1 1 00:00:00,960 --> 00:00:02,620 Hi guys, and welcome back! 2 2 00:00:02,620 --> 00:00:04,350 In this video, we're going to start talking 3 3 00:00:04,350 --> 00:00:06,780 about class inheritance. 4 4 00:00:06,780 --> 00:00:10,800 Inheritance allows one class to take some methods 5 5 00:00:10,800 --> 00:00:12,950 and properties from another class. 6 6 00:00:12,950 --> 00:00:14,660 Let's have a look. 7 7 00:00:14,660 --> 00:00:17,060 Imagine you've got a class called Device, 8 8 00:00:17,060 --> 00:00:20,150 such as a USB device or something like that. 9 9 00:00:20,150 --> 00:00:23,070 And you want it to represent just anything 10 10 00:00:23,070 --> 00:00:25,560 you might plug into your computer. 11 11 00:00:25,560 --> 00:00:27,720 So I'm going to define my init method, 12 12 00:00:27,720 --> 00:00:31,420 and then we're going to be able to give these devices names 13 13 00:00:31,420 --> 00:00:34,700 and how they are connected, such as by a USB. 14 14 00:00:34,700 --> 00:00:36,763 So I'll say, connected by. 15 15 00:00:37,670 --> 00:00:40,090 Then we'll say self.name=name, 16 16 00:00:40,090 --> 00:00:43,870 self.connected by=connected by, 17 17 00:00:43,870 --> 00:00:45,740 and we're also going to assume 18 18 00:00:45,740 --> 00:00:49,270 that when you create a device object, 19 19 00:00:49,270 --> 00:00:52,240 that is going to represent a connected device, 20 20 00:00:52,240 --> 00:00:54,690 something that is already connected to your computer. 21 21 00:00:54,690 --> 00:00:57,143 So we'll say self.connecte=true. 22 22 00:00:58,150 --> 00:01:00,690 We are going to assume that it's connected 23 23 00:01:00,690 --> 00:01:03,810 and we don't have to pass in a parameter for that. 24 24 00:01:03,810 --> 00:01:05,373 We can just say true here. 25 25 00:01:06,370 --> 00:01:11,370 Then, we will type str and we're gonna say return f 26 26 00:01:12,180 --> 00:01:16,390 device self.name, and we're going to put !r 27 27 00:01:17,730 --> 00:01:19,840 and we're gonna say self.connected_by. 28 28 00:01:19,840 --> 00:01:21,360 The exclamation mark r, 29 29 00:01:21,360 --> 00:01:25,300 calls the repr method of self.name, 30 30 00:01:25,300 --> 00:01:28,260 so that it shows up as having the quotes already. 31 31 00:01:28,260 --> 00:01:29,740 So this is just a little bit nicer 32 32 00:01:29,740 --> 00:01:31,060 than putting the quotes in yourself. 33 33 00:01:31,060 --> 00:01:34,880 Then we're also going to add a disconnect method, 34 34 00:01:34,880 --> 00:01:38,290 which just says self.connected=false 35 35 00:01:38,290 --> 00:01:40,570 and the prints out disconnected. 36 36 00:01:40,570 --> 00:01:43,580 So you have this class device here. 37 37 00:01:43,580 --> 00:01:45,580 This device doesn't do much at the moment, 38 38 00:01:45,580 --> 00:01:49,020 but you could do something like printer=device 39 39 00:01:49,020 --> 00:01:52,550 and call it Printer and say it's connected via USB, 40 40 00:01:52,550 --> 00:01:54,820 and then you can print it out. 41 41 00:01:54,820 --> 00:01:57,740 Finally, you can also disconnect it. 42 42 00:01:57,740 --> 00:01:59,390 All right, let's press play. 43 43 00:01:59,390 --> 00:02:02,740 And you can see that you get Device, Printer, USB, 44 44 00:02:02,740 --> 00:02:05,370 and then disconnected, so very good. 45 45 00:02:05,370 --> 00:02:09,560 Now, let's say you want to add functionality to this class 46 46 00:02:09,560 --> 00:02:13,130 that allows it to print things out. 47 47 00:02:13,130 --> 00:02:15,120 For example, it can print a certain number of pages, 48 48 00:02:15,120 --> 00:02:16,650 or something like that. 49 49 00:02:16,650 --> 00:02:20,930 This class here is not a printer, this is a device. 