WEBVTT 1 00:00:00.960 --> 00:00:02.620 Hi guys, and welcome back! 2 00:00:02.620 --> 00:00:04.350 In this video, we're going to start talking 3 00:00:04.350 --> 00:00:06.780 about class inheritance. 4 00:00:06.780 --> 00:00:10.800 Inheritance allows one class to take some methods 5 00:00:10.800 --> 00:00:12.950 and properties from another class. 6 00:00:12.950 --> 00:00:14.660 Let's have a look. 7 00:00:14.660 --> 00:00:17.060 Imagine you've got a class called Device, 8 00:00:17.060 --> 00:00:20.150 such as a USB device or something like that. 9 00:00:20.150 --> 00:00:23.070 And you want it to represent just anything 10 00:00:23.070 --> 00:00:25.560 you might plug into your computer. 11 00:00:25.560 --> 00:00:27.720 So I'm going to define my init method, 12 00:00:27.720 --> 00:00:31.420 and then we're going to be able to give these devices names 13 00:00:31.420 --> 00:00:34.700 and how they are connected, such as by a USB. 14 00:00:34.700 --> 00:00:36.763 So I'll say, connected by. 15 00:00:37.670 --> 00:00:40.090 Then we'll say self.name=name, 16 00:00:40.090 --> 00:00:43.870 self.connected by=connected by, 17 00:00:43.870 --> 00:00:45.740 and we're also going to assume 18 00:00:45.740 --> 00:00:49.270 that when you create a device object, 19 00:00:49.270 --> 00:00:52.240 that is going to represent a connected device, 20 00:00:52.240 --> 00:00:54.690 something that is already connected to your computer. 21 00:00:54.690 --> 00:00:57.143 So we'll say self.connecte=true. 22 00:00:58.150 --> 00:01:00.690 We are going to assume that it's connected 23 00:01:00.690 --> 00:01:03.810 and we don't have to pass in a parameter for that. 24 00:01:03.810 --> 00:01:05.373 We can just say true here. 25 00:01:06.370 --> 00:01:11.370 Then, we will type str and we're gonna say return f 26 00:01:12.180 --> 00:01:16.390 device self.name, and we're going to put !r 27 00:01:17.730 --> 00:01:19.840 and we're gonna say self.connected_by. 28 00:01:19.840 --> 00:01:21.360 The exclamation mark r, 29 00:01:21.360 --> 00:01:25.300 calls the repr method of self.name, 30 00:01:25.300 --> 00:01:28.260 so that it shows up as having the quotes already. 31 00:01:28.260 --> 00:01:29.740 So this is just a little bit nicer 32 00:01:29.740 --> 00:01:31.060 than putting the quotes in yourself. 33 00:01:31.060 --> 00:01:34.880 Then we're also going to add a disconnect method, 34 00:01:34.880 --> 00:01:38.290 which just says self.connected=false 35 00:01:38.290 --> 00:01:40.570 and the prints out disconnected. 36 00:01:40.570 --> 00:01:43.580 So you have this class device here. 37 00:01:43.580 --> 00:01:45.580 This device doesn't do much at the moment, 38 00:01:45.580 --> 00:01:49.020 but you could do something like printer=device 39 00:01:49.020 --> 00:01:52.550 and call it Printer and say it's connected via USB, 40 00:01:52.550 --> 00:01:54.820 and then you can print it out. 41 00:01:54.820 --> 00:01:57.740 Finally, you can also disconnect it. 42 00:01:57.740 --> 00:01:59.390 All right, let's press play. 43 00:01:59.390 --> 00:02:02.740 And you can see that you get Device, Printer, USB, 44 00:02:02.740 --> 00:02:05.370 and then disconnected, so very good. 45 00:02:05.370 --> 00:02:09.560 Now, let's say you want to add functionality to this class 46 00:02:09.560 --> 00:02:13.130 that allows it to print things out. 47 00:02:13.130 --> 00:02:15.120 For example, it can print a certain number of pages, 48 00:02:15.120 --> 00:02:16.650 or something like that. 49 00:02:16.650 --> 00:02:20.930 This class here is not a printer, this is a device. 