1 1 00:00:01,190 --> 00:00:02,060 line:15% Hi, and welcome back. 2 2 00:00:02,060 --> 00:00:04,310 line:15% Another video in inheritance, and in this one 3 3 00:00:04,310 --> 00:00:06,360 line:15% we're going to be moving methods 4 4 00:00:06,360 --> 00:00:11,230 from our models into the model parent class. 5 5 00:00:11,230 --> 00:00:14,660 Everything that's duplicated between those two item 6 6 00:00:14,660 --> 00:00:17,650 and alert models can usually move to a parent class 7 7 00:00:17,650 --> 00:00:22,400 as long as we make it generic enough that it can be moved. 8 8 00:00:22,400 --> 00:00:25,080 In this video we're going to learn how to do that. 9 9 00:00:25,080 --> 00:00:27,300 So here's a method that is duplicated 10 10 00:00:27,300 --> 00:00:32,300 between the two models, and that is this all method. 11 11 00:00:32,770 --> 00:00:36,320 So, so far we have an all method inside the item model, 12 12 00:00:36,320 --> 00:00:39,720 and we also have an all method inside the alert model, 13 13 00:00:39,720 --> 00:00:41,640 and it does basically the same thing. 14 14 00:00:41,640 --> 00:00:43,540 It gets stuff from a database 15 15 00:00:43,540 --> 00:00:47,200 from the appropriate collection, and then it returns a list 16 16 00:00:47,200 --> 00:00:51,520 of objects of the type that is being called. 17 17 00:00:51,520 --> 00:00:53,290 So if you call it with the item class, 18 18 00:00:53,290 --> 00:00:54,793 it'll return item models. 19 19 00:00:56,069 --> 00:00:57,560 If you call it with the alert class, 20 20 00:00:57,560 --> 00:01:00,260 it will return alert models. 21 21 00:01:00,260 --> 00:01:02,930 We can move this to the parent class, 22 22 00:01:02,930 --> 00:01:07,930 and very important, at this point you wanna be using cls. 23 23 00:01:08,250 --> 00:01:13,250 If you do alert, and you move this to the model, of course, 24 24 00:01:15,093 --> 00:01:18,527 both item and alert are gonna return alert objects. 25 25 00:01:18,527 --> 00:01:20,933 I'm gonna come back to that in just a moment. 26 26 00:01:22,100 --> 00:01:26,310 First of all, let's copy that and go over to our model, 27 27 00:01:26,310 --> 00:01:29,823 and I'm going to put it down here. 28 28 00:01:31,870 --> 00:01:33,330 Couple of things that this is gonna do 29 29 00:01:33,330 --> 00:01:38,150 is it is going to use database and list, 30 30 00:01:38,150 --> 00:01:43,150 so we do have to do from typing import list, 31 31 00:01:43,280 --> 00:01:48,220 and from common database, import database. 32 32 00:01:48,220 --> 00:01:51,050 So we do have to add those imports in there, 33 33 00:01:51,050 --> 00:01:54,060 and then we have a small problem. 34 34 00:01:54,060 --> 00:01:58,020 We have the problem where this method 35 35 00:01:58,020 --> 00:02:01,460 is accessing the alerts collection, 36 36 00:02:01,460 --> 00:02:03,990 but we don't want it to access the alerts collection 37 37 00:02:03,990 --> 00:02:06,070 because ideally what we wanna do 38 38 00:02:06,070 --> 00:02:11,070 is if we call this method like so, item.all, 39 39 00:02:13,620 --> 00:02:18,370 then obviously we wanna get back a list of items. 40 40 00:02:18,370 --> 00:02:23,323 If we do alert.all, we wanna get back a list of alerts. 41 41 00:02:24,960 --> 00:02:26,893 So how are we gonna do that? 42 42 00:02:26,893 --> 00:02:29,790 Well, this is a class method. 43 43 00:02:29,790 --> 00:02:33,120 That means that when we call this method, 44 44 00:02:33,120 --> 00:02:37,380 we have access to things that are defined in the class. 45 45 00:02:37,380 --> 00:02:38,960 We don't have access to things 46 46 00:02:38,960 --> 00:02:42,570 that are defined in any particular object. 47 47 00:02:42,570 --> 00:02:47,173 So, for example, when we create an item object, 48 48 00:02:48,050 --> 00:02:51,570 we define the url, tag name, and query of that object, 49 49 00:02:51,570 --> 00:02:54,100 and then once we have the object, 50 50 00:02:54,100 --> 00:02:58,210 we can access the self.url property, self.tag_name property, 51 51 00:02:58,210 --> 00:03:02,513 and so on, including self.collection. 