1 00:00:05,310 --> 00:00:09,360 Welcome back everyone in this lecture we're going to finish our discussion of object oriented programming 2 00:00:09,630 --> 00:00:11,450 by talking about special methods. 3 00:00:11,460 --> 00:00:15,400 These are also called Dunder methods because of their use of double underscores. 4 00:00:15,600 --> 00:00:18,520 Let's head over to the text editor and finish off our discussion. 5 00:00:18,750 --> 00:00:19,400 All right. 6 00:00:19,470 --> 00:00:22,790 As we already know there's built in functions in Python. 7 00:00:22,860 --> 00:00:28,320 For example there's the built in length function that can check the length of a list or there's the 8 00:00:28,320 --> 00:00:31,530 built in print function that can print out a list. 9 00:00:31,860 --> 00:00:38,730 But notice that Lenk can check the length of a list but it could also check the length of a string. 10 00:00:38,730 --> 00:00:39,660 So how does it do this. 11 00:00:39,660 --> 00:00:44,730 How does length as a built in method know what to return when you ask the length of the list or the 12 00:00:44,730 --> 00:00:47,480 length of the string or the length of any other object. 13 00:00:47,610 --> 00:00:53,250 And what if we want our own objects to be able to interact with built in functions such as Eliane and 14 00:00:53,250 --> 00:00:54,230 Prince. 15 00:00:54,330 --> 00:00:57,090 In that case we can use what's known as special methods. 16 00:00:57,240 --> 00:01:01,980 So we're going to be creating special methods within our own classes or objects in order to be able 17 00:01:01,980 --> 00:01:07,220 to interact with these built in length and print functionalities. 18 00:01:07,300 --> 00:01:15,170 I'm going to create a class and we're going to call this class book and we're going to have an in it 19 00:01:15,170 --> 00:01:25,980 method and our book is going to take in a tidal an author and the number of pages and then we'll say 20 00:01:26,040 --> 00:01:36,450 self-taught title is equal to title self-taught author is equal to author and then self-taught pages 21 00:01:37,020 --> 00:01:40,180 is equal to the pages. 22 00:01:40,220 --> 00:01:46,190 OK so there's our book save this and then let's see what happens if I create an instance of the book 23 00:01:46,330 --> 00:01:52,870 so I'll say my book is equal to an instance of the book and I need to pass in the attributes title author 24 00:01:52,880 --> 00:01:53,810 pages. 25 00:01:54,050 --> 00:01:57,000 So let's say the title is Jose. 26 00:01:57,060 --> 00:02:03,570 Actually let's say the title is python rocks the author is Jose. 27 00:02:03,700 --> 00:02:08,760 And then we'll say number of pages is there's 250 pages in this book so I'm going to save that. 28 00:02:09,010 --> 00:02:10,540 So I've already created my book. 29 00:02:10,540 --> 00:02:15,950 And then what I'm going to do is I'm going to print out my book. 30 00:02:16,030 --> 00:02:18,970 Now what do you think is actually going to print out when I print my book. 31 00:02:18,970 --> 00:02:19,650 Let's find out. 32 00:02:20,560 --> 00:02:25,160 We'll say pass on my example that pi and Currently when you print out my book right now I'm just going 33 00:02:25,160 --> 00:02:29,640 to say hey you have a book object at this location in memory. 34 00:02:29,870 --> 00:02:35,060 Now you can actually decide what gets displayed when you print out my book and the way you can do that 35 00:02:35,110 --> 00:02:39,290 is through the use of a special method and the special method for that. 36 00:02:39,290 --> 00:02:41,000 There's actually two options. 37 00:02:41,030 --> 00:02:46,340 One is underscore underscore SDR underscore underscore that the one we're going to be using since it's 38 00:02:46,340 --> 00:02:54,120 used more often in the flash documentation is our R and that's basically going to ask for what's the 39 00:02:54,120 --> 00:02:58,800 representation for this object when you want it to be printed out. 40 00:02:58,960 --> 00:03:01,790 So I'll say self. 41 00:03:02,030 --> 00:03:08,150 And then we can return whatever we want to be printed out when you print out a book object. 