50 00:02:20.930 --> 00:02:23.260 Could be a printer, but it could also be a webcam, 51 00:02:23.260 --> 00:02:25.150 or a microphone, it could be anything. 52 00:02:25.150 --> 00:02:26.670 So we don't want to add 53 00:02:26.670 --> 00:02:30.570 printer specific functionality to this class. 54 00:02:30.570 --> 00:02:34.190 You could rename the class and turn it into a Printer class, 55 00:02:34.190 --> 00:02:36.740 but you can also create another class 56 00:02:36.740 --> 00:02:41.420 that uses all of these methods and has more, 57 00:02:41.420 --> 00:02:44.350 and that is where inheritance comes in. 58 00:02:44.350 --> 00:02:46.940 So we're going to create a printer class now, 59 00:02:46.940 --> 00:02:48.150 and we're going to do something new, 60 00:02:48.150 --> 00:02:50.750 which is put a pair of brackets after it, 61 00:02:50.750 --> 00:02:53.430 and here we're going to put Device. 62 00:02:53.430 --> 00:02:54.480 And what this tells Python 63 00:02:54.480 --> 00:02:56.670 is that you're creating a new class called Printer 64 00:02:56.670 --> 00:02:59.820 and it inherits from Device, which means that it will 65 00:02:59.820 --> 00:03:03.190 essentially copy and paste all of these methods. 66 00:03:03.190 --> 00:03:04.680 It won't quite do that, 67 00:03:04.680 --> 00:03:07.990 but you will be able to use these methods from this class. 68 00:03:07.990 --> 00:03:10.940 So, let's start by defining an init method. 69 00:03:10.940 --> 00:03:14.480 Now, this printer object is going to need a name, 70 00:03:14.480 --> 00:03:18.640 just like a device, e-connected by just like the device, 71 00:03:18.640 --> 00:03:21.850 and it's also going to have a capacity. 72 00:03:21.850 --> 00:03:24.280 How many pages left of ink? 73 00:03:24.280 --> 00:03:26.380 Then, what we're going to do is, 74 00:03:26.380 --> 00:03:28.690 instead of doing self.name=name, 75 00:03:28.690 --> 00:03:33.690 self.connect by=connected by, self.connected=true. 76 00:03:34.030 --> 00:03:35.210 Instead of doing that, 77 00:03:35.210 --> 00:03:37.910 which as you can see is a copy paste of this method here, 78 00:03:37.910 --> 00:03:42.820 we can just call the parent class device init method. 79 00:03:42.820 --> 00:03:47.053 So, what to do in Python 3, is to do super.init. 80 00:03:49.040 --> 00:03:52.440 So, super and then a pair of brackets, 81 00:03:52.440 --> 00:03:53.560 and what this does in Python 82 00:03:53.560 --> 00:03:56.760 is it gets the super class, or the parent class, 83 00:03:56.760 --> 00:03:58.600 which in this case is Device, 84 00:03:58.600 --> 00:04:02.300 and it's going to call the init method of that super class.. 85 00:04:02.300 --> 00:04:06.260 Of course, the init-method needs a name and a connected by, 86 00:04:06.260 --> 00:04:08.850 so we would pass a name and connected by. 87 00:04:08.850 --> 00:04:10.260 These are two parameters. 88 00:04:10.260 --> 00:04:11.960 We still have capacity. 89 00:04:11.960 --> 00:04:15.550 So, we will say self.capacity=capacity 90 00:04:16.500 --> 00:04:20.260 and self.remaining pages=capacity. 91 00:04:20.260 --> 00:04:24.310 Why am I doing two properties with the same value? 