WEBVTT 1 00:00:00.032 --> 00:00:01.300 And welcome back. 2 00:00:01.300 --> 00:00:03.010 In this section, we're talking about 3 00:00:03.010 --> 00:00:06.730 some more advanced object oriented principles. 4 00:00:06.730 --> 00:00:08.860 So I've created this sample project here, 5 00:00:08.860 --> 00:00:10.170 called Advanced OOP, 6 00:00:10.170 --> 00:00:11.290 just a small project 7 00:00:11.290 --> 00:00:12.570 where we're gonna create a few files 8 00:00:12.570 --> 00:00:14.800 to explain a couple of these concepts. 9 00:00:15.820 --> 00:00:17.990 There are two files in here right now. 10 00:00:17.990 --> 00:00:19.360 We've got user.py, 11 00:00:20.320 --> 00:00:22.343 and in this file, all we have 12 00:00:22.343 --> 00:00:24.941 is a short class 13 00:00:24.941 --> 00:00:27.000 that represents a user. 14 00:00:27.000 --> 00:00:28.870 So you've got an init method, 15 00:00:28.870 --> 00:00:30.200 fixing a username and a password 16 00:00:30.200 --> 00:00:31.930 and sort of sets them as the properties 17 00:00:31.930 --> 00:00:33.850 of the object you've created, 18 00:00:33.850 --> 00:00:35.770 and then it has two other methods. 19 00:00:35.770 --> 00:00:39.260 A login methods, just returns that you've logged in. 20 00:00:39.260 --> 00:00:41.090 Potentially this should do something like 21 00:00:41.090 --> 00:00:42.980 check a database, compare 22 00:00:42.980 --> 00:00:44.140 the password that you've given 23 00:00:44.140 --> 00:00:45.260 with the password in the database, 24 00:00:45.260 --> 00:00:47.029 or something like that, 25 00:00:47.029 --> 00:00:48.990 and it's got a repr method. 26 00:00:48.990 --> 00:00:51.860 Fairly normal class, you know, 27 00:00:51.860 --> 00:00:53.340 you'll create things like these 28 00:00:53.340 --> 00:00:55.530 all the time in your project, 29 00:00:55.530 --> 00:00:59.000 and then we've got an admin.py class, 30 00:00:59.000 --> 00:01:01.850 and this inherits from user. 31 00:01:01.850 --> 00:01:04.230 So it takes in all of the methods of the user, 32 00:01:05.436 --> 00:01:07.290 and then what we've got is on another init method 33 00:01:07.290 --> 00:01:08.995 in this admin class, 34 00:01:08.995 --> 00:01:10.440 it takes in a username, a password, 35 00:01:10.440 --> 00:01:11.780 and an access level, 36 00:01:12.670 --> 00:01:15.230 and then that calls the user's init 37 00:01:15.230 --> 00:01:17.530 and sets the access level property. 38 00:01:18.408 --> 00:01:21.140 Potentially this access level could be something like 39 00:01:21.140 --> 00:01:24.680 superuser or manager or something like that, 40 00:01:24.680 --> 00:01:27.020 different levels of admin privilege. 41 00:01:27.900 --> 00:01:31.430 Then the repr method here has the admin, 42 00:01:31.430 --> 00:01:33.350 username, and the access, 43 00:01:33.350 --> 00:01:36.535 and finally it also includes a to_dict method, 44 00:01:36.535 --> 00:01:39.870 which returns a dictionary representation 45 00:01:39.870 --> 00:01:41.940 of this admin object. 46 00:01:41.940 --> 00:01:44.480 So it returns the username as self.username, 47 00:01:44.480 --> 00:01:45.620 password as self.password, 48 00:01:45.620 --> 00:01:47.890 and access, self.access. 49 00:01:47.890 --> 00:01:51.080 Just returns a dictionary and using this dictionary, 50 00:01:51.080 --> 00:01:53.820 we could recreate the object. 