52 52 00:03:03,500 --> 00:03:07,270 So, instead of hard coding items here, 53 53 00:03:07,270 --> 00:03:10,740 we could do potentially something like self.collection 54 54 00:03:11,730 --> 00:03:16,420 if this method was an instance method, but it's not. 55 55 00:03:16,420 --> 00:03:19,170 It's a class method so we can't do this. 56 56 00:03:19,170 --> 00:03:21,400 What alternative do we have? 57 57 00:03:21,400 --> 00:03:26,400 Since collection is always the same in all item objects, 58 58 00:03:26,630 --> 00:03:31,110 essentially it's always items, we can move this collection 59 59 00:03:31,110 --> 00:03:33,320 so that it stops being an instance property, 60 60 00:03:33,320 --> 00:03:36,880 stops being self.something, and instead 61 61 00:03:36,880 --> 00:03:39,500 is defined at the class level. 62 62 00:03:39,500 --> 00:03:43,700 That way, all items will have the same property, 63 63 00:03:43,700 --> 00:03:47,030 and we no longer need to have self 64 64 00:03:47,030 --> 00:03:48,770 in order to use the property. 65 65 00:03:48,770 --> 00:03:49,623 As you can see, we can't use it here 66 66 00:03:49,623 --> 00:03:51,310 because we don't have self. 67 67 00:03:51,310 --> 00:03:52,840 We have a class method. 68 68 00:03:52,840 --> 00:03:55,800 We have cls which is the class. 69 69 00:03:55,800 --> 00:03:58,890 That means, if we put the collection here 70 70 00:03:59,830 --> 00:04:04,830 and we take it away from here, now we can do cls.collection. 71 71 00:04:09,540 --> 00:04:11,200 Similarly, in the alert model, 72 72 00:04:11,200 --> 00:04:15,470 we can put collection here as alerts. 73 73 00:04:15,470 --> 00:04:17,630 We can remove it from there, 74 74 00:04:17,630 --> 00:04:20,773 and here we can use cls.collection. 75 75 00:04:22,610 --> 00:04:26,250 Now, by doing this, by putting the collection 76 76 00:04:26,250 --> 00:04:29,380 at the top level of the class level, that means that we 77 77 00:04:29,380 --> 00:04:34,380 no longer have this dependent on the specific objects, 78 78 00:04:34,570 --> 00:04:37,330 so we could do the same here, cls.collection. 79 79 00:04:37,330 --> 00:04:42,330 Instead of doing cls(**alert), what we're gonna do is simply 80 80 00:04:44,350 --> 00:04:46,663 take this and make it a bit more generic. 81 81 00:04:54,830 --> 00:04:58,300 Now there's a small problem which is that PyCharm will tell 82 82 00:04:58,300 --> 00:05:03,300 you that cls.collection is not defined, and it's correct. 83 83 00:05:03,520 --> 00:05:06,920 The model class does not have a collection property 84 84 00:05:06,920 --> 00:05:08,930 defined in it, so this warning, 85 85 00:05:08,930 --> 00:05:11,000 it tells you that it's not there. 86 86 00:05:11,000 --> 00:05:15,860 However, the subclasses do have a collection property. 87 87 00:05:15,860 --> 00:05:17,943 That's the power of cls. 88 88 00:05:18,860 --> 00:05:21,280 The subclass can have different properties 89 89 00:05:21,280 --> 00:05:24,320 from the parent class, and using cls means 90 90 00:05:24,320 --> 00:05:28,660 that if you call item.all, really what you're going 91 91 00:05:28,660 --> 00:05:33,480 is you're going to item.all, and that is going 92 92 00:05:33,480 --> 00:05:37,083 to use item.collection which is defined. 93 93 00:05:38,150 --> 00:05:43,150 We are never going to call model.all just because 94 94 00:05:43,870 --> 00:05:46,610 it doesn't make sense in our application to do that. 95 95 00:05:46,610 --> 00:05:48,430 However, you're still going to get this warning, 96 96 00:05:48,430 --> 00:05:51,910 and you can call model.all if you want, 97 97 00:05:51,910 --> 00:05:55,570 and if you call model.all, you're gonna have a problem 98 98 00:05:55,570 --> 00:05:57,490 because there is no collection property, 99 99 00:05:57,490 --> 00:05:59,710 so you're going to have a problem there, 100 100 00:05:59,710 --> 00:06:04,710 and also you cannot create model objects. 101 101 00:06:05,170 --> 00:06:07,840 There are two problems with the model-object creation. 