92 00:04:24.310 --> 00:04:28.410 The capacity is the maximum amount of pages 93 00:04:28.410 --> 00:04:31.820 that a printer could potentially print, if it had full ink. 94 00:04:31.820 --> 00:04:34.280 Whereas the remaining pages is how many are left 95 00:04:34.280 --> 00:04:36.100 after printing some. 96 00:04:36.100 --> 00:04:38.340 So, you can think of this as maximum capacity 97 00:04:38.340 --> 00:04:39.933 and this as current capacity. 98 00:04:40.900 --> 00:04:43.793 Then, we are going to define an str method, 99 00:04:45.040 --> 00:04:47.160 and here we're going to return 100 00:04:48.070 --> 00:04:51.240 the super classes str method. 101 00:04:51.240 --> 00:04:56.240 And then we're also going to put in self.remaining pages, 102 00:04:56.560 --> 00:04:58.690 pages remaining, something like that. 103 00:04:58.690 --> 00:05:01.010 Of course, do remember to put in your quotation marks. 104 00:05:01.010 --> 00:05:03.600 You will need them and your f-string. 105 00:05:03.600 --> 00:05:06.230 So now, we're going to print out this entire thing, 106 00:05:06.230 --> 00:05:08.630 Device printer, USB, 107 00:05:08.630 --> 00:05:12.283 and then we're gonna say remaining pages 100, or something. 108 00:05:13.240 --> 00:05:14.430 And finally, we're going to put 109 00:05:14.430 --> 00:05:18.043 in our printer specific method, which is the print method. 110 00:05:19.550 --> 00:05:20.470 Here, we're going to be able 111 00:05:20.470 --> 00:05:22.530 to print a certain number of pages. 112 00:05:22.530 --> 00:05:27.080 So we will say that if not self.connected, 113 00:05:27.080 --> 00:05:30.900 then we will print your printer is not connected. 114 00:05:30.900 --> 00:05:35.900 Otherwise, we're going to say print printing pages that, 115 00:05:35.950 --> 00:05:39.523 and then self.remaining pages-=pages. 116 00:05:40.360 --> 00:05:42.880 Of course, we do have to return here, 117 00:05:42.880 --> 00:05:45.640 so that we exit the function if we're not connected. 118 00:05:45.640 --> 00:05:48.270 Otherwise, we will print this and then that, 119 00:05:48.270 --> 00:05:49.990 which is not what we want. 120 00:05:49.990 --> 00:05:51.730 So, now we have our two classes; 121 00:05:51.730 --> 00:05:53.570 the printer and the device class. 122 00:05:53.570 --> 00:05:55.890 You can still use it, Device class on its own if you want. 123 00:05:55.890 --> 00:05:58.310 It's in no way related to the Printer class, 124 00:05:58.310 --> 00:06:01.460 so the Device class knows nothing about Printer. 125 00:06:01.460 --> 00:06:03.650 But the Printer class is related to the Device. 126 00:06:03.650 --> 00:06:06.770 It knows about it, it is the child of it. 127 00:06:06.770 --> 00:06:08.520 So, you can now create a printer and say 128 00:06:08.520 --> 00:06:13.520 Printer, printer, USB, and say 500 pages of capacity. 129 00:06:14.300 --> 00:06:18.890 And then, you can do printer.print and print 20 pages out, 130 00:06:18.890 --> 00:06:20.520 and then you can print your printer. 131 00:06:20.520 --> 00:06:21.580 This is all a bit confusing 132 00:06:21.580 --> 00:06:23.410 because I've gone for the name printer, 133 00:06:23.410 --> 00:06:25.770 which may not be the best one, but oh, well. 134 00:06:25.770 --> 00:06:27.420 So, we forgot an f-string here. 135 00:06:27.420 --> 00:06:28.660 Make sure to add that in. 136 00:06:28.660 --> 00:06:30.560 Then, we're gonna run it again. 137 00:06:30.560 --> 00:06:32.300 So, you get Printing 20 pages, 138 00:06:32.300 --> 00:06:36.550 and then your Device, printer, USB, 480 pages remaining. 