51 00:01:53.820 --> 00:01:57.000 So this is similar to, for example, 52 00:01:57.000 --> 00:01:58.970 if you wanted to save this to a database, 53 00:01:58.970 --> 00:02:00.840 you would just take each of the properties 54 00:02:00.840 --> 00:02:01.673 in this dictionary, 55 00:02:01.673 --> 00:02:03.540 you could put them in a database, for example. 56 00:02:04.640 --> 00:02:07.296 Now, let's say that we do want to create 57 00:02:07.296 --> 00:02:08.610 some sort of database. 58 00:02:08.610 --> 00:02:11.428 I'm gonna create an in memory database file, 59 00:02:11.428 --> 00:02:13.280 call it database.py, 60 00:02:13.280 --> 00:02:14.790 and just code it live in front of you, 61 00:02:14.790 --> 00:02:17.610 because I think this is an interesting thing to go through. 62 00:02:17.610 --> 00:02:18.700 In creating this database, 63 00:02:18.700 --> 00:02:20.330 we're then going to use it 64 00:02:20.330 --> 00:02:23.460 to explain how multiple inheritance works. 65 00:02:24.500 --> 00:02:26.920 So I'm gonna create this class database, 66 00:02:26.920 --> 00:02:29.360 and the database class is going to 67 00:02:29.360 --> 00:02:32.240 have a class variable called content, 68 00:02:33.320 --> 00:02:35.860 and a class variable is just a variable that exists 69 00:02:35.860 --> 00:02:38.870 within the class as opposed to within the object, 70 00:02:39.880 --> 00:02:43.660 so you can access this as database.content, 71 00:02:43.660 --> 00:02:45.860 and it will always give you this dictionary. 72 00:02:47.190 --> 00:02:49.440 Then we're gonna have a couple of methods, 73 00:02:49.440 --> 00:02:50.600 a couple of class methods, 74 00:02:50.600 --> 00:02:54.160 insert, that is gonna take in some data, 75 00:02:54.160 --> 00:02:56.390 and is going to insert it into 76 00:02:56.390 --> 00:03:00.960 database.content('users').append(data). 77 00:03:03.400 --> 00:03:07.263 So again, database.content is this dictionary here, 78 00:03:07.263 --> 00:03:10.990 and all we're doing is we're appending to the user's list 79 00:03:10.990 --> 00:03:13.320 whatever data we received 80 00:03:13.320 --> 00:03:15.240 in the insert method. 81 00:03:15.240 --> 00:03:16.360 Because this is a class method 82 00:03:16.360 --> 00:03:18.107 we can of course replace database 83 00:03:18.107 --> 00:03:23.107 by cls, since cls and database are the same thing. 84 00:03:24.720 --> 00:03:26.980 Then we're gonna have another class method 85 00:03:26.980 --> 00:03:28.340 that is gonna be remove, 86 00:03:29.790 --> 00:03:33.280 and this is going to take in a finder function 87 00:03:33.280 --> 00:03:34.970 and all it's going to do is it's going to do 88 00:03:34.970 --> 00:03:39.970 cls.content('users') equal user for user 89 00:03:40.750 --> 00:03:43.430 in cls.content('users') 90 00:03:43.430 --> 00:03:45.030 if not finder(user). 91 00:03:47.140 --> 00:03:49.340 Pretty, you know, standard way 92 00:03:49.340 --> 00:03:51.130 of removing something from a list. 93 00:03:51.130 --> 00:03:52.770 Do a list comprehension, 94 00:03:52.770 --> 00:03:56.091 and put all the elements of that list into a new list 95 00:03:56.091 --> 00:04:00.330 for as long as they don't match this finder function. 