll see you in the next video.