102 102 00:06:07,840 --> 00:06:11,070 The first one is that you don't have a constructor 103 103 00:06:11,070 --> 00:06:14,680 in your model, so even if you found some data 104 104 00:06:14,680 --> 00:06:16,370 from the database, you're not gonna be able 105 105 00:06:16,370 --> 00:06:19,260 to create the model because there is no constructor 106 106 00:06:19,260 --> 00:06:22,120 that can accept the data coming from the database. 107 107 00:06:22,120 --> 00:06:23,603 There's no init method. 108 108 00:06:24,800 --> 00:06:27,570 Second problem, even if there was an init method, 109 109 00:06:27,570 --> 00:06:29,750 you still wouldn't be able to create a model object 110 110 00:06:29,750 --> 00:06:31,930 because you have this abstract method here, 111 111 00:06:31,930 --> 00:06:35,730 and as you know from the last video, you can't create 112 112 00:06:35,730 --> 00:06:39,300 objects of classes that have abstract methods. 113 113 00:06:39,300 --> 00:06:43,353 So these are a few issues with this help 114 114 00:06:44,380 --> 00:06:47,810 that you're getting from PyCharm and abstract classes, 115 115 00:06:47,810 --> 00:06:49,530 and I haven't really found a good, 116 116 00:06:49,530 --> 00:06:52,690 simple-enough way of avoiding these problems. 117 117 00:06:52,690 --> 00:06:55,880 So what you can do is you can define a collection 118 118 00:06:57,420 --> 00:07:00,963 variable here, and that get rids of this error, 119 119 00:07:01,920 --> 00:07:05,670 and you can define a dunder init method if you want 120 120 00:07:05,670 --> 00:07:10,580 with unlimited argument and keyword arguments, and make it 121 121 00:07:10,580 --> 00:07:14,660 do nothing, and that gets rid of this other error. 122 122 00:07:14,660 --> 00:07:17,700 This I think is fine to do. 123 123 00:07:17,700 --> 00:07:21,260 It's not perfect because this code here 124 124 00:07:21,260 --> 00:07:24,430 is only there to get rid of the warnings. 125 125 00:07:24,430 --> 00:07:26,330 It's not there to do anything, 126 126 00:07:26,330 --> 00:07:28,250 and I'm not a big fan of that. 127 127 00:07:28,250 --> 00:07:29,860 However, as I said, I haven't found 128 128 00:07:29,860 --> 00:07:32,850 another easy way of fixing the issue. 129 129 00:07:32,850 --> 00:07:36,380 There is a way of defining an abstract property 130 130 00:07:36,380 --> 00:07:40,590 which means that the subclasses would need to define 131 131 00:07:40,590 --> 00:07:45,590 collection, but this base class does not define collection. 132 132 00:07:45,740 --> 00:07:47,880 There is a way to do that but it's a bit more complicated. 133 133 00:07:47,880 --> 00:07:49,230 We may look at it later on. 134 134 00:07:50,740 --> 00:07:54,000 And there is no real way around the init method 135 135 00:07:54,000 --> 00:07:56,030 and this warning that you were getting here. 136 136 00:07:56,030 --> 00:07:58,030 So for now we're going to leave it as this. 137 137 00:07:58,030 --> 00:08:01,610 Later on, we're going to look at an improvement on it. 138 138 00:08:01,610 --> 00:08:03,950 Just remember that you're never going to be able 139 139 00:08:03,950 --> 00:08:07,040 to create objects of class model, 140 140 00:08:07,040 --> 00:08:09,500 so this code here is sort of superfluous. 141 141 00:08:09,500 --> 00:08:11,640 It's not really necessary other than 142 142 00:08:13,895 --> 00:08:16,160 to avoid the warnings, and the reason you wanna avoid 143 143 00:08:16,160 --> 00:08:19,590 the warnings, by the way, is because if you have a code 144 144 00:08:19,590 --> 00:08:21,760 that's full of warnings, you're probably not gonna 145 145 00:08:21,760 --> 00:08:24,000 pay attention to them just because you're gonna be like, 146 146 00:08:24,000 --> 00:08:26,370 oh yeah, there's another warning, whatever, 147 147 00:08:26,370 --> 00:08:28,790 and not paying attention to warnings can be a little bit 148 148 00:08:28,790 --> 00:08:31,460 dangerous 'cause often the warnings are helpful, 149 149 00:08:31,460 --> 00:08:33,260 so that's why I'm adding this here 150 150 00:08:33,260 --> 00:08:34,730 even though I'm not a big fan of it. 151 151 00:08:34,730 --> 00:08:37,070 I'm adding it there to avoid the warnings 152 152 00:08:37,070 --> 00:08:40,020 that I know I can get rid of without a problem 153 153 00:08:40,020 --> 00:08:42,210 so that if I do get any other warnings, 154 154 00:08:42,210 --> 00:08:43,780 I will pay attention to them because they will 155 155 00:08:43,780 --> 00:08:45,840 be the only warnings that are there. 