96 00:04:00.330 --> 00:04:01.640 So the finder function potentially 97 00:04:01.640 --> 00:04:05.600 could be something like lambda x, x username 98 00:04:07.430 --> 00:04:09.120 is not Rolf. 99 00:04:10.970 --> 00:04:12.310 So this could be the finder function, 100 00:04:12.310 --> 00:04:15.230 and then if it doesn't match that, 101 00:04:15.230 --> 00:04:16.850 we will remove it. 102 00:04:19.640 --> 00:04:21.730 Finally, just for completeness, 103 00:04:21.730 --> 00:04:24.310 and because of course we will need this, 104 00:04:24.310 --> 00:04:27.110 let's add a final class method, 105 00:04:27.110 --> 00:04:28.730 which is gonna be a find. 106 00:04:30.130 --> 00:04:32.070 Okay, and this is gonna return 107 00:04:32.070 --> 00:04:35.850 user for user in cls.content('users') 108 00:04:35.850 --> 00:04:37.010 if finder(user). 109 00:04:38.166 --> 00:04:40.816 So this time it's gonna find all the users that match 110 00:04:41.749 --> 00:04:43.370 the lambda function. 111 00:04:43.370 --> 00:04:47.050 So again lambda x, 112 00:04:47.050 --> 00:04:48.340 x username 113 00:04:50.030 --> 00:04:51.040 is Rolf, 114 00:04:51.040 --> 00:04:54.290 this is gonna find all the users in the database 115 00:04:54.290 --> 00:04:56.240 that do match this lambda function, 116 00:04:56.240 --> 00:04:57.840 so it's gonna find all the users 117 00:04:57.840 --> 00:04:59.790 that have username of Rolf. 118 00:05:01.220 --> 00:05:02.053 Okay. 119 00:05:03.520 --> 00:05:06.360 This is our sample database. 120 00:05:06.360 --> 00:05:10.410 So, when we import this class, 121 00:05:10.410 --> 00:05:13.490 it's going to initialise this content variable, 122 00:05:13.490 --> 00:05:16.330 and then we can insert, remove, or find data 123 00:05:16.330 --> 00:05:19.300 within this class variable there. 124 00:05:19.300 --> 00:05:21.620 Notice that if we create multiple objects 125 00:05:21.620 --> 00:05:23.890 of type database, 126 00:05:23.890 --> 00:05:27.070 they're all going to have the same content variable. 127 00:05:27.070 --> 00:05:28.840 This variable is a class variable, 128 00:05:28.840 --> 00:05:31.200 it's not an instance variable, 129 00:05:31.200 --> 00:05:33.080 so it doesn't change per object, 130 00:05:33.080 --> 00:05:35.780 all objects have the same instance variable. 131 00:05:35.780 --> 00:05:40.640 So, going back to our admin.py, 132 00:05:40.640 --> 00:05:44.310 let's say we want to insert this into our database. 133 00:05:44.310 --> 00:05:45.180 What could we do? 134 00:05:46.130 --> 00:05:48.480 Well, we could say something like save, 135 00:05:49.541 --> 00:05:52.890 and then do something like database.insert(self.to_dict()). 136 00:05:55.360 --> 00:05:57.460 And we'll have to import database as well. 137 00:06:01.597 --> 00:06:04.230 Okay, this is the save method 138 00:06:04.230 --> 00:06:06.740 implemented into our admin. 139 00:06:06.740 --> 00:06:10.250 Now if we created another file, let's call it app.py, 140 00:06:12.300 --> 00:06:13.990 and we want it to test this out 141 00:06:13.990 --> 00:06:15.631 as the admin out 142 00:06:15.631 --> 00:06:17.231 by creating an object and then saving it 143 00:06:17.231 --> 00:06:18.600 to the database, and then maybe finding it back, 144 00:06:18.600 --> 00:06:19.510 we could do that. 145 00:06:21.380 --> 00:06:22.690 We can import the admin class, 146 00:06:22.690 --> 00:06:25.080 say that a is an admin, 147 00:06:25.080 --> 00:06:28.470 rolf and, one two three four, 148 00:06:28.470 --> 00:06:31.130 and the access level's gonna be, let's say three. 149 00:06:31.130 --> 00:06:33.460 Then we can do a.save, 150 00:06:33.460 --> 00:06:38.370 and then we can do print(Database.find(lambda x: 151 00:06:38.370 --> 00:06:42.300 x('username') is 'rolf')). 