139 00:06:36.550 --> 00:06:39.760 Here, we've used inheritance to build out our methods, 140 00:06:39.760 --> 00:06:42.350 like our init methods and our str method 141 00:06:42.350 --> 00:06:47.270 by using the parent-class-methods, such as here and here. 142 00:06:47.270 --> 00:06:49.170 In addition, this printer 143 00:06:50.360 --> 00:06:53.710 can still use the disconnect method 144 00:06:53.710 --> 00:06:56.220 that was defined in the Device class. 145 00:06:56.220 --> 00:06:59.030 And what happens when you do printer.disconnect 146 00:06:59.030 --> 00:07:02.690 is that Python will try to look for the disconnect method 147 00:07:02.690 --> 00:07:05.320 inside the Printer class. 148 00:07:05.320 --> 00:07:07.470 But upon not finding it, 149 00:07:07.470 --> 00:07:10.210 it will try to look in the next parent class, 150 00:07:10.210 --> 00:07:13.250 which is Device, and it will find it there. 151 00:07:13.250 --> 00:07:16.020 The Device class actually inherits from another class, 152 00:07:16.020 --> 00:07:18.480 as well, which is the Default inheritance, 153 00:07:18.480 --> 00:07:21.143 and that is Python's object class. 154 00:07:21.980 --> 00:07:26.310 So, if you call a method that is not in the Printer 155 00:07:26.310 --> 00:07:29.600 or the Device class, Python will then go to the Object class 156 00:07:29.600 --> 00:07:31.050 and see if it's there. 157 00:07:31.050 --> 00:07:33.660 And if it's not there, then you'll get an error. 158 00:07:33.660 --> 00:07:36.310 So those are the three levels of calling 159 00:07:36.310 --> 00:07:38.780 that you will see when you use the Printer class. 160 00:07:38.780 --> 00:07:40.590 First, the printer will be checked. 161 00:07:40.590 --> 00:07:42.210 Then, the Device will be checked. 162 00:07:42.210 --> 00:07:44.320 Finally, the object will be checked, 163 00:07:44.320 --> 00:07:46.360 and if none of them has the method 164 00:07:46.360 --> 00:07:48.060 or the property you're trying to access, 165 00:07:48.060 --> 00:07:49.650 then you'll get an error. 166 00:07:49.650 --> 00:07:51.000 Let me just run that, 167 00:07:51.000 --> 00:07:53.410 and you can see that you get disconnected there. 168 00:07:53.410 --> 00:07:56.230 Of course, if you do printer.print, and you try to print 169 00:07:56.230 --> 00:07:58.820 30 after you've disconnected, 170 00:07:58.820 --> 00:08:01.963 then you will see your printer is not connected. 171 00:08:02.950 --> 00:08:04.200 Hopefully, all of this makes sense. 172 00:08:04.200 --> 00:08:06.470 Inheritance in Python is much simpler 173 00:08:06.470 --> 00:08:08.280 than in other programming languages. 174 00:08:08.280 --> 00:08:11.850 All that happens is there is a hierarchy for calling things. 175 00:08:11.850 --> 00:08:14.010 When you try to call a method, 176 00:08:14.010 --> 00:08:15.920 or access a property of a class, 177 00:08:15.920 --> 00:08:19.610 that just goes up the hierarchy, and that's it. 178 00:08:19.610 --> 00:08:23.060 You can always use the Super class within a class. 179 00:08:23.060 --> 00:08:25.700 So, you can access that class' methods, 180 00:08:25.700 --> 00:08:28.140 such as what we're doing here and here. 181 00:08:28.140 --> 00:08:29.630 Thanks for joining me in this video. 182 00:08:29.630 --> 00:08:30.610 I hope you've enjoyed it, 183 00:08:30.610 --> 00:08:32.260 and I'll see you in the next one.