156 156 00:08:45,840 --> 00:08:48,060 If I allow this to build up warnings, 157 157 00:08:48,060 --> 00:08:50,870 I'm eventually just not gonna care about them. 158 158 00:08:50,870 --> 00:08:52,740 That's known as the broken windows problem. 159 159 00:08:52,740 --> 00:08:54,180 If you're not familiar with it, 160 160 00:08:54,180 --> 00:08:56,513 something you wanna avoid in your code. 161 161 00:08:57,460 --> 00:08:59,080 All right, that's everything for this video. 162 162 00:08:59,080 --> 00:09:01,260 I just wanted to move the all method 163 163 00:09:01,260 --> 00:09:04,410 to teach you how you can think of this move 164 164 00:09:04,410 --> 00:09:07,350 from individual models into a base class, 165 165 00:09:07,350 --> 00:09:09,670 and of course we can go down here 166 166 00:09:09,670 --> 00:09:13,350 to our models and delete the all method. 167 167 00:09:13,350 --> 00:09:17,200 Now, what this is doing is, when you delete it, 168 168 00:09:17,200 --> 00:09:18,790 it's just doing exactly the same thing. 169 169 00:09:18,790 --> 00:09:21,320 Just think of it as a copy and paste 170 170 00:09:21,320 --> 00:09:23,370 because we're using cls. 171 171 00:09:23,370 --> 00:09:24,400 Because we're using cls, 172 172 00:09:24,400 --> 00:09:25,620 it's essentially a copy and paste. 173 173 00:09:25,620 --> 00:09:27,620 When you call it in the alert model, 174 174 00:09:27,620 --> 00:09:29,500 you're using alert.collection, 175 175 00:09:29,500 --> 00:09:31,810 and you're using the alert constructor. 176 176 00:09:31,810 --> 00:09:33,220 If you call it from the item model, 177 177 00:09:33,220 --> 00:09:36,100 you're using item.collection and the item constructor 178 178 00:09:36,100 --> 00:09:39,453 so it'll work in exactly the same way as it does here. 179 179 00:09:40,420 --> 00:09:43,410 Now, something else that you wanna 180 180 00:09:43,410 --> 00:09:45,790 think about is this init method. 181 181 00:09:45,790 --> 00:09:48,580 It says the call to the superclass is missed, 182 182 00:09:48,580 --> 00:09:53,270 so if you want to call the superclass, just do this. 183 183 00:09:53,270 --> 00:09:54,370 What does this do? 184 184 00:09:54,370 --> 00:09:57,760 Well, it goes over to the model class 185 185 00:09:57,760 --> 00:09:59,990 and calls the init method. 186 186 00:09:59,990 --> 00:10:02,780 This is something that Python expects you to do 187 187 00:10:02,780 --> 00:10:07,690 just so you don't miss anything in the chain of inheritance, 188 188 00:10:07,690 --> 00:10:09,104 and in this case, doing this does nothing 189 189 00:10:09,104 --> 00:10:10,533 for us but you still have to do it. 190 190 00:10:10,533 --> 00:10:13,360 This is one more reason why I'm not a big fan 191 191 00:10:13,360 --> 00:10:16,010 of that solution but for now, again, we're gonna put 192 192 00:10:16,010 --> 00:10:19,060 it there just because we do have a parent class, 193 193 00:10:19,060 --> 00:10:21,750 and we do want to call the init method 194 194 00:10:21,750 --> 00:10:24,710 in case we do add something here later on. 195 195 00:10:24,710 --> 00:10:25,890 We don't miss that later 196 196 00:10:25,890 --> 00:10:27,640 so we're gonna put it there for now. 197 197 00:10:27,640 --> 00:10:30,433 Later on, if we make changes, we can change that. 198 198 00:10:31,370 --> 00:10:33,030 Do note that you can put things 199 199 00:10:33,030 --> 00:10:34,500 in the init method if you want it. 200 200 00:10:34,500 --> 00:10:36,040 There's something that you want to share 201 201 00:10:36,040 --> 00:10:38,880 across all objects, all your models, then you can put 202 202 00:10:38,880 --> 00:10:41,130 it there and that's totally fine thing to do. 203 203 00:10:42,010 --> 00:10:44,090 All right, thanks for joining me in this video. 204 204 00:10:44,090 --> 00:10:45,540 I'll see you on the next one.