152 00:06:42.300 --> 00:06:44.950 We'll of course have to import database as well. 153 00:06:47.610 --> 00:06:49.780 Okay, let's run app.py. 154 00:06:51.108 --> 00:06:52.870 And as you can see we get back 155 00:06:52.870 --> 00:06:54.870 what you'd expect. 156 00:06:54.870 --> 00:06:58.720 The result of this database.find. 157 00:07:00.530 --> 00:07:02.920 So that's well and good. 158 00:07:02.920 --> 00:07:04.450 What if 159 00:07:04.450 --> 00:07:07.000 we wanted to create another thing 160 00:07:07.000 --> 00:07:09.180 that we could save to the database? 161 00:07:09.180 --> 00:07:11.230 Let's say for example we wanted to create 162 00:07:14.450 --> 00:07:16.460 something like a store.py. 163 00:07:16.460 --> 00:07:18.350 This is gonna be some class store, 164 00:07:18.350 --> 00:07:20.310 and it's gonna have all its stuff, 165 00:07:20.310 --> 00:07:22.560 and then it's gonna have a to_dict, 166 00:07:23.800 --> 00:07:25.360 and that is gonna do something, 167 00:07:25.360 --> 00:07:27.120 and it's gonna have a save, 168 00:07:27.120 --> 00:07:28.450 and that is gonna do 169 00:07:29.673 --> 00:07:31.680 database.insert(self.to_dict()). 170 00:07:38.850 --> 00:07:40.120 As you can see, 171 00:07:41.580 --> 00:07:45.540 we've got some duplication between admin and store. 172 00:07:46.860 --> 00:07:49.640 There is one thing that is fully duplicated, 173 00:07:49.640 --> 00:07:52.500 and one thing that is partially duplicated. 174 00:07:52.500 --> 00:07:54.160 The one thing that's fully duplicated, 175 00:07:54.160 --> 00:07:57.840 and the thing we are going to solve in this video, 176 00:07:57.840 --> 00:07:59.130 is this save method. 177 00:07:59.130 --> 00:08:02.950 The save method is fully duplicated between admin and store. 178 00:08:02.950 --> 00:08:04.240 So as you can see, 179 00:08:04.240 --> 00:08:06.172 you've got this save, 180 00:08:06.172 --> 00:08:07.190 and it's database.insert(self.to_dict()), 181 00:08:08.680 --> 00:08:11.000 and in the store you've got exactly the same thing. 182 00:08:11.000 --> 00:08:13.100 Def save, Database.insert(self.to_dict()). 183 00:08:14.690 --> 00:08:16.330 So in both cases the save method, 184 00:08:16.330 --> 00:08:18.720 all it's doing, is it's taking the current object, 185 00:08:18.720 --> 00:08:20.060 whatever object you're talking about, 186 00:08:20.060 --> 00:08:22.600 be it a store, or an admin, 187 00:08:22.600 --> 00:08:23.870 it's converting it to a dictionary 188 00:08:23.870 --> 00:08:27.238 using its to_dict method, 189 00:08:27.238 --> 00:08:29.220 and then it's putting it into the database. 190 00:08:30.210 --> 00:08:31.840 Same thing, same code. 191 00:08:31.840 --> 00:08:33.480 The thing that's partially duplicated 192 00:08:33.480 --> 00:08:35.930 is this to_dict method. 193 00:08:35.930 --> 00:08:38.360 As you can see, both admin and store 194 00:08:38.360 --> 00:08:41.240 have a to_dict method, 195 00:08:41.240 --> 00:08:44.830 but the implementation of the method, what it does, 196 00:08:44.830 --> 00:08:46.330 is different between both. 197 00:08:46.330 --> 00:08:48.390 The store here will probably return, 198 00:08:48.390 --> 00:08:50.010 I don't know, what items are in it, 199 00:08:50.010 --> 00:08:51.020 or the name of the store, 200 00:08:51.020 --> 00:08:52.700 or the address, et cetera. 