50 50 00:02:20,930 --> 00:02:23,260 Could be a printer, but it could also be a webcam, 51 51 00:02:23,260 --> 00:02:25,150 or a microphone, it could be anything. 52 52 00:02:25,150 --> 00:02:26,670 So we don't want to add 53 53 00:02:26,670 --> 00:02:30,570 printer specific functionality to this class. 54 54 00:02:30,570 --> 00:02:34,190 You could rename the class and turn it into a Printer class, 55 55 00:02:34,190 --> 00:02:36,740 but you can also create another class 56 56 00:02:36,740 --> 00:02:41,420 that uses all of these methods and has more, 57 57 00:02:41,420 --> 00:02:44,350 and that is where inheritance comes in. 58 58 00:02:44,350 --> 00:02:46,940 So we're going to create a printer class now, 59 59 00:02:46,940 --> 00:02:48,150 and we're going to do something new, 60 60 00:02:48,150 --> 00:02:50,750 which is put a pair of brackets after it, 61 61 00:02:50,750 --> 00:02:53,430 and here we're going to put Device. 62 62 00:02:53,430 --> 00:02:54,480 And what this tells Python 63 63 00:02:54,480 --> 00:02:56,670 is that you're creating a new class called Printer 64 64 00:02:56,670 --> 00:02:59,820 and it inherits from Device, which means that it will 65 65 00:02:59,820 --> 00:03:03,190 essentially copy and paste all of these methods. 66 66 00:03:03,190 --> 00:03:04,680 It won't quite do that, 67 67 00:03:04,680 --> 00:03:07,990 but you will be able to use these methods from this class. 68 68 00:03:07,990 --> 00:03:10,940 So, let's start by defining an init method. 69 69 00:03:10,940 --> 00:03:14,480 Now, this printer object is going to need a name, 70 70 00:03:14,480 --> 00:03:18,640 just like a device, e-connected by just like the device, 71 71 00:03:18,640 --> 00:03:21,850 and it's also going to have a capacity. 72 72 00:03:21,850 --> 00:03:24,280 How many pages left of ink? 73 73 00:03:24,280 --> 00:03:26,380 Then, what we're going to do is, 74 74 00:03:26,380 --> 00:03:28,690 instead of doing self.name=name, 75 75 00:03:28,690 --> 00:03:33,690 self.connect by=connected by, self.connected=true. 76 76 00:03:34,030 --> 00:03:35,210 Instead of doing that, 77 77 00:03:35,210 --> 00:03:37,910 which as you can see is a copy paste of this method here, 78 78 00:03:37,910 --> 00:03:42,820 we can just call the parent class device init method. 79 79 00:03:42,820 --> 00:03:47,053 So, what to do in Python 3, is to do super.init. 80 80 00:03:49,040 --> 00:03:52,440 So, super and then a pair of brackets, 81 81 00:03:52,440 --> 00:03:53,560 and what this does in Python 82 82 00:03:53,560 --> 00:03:56,760 is it gets the super class, or the parent class, 83 83 00:03:56,760 --> 00:03:58,600 which in this case is Device, 84 84 00:03:58,600 --> 00:04:02,300 and it's going to call the init method of that super class.. 85 85 00:04:02,300 --> 00:04:06,260 Of course, the init-method needs a name and a connected by, 86 86 00:04:06,260 --> 00:04:08,850 so we would pass a name and connected by. 87 87 00:04:08,850 --> 00:04:10,260 These are two parameters. 88 88 00:04:10,260 --> 00:04:11,960 We still have capacity. 89 89 00:04:11,960 --> 00:04:15,550 So, we will say self.capacity=capacity 90 90 00:04:16,500 --> 00:04:20,260 and self.remaining pages=capacity. 91 91 00:04:20,260 --> 00:04:24,310 Why am I doing two properties with the same value? 