42 00:03:08,150 --> 00:03:12,320 And notice here I'm saying return not print. 43 00:03:12,460 --> 00:03:16,440 So we're going to say using string literal notation. 44 00:03:16,840 --> 00:03:30,790 The title is self-taught title and the author is self-taught author save that. 45 00:03:31,140 --> 00:03:35,930 OK so what I'm doing here is going to zoom out so you can see the whole picture. 46 00:03:36,020 --> 00:03:41,210 I have the special method underscore underscore R E P R underscore underscore. 47 00:03:41,390 --> 00:03:47,480 And what's going to happen is when you called print on my book print is going to ask hey do you have 48 00:03:47,480 --> 00:03:50,570 a string representation of this book object I can use. 49 00:03:50,570 --> 00:03:56,360 And then this special method is going to say yeah no problem here's the representation and it's going 50 00:03:56,360 --> 00:04:00,800 to pasand the string and the string is using formatting to inject. 51 00:04:00,800 --> 00:04:06,500 Self-taught title the title provided appear and self the author the author provided appear. 52 00:04:06,500 --> 00:04:09,570 So now let's save this and try printing out our book again. 53 00:04:10,700 --> 00:04:15,990 And now when I print out my book object I get title is python rocks author Jose. 54 00:04:16,280 --> 00:04:20,840 So it's totally up to you what you want the actual representation of your object to be when you print 55 00:04:20,840 --> 00:04:21,330 it out. 56 00:04:21,560 --> 00:04:28,160 All you need to do is call this special method underscore underscore our EPR underscore underscore self 57 00:04:28,610 --> 00:04:31,830 and then return whatever string you want to be printed out. 58 00:04:31,940 --> 00:04:34,310 Again we're returning it not printing it here. 59 00:04:34,430 --> 00:04:35,150 You print it out. 60 00:04:35,180 --> 00:04:41,110 When you actually call that function just to make it very clear what's happening here I'll say print 61 00:04:41,110 --> 00:04:41,660 out. 62 00:04:43,100 --> 00:04:45,420 Yep actually returning. 63 00:04:45,770 --> 00:04:52,640 So in saying F are EPR return up and I want to print out my book for with to run this again I'll just 64 00:04:52,640 --> 00:04:57,590 say up and I'll see you up for any book no matter what title is what the author is and what the number 65 00:04:57,590 --> 00:04:58,520 of pages are. 66 00:04:58,850 --> 00:05:03,710 So it probably makes more sense to go for our original thing which was inject self-taught title and 67 00:05:03,800 --> 00:05:05,000 author. 68 00:05:05,120 --> 00:05:10,100 Again all we're doing here is when you print out the book object it's going to look down and say what's 69 00:05:10,100 --> 00:05:12,620 the representation provided. 70 00:05:12,640 --> 00:05:15,570 Now let's try another function that's built in. 71 00:05:15,800 --> 00:05:25,500 Let's see if we can check the length of the book and we'll say Lankes book is equal to length of my 72 00:05:25,500 --> 00:05:26,060 book. 73 00:05:26,370 --> 00:05:30,940 And then at the very end I'll just print out the length of book. 74 00:05:30,970 --> 00:05:33,400 Now let's see what happens if we actually try to run this. 75 00:05:33,760 --> 00:05:37,330 You'll get an error and it says object of type book has no length. 76 00:05:37,420 --> 00:05:38,800 And that actually makes sense. 77 00:05:38,980 --> 00:05:41,960 What should Python use to represent the length of your book. 78 00:05:41,980 --> 00:05:46,810 You haven't really told it so it can't just go in and guess because all it knows is that a book has 79 00:05:46,810 --> 00:05:48,680 a title author and pages. 80 00:05:48,790 --> 00:05:52,700 It's up to you to decide what the