201 00:08:52.700 --> 00:08:54.300 The admin is returning the username, 202 00:08:54.300 --> 00:08:56.320 password, and the access level. 203 00:08:56.320 --> 00:08:59.050 But as you can see, they both have a to_dict. 204 00:09:00.390 --> 00:09:01.770 So throughout the next section 205 00:09:01.770 --> 00:09:03.190 we're going to learn these techniques 206 00:09:03.190 --> 00:09:05.910 that are going to allow us to reduce this duplication, 207 00:09:05.910 --> 00:09:08.410 and also improve the stability 208 00:09:08.410 --> 00:09:10.210 and the consistency of our programmes. 209 00:09:11.120 --> 00:09:15.610 In order to solve this save problem, 210 00:09:15.610 --> 00:09:17.630 we could do, of course, one thing. 211 00:09:17.630 --> 00:09:20.530 The first thing we could do is we could cut it from there. 212 00:09:21.832 --> 00:09:25.840 If we wanted to remove it from the admin class, 213 00:09:25.840 --> 00:09:27.590 we could put it into user, right? 214 00:09:30.100 --> 00:09:33.660 Now of course we'll have to import database from here, 215 00:09:33.660 --> 00:09:35.290 in order to make that work, 216 00:09:35.290 --> 00:09:38.150 but the truth is the store does not inherit from user, 217 00:09:38.150 --> 00:09:40.660 so it's not going to have 218 00:09:40.660 --> 00:09:44.360 that to_dict method available. 219 00:09:44.360 --> 00:09:46.230 I'm sorry, that save method available. 220 00:09:46.230 --> 00:09:48.490 So we don't want to do that. 221 00:09:50.710 --> 00:09:55.020 What we want to do is have a third class, 222 00:09:55.020 --> 00:09:56.380 or rather a fourth class, 223 00:09:56.380 --> 00:09:59.500 that contains this save method only. 224 00:09:59.500 --> 00:10:00.850 So I'm gonna go and do just that, 225 00:10:00.850 --> 00:10:02.600 I'm gonna create a new Python file, 226 00:10:03.750 --> 00:10:05.890 I'm gonna call it saveable.py, 227 00:10:05.890 --> 00:10:08.510 and there's a reason why I'm calling it saveable.py, 228 00:10:09.490 --> 00:10:12.210 as I'll explain later on in the course, 229 00:10:13.370 --> 00:10:15.750 and this is gonna be some class saveable, 230 00:10:15.750 --> 00:10:17.290 and all it's going to do 231 00:10:17.290 --> 00:10:19.980 is it's going to implement this method. 232 00:10:23.730 --> 00:10:26.390 So we have to import that. 233 00:10:26.390 --> 00:10:27.800 Notice how it says, 234 00:10:27.800 --> 00:10:30.450 I don't know what a to_dict method is, 235 00:10:30.450 --> 00:10:33.880 because we don't have it defined inside this class, 236 00:10:33.880 --> 00:10:35.160 and we are going to fix this 237 00:10:35.160 --> 00:10:36.390 later on in this section, 238 00:10:36.390 --> 00:10:38.368 but for now, all we need 239 00:10:38.368 --> 00:10:40.580 is to go over to admin. 240 00:10:41.720 --> 00:10:44.460 No longer need to import database here, 241 00:10:44.460 --> 00:10:47.670 but from saveable, import saveable, 242 00:10:48.570 --> 00:10:50.080 and now after user, 243 00:10:54.640 --> 00:10:55.870 include saveable there, 244 00:10:55.870 --> 00:10:58.190 now we no longer need to import database, 245 00:10:58.190 --> 00:11:00.040 we don't need a save method there. 