92 92 00:04:24,310 --> 00:04:28,410 The capacity is the maximum amount of pages 93 93 00:04:28,410 --> 00:04:31,820 that a printer could potentially print, if it had full ink. 94 94 00:04:31,820 --> 00:04:34,280 Whereas the remaining pages is how many are left 95 95 00:04:34,280 --> 00:04:36,100 after printing some. 96 96 00:04:36,100 --> 00:04:38,340 So, you can think of this as maximum capacity 97 97 00:04:38,340 --> 00:04:39,933 and this as current capacity. 98 98 00:04:40,900 --> 00:04:43,793 Then, we are going to define an str method, 99 99 00:04:45,040 --> 00:04:47,160 and here we're going to return 100 100 00:04:48,070 --> 00:04:51,240 the super classes str method. 101 101 00:04:51,240 --> 00:04:56,240 And then we're also going to put in self.remaining pages, 102 102 00:04:56,560 --> 00:04:58,690 pages remaining, something like that. 103 103 00:04:58,690 --> 00:05:01,010 Of course, do remember to put in your quotation marks. 104 104 00:05:01,010 --> 00:05:03,600 You will need them and your f-string. 105 105 00:05:03,600 --> 00:05:06,230 So now, we're going to print out this entire thing, 106 106 00:05:06,230 --> 00:05:08,630 Device printer, USB, 107 107 00:05:08,630 --> 00:05:12,283 and then we're gonna say remaining pages 100, or something. 108 108 00:05:13,240 --> 00:05:14,430 And finally, we're going to put 109 109 00:05:14,430 --> 00:05:18,043 in our printer specific method, which is the print method. 110 110 00:05:19,550 --> 00:05:20,470 Here, we're going to be able 111 111 00:05:20,470 --> 00:05:22,530 to print a certain number of pages. 112 112 00:05:22,530 --> 00:05:27,080 So we will say that if not self.connected, 113 113 00:05:27,080 --> 00:05:30,900 then we will print your printer is not connected. 114 114 00:05:30,900 --> 00:05:35,900 Otherwise, we're going to say print printing pages that, 115 115 00:05:35,950 --> 00:05:39,523 and then self.remaining pages-=pages. 116 116 00:05:40,360 --> 00:05:42,880 Of course, we do have to return here, 117 117 00:05:42,880 --> 00:05:45,640 so that we exit the function if we're not connected. 118 118 00:05:45,640 --> 00:05:48,270 Otherwise, we will print this and then that, 119 119 00:05:48,270 --> 00:05:49,990 which is not what we want. 120 120 00:05:49,990 --> 00:05:51,730 So, now we have our two classes; 121 121 00:05:51,730 --> 00:05:53,570 the printer and the device class. 122 122 00:05:53,570 --> 00:05:55,890 You can still use it, Device class on its own if you want. 123 123 00:05:55,890 --> 00:05:58,310 It's in no way related to the Printer class, 124 124 00:05:58,310 --> 00:06:01,460 so the Device class knows nothing about Printer. 125 125 00:06:01,460 --> 00:06:03,650 But the Printer class is related to the Device. 126 126 00:06:03,650 --> 00:06:06,770 It knows about it, it is the child of it. 127 127 00:06:06,770 --> 00:06:08,520 So, you can now create a printer and say 128 128 00:06:08,520 --> 00:06:13,520 Printer, printer, USB, and say 500 pages of capacity. 129 129 00:06:14,300 --> 00:06:18,890 And then, you can do printer.print and print 20 pages out, 130 130 00:06:18,890 --> 00:06:20,520 and then you can print your printer. 131 131 00:06:20,520 --> 00:06:21,580 This is all a bit confusing 132 132 00:06:21,580 --> 00:06:23,410 because I've gone for the name printer, 133 133 00:06:23,410 --> 00:06:25,770 which may not be the best one, but oh, well. 134 134 00:06:25,770 --> 00:06:27,420 So, we forgot an f-string here. 135 135 00:06:27,420 --> 00:06:28,660 Make sure to add that in. 136 136 00:06:28,660 --> 00:06:30,560 Then, we're gonna run it again. 137 137 00:06:30,560 --> 00:06:32,300 So, you get Printing 20 pages, 138 138 00:06:32,300 --> 00:06:36,550 and then your Device, printer, USB, 480 pages remaining. 139 139 00:06:36,550 --> 00:06:39,760 Here, we've used inheritance to build out our methods, 140 140 00:06:39,760 --> 00:06:42,350 like our init methods and our str method 141 141 00:06:42,350 --> 00:06:47,270 by using the parent-class-methods, such as here and here. 142 142 00:06:47,270 --> 00:06:49,170 In addition, this printer 143 143 00:06:50,360 --> 00:06:53,710 can still use the disconnect method 144 144 00:06:53,710 --> 00:06:56,220 that was defined in the Device class. 145 145 00:06:56,220 --> 00:06:59,030 And what happens when you do printer.disconnect 146 146 00:06:59,030 --> 00:07:02,690 is that Python will try to look for the disconnect method 147 147 00:07:02,690 --> 00:07:05,320 inside the Printer class. 148 148 00:07:05,320 --> 00:07:07,470 But upon not finding it, 149 149 00:07:07,470 --> 00:07:10,210 it will try to look in the next parent class, 150 150 00:07:10,210 --> 00:07:13,250 which is Device, and it will find it there. 151 151 00:07:13,250 --> 00:07:16,020 The Device class actually inherits from another class, 152 152 00:07:16,020 --> 00:07:18,480 as well, which is the Default inheritance, 153 153 00:07:18,480 --> 00:07:21,143 and that is Python's object class. 154 154 00:07:21,980 --> 00:07:26,310 So, if you call a method that is not in the Printer 155 155 00:07:26,310 --> 00:07:29,600 or the Device class, Python will then go to the Object class 156 156 00:07:29,600 --> 00:07:31,050 and see if it's there. 157 157 00:07:31,050 --> 00:07:33,660 And if it's not there, then you'll get an error. 158 158 00:07:33,660 --> 00:07:36,310 So those are the three levels of calling 159 159 00:07:36,310 --> 00:07:38,780 that you will see when you use the Printer class. 160 160 00:07:38,780 --> 00:07:40,590 First, the printer will be checked. 161 161 00:07:40,590 --> 00:07:42,210 Then, the Device will be checked. 162 162 00:07:42,210 --> 00:07:44,320 Finally, the object will be checked, 163 163 00:07:44,320 --> 00:07:46,360 and if none of them has the method 164 164 00:07:46,360 --> 00:07:48,060 or the property you're trying to access, 165 165 00:07:48,060 --> 00:07:49,650 then you'll get an error. 166 166 00:07:49,650 --> 00:07:51,000 Let me just run that, 167 167 00:07:51,000 --> 00:07:53,410 and you can see that you get disconnected there. 168 168 00:07:53,410 --> 00:07:56,230 Of course, if you do printer.print, and you try to print 169 169 00:07:56,230 --> 00:07:58,820 30 after you've disconnected, 170 170 00:07:58,820 --> 00:08:01,963 then you will see your printer is not connected. 171 171 00:08:02,950 --> 00:08:04,200 Hopefully, all of this makes sense. 172 172 00:08:04,200 --> 00:08:06,470 Inheritance in Python is much simpler 173 173 00:08:06,470 --> 00:08:08,280 than in other programming languages. 174 174 00:08:08,280 --> 00:08:11,850 All that happens is there is a hierarchy for calling things. 175 175 00:08:11,850 --> 00:08:14,010 When you try to call a method, 176 176 00:08:14,010 --> 00:08:15,920 or access a property of a class, 177 177 00:08:15,920 --> 00:08:19,610 that just goes up the hierarchy, and that's it. 178 178 00:08:19,610 --> 00:08:23,060 You can always use the Super class within a class. 179 179 00:08:23,060 --> 00:08:25,700 So, you can access that class' methods, 180 180 00:08:25,700 --> 00:08:28,140 such as what we're doing here and here. 181 181 00:08:28,140 --> 00:08:29,630 Thanks for joining me in this video. 182 182 00:08:29,630 --> 00:08:30,610 I hope you've enjoyed it, 183 183 00:08:30,610 --> 00:08:32,260 and I'll see you in the next one.