246 00:11:01.330 --> 00:11:04.180 Remember, inheritance is very similar 247 00:11:04.180 --> 00:11:07.070 to copying the code and putting it into this class, 248 00:11:07.070 --> 00:11:11.300 so as soon as you inherit from both user and saveable, 249 00:11:11.300 --> 00:11:13.600 all that Python does is 250 00:11:13.600 --> 00:11:16.580 when you call a method, like self.save, 251 00:11:16.580 --> 00:11:19.670 it goes to user, tries to find that method, 252 00:11:19.670 --> 00:11:21.450 and if it doesn't exist in user, 253 00:11:21.450 --> 00:11:24.240 then it goes to saveable, and it tries to find it there. 254 00:11:24.240 --> 00:11:26.300 So it's gonna find self.save 255 00:11:26.300 --> 00:11:27.660 inside this superclass, 256 00:11:27.660 --> 00:11:28.910 and it's gonna call it, 257 00:11:28.910 --> 00:11:31.930 but it's gonna be able to find self.to_dict 258 00:11:31.930 --> 00:11:33.050 in the admin class. 259 00:11:34.820 --> 00:11:35.653 So again, 260 00:11:37.790 --> 00:11:42.470 self.save will be searched in admin, 261 00:11:43.530 --> 00:11:44.710 then in user, 262 00:11:45.550 --> 00:11:46.670 then in saveable. 263 00:11:48.260 --> 00:11:49.410 Where it will be found. 264 00:11:51.950 --> 00:11:55.130 Self.save uses self.to_dict, 265 00:11:56.670 --> 00:12:01.670 and again self.to_dict will be searched for in admin, 266 00:12:03.050 --> 00:12:04.200 where it will be found. 267 00:12:05.740 --> 00:12:07.300 And that's it. 268 00:12:07.300 --> 00:12:09.300 So there's a tree of, 269 00:12:10.295 --> 00:12:12.320 of sort of method calls that happens 270 00:12:12.320 --> 00:12:14.230 when you have inheritance, 271 00:12:14.230 --> 00:12:16.300 and that's the first class that you inherit from, 272 00:12:16.300 --> 00:12:19.380 the one on the left is, well, of course, 273 00:12:19.380 --> 00:12:22.100 the class you're in is the first one that gets searched, 274 00:12:22.100 --> 00:12:23.430 and then the one at the left, 275 00:12:23.430 --> 00:12:26.320 and then the one next to it, and so forth. 276 00:12:26.320 --> 00:12:29.290 So by using multiple inheritance like this, 277 00:12:29.290 --> 00:12:31.880 you can actually extract functionality 278 00:12:31.880 --> 00:12:34.570 that is shared amongst similar objects, 279 00:12:34.570 --> 00:12:36.900 and similar classes, like user to admin, 280 00:12:36.900 --> 00:12:40.510 but then also between more abstract classes, 281 00:12:40.510 --> 00:12:42.660 like saveable class isn't a user, 282 00:12:42.660 --> 00:12:44.960 it could be anything in your application. 283 00:12:44.960 --> 00:12:47.577 So you can extract specific bits of functionality 284 00:12:47.577 --> 00:12:49.720 into a class like this one. 285 00:12:51.290 --> 00:12:52.780 Of course it's not great, 286 00:12:52.780 --> 00:12:56.060 because you've got this unresolved attribute reference, 287 00:12:56.060 --> 00:12:59.110 so we're going to look at how to fix this very soon. 288 00:13:00.190 --> 00:13:03.010 Again, if you run app.py, 289 00:13:03.010 --> 00:13:04.320 you'll see that it all works fine, 290 00:13:04.320 --> 00:13:06.380 it doesn't complain, it doesn't give you an error, 291 00:13:06.380 --> 00:13:09.370 because exactly what I said earlier is what's happening. 292 00:13:09.370 --> 00:13:11.730 The save method is in saveable, 293 00:13:11.730 --> 00:13:13.320 the to_dict method is in admin, 294 00:13:13.320 --> 00:13:15.070 and they all sort of work together. 295 00:13:16.160 --> 00:13:17.050 That's it for this video, 296 00:13:17.050 --> 00:13:18.590 I wanted to give you a quick intro 297 00:13:18.590 --> 00:13:20.140 to multiple inheritance, 298 00:13:20.140 --> 00:13:21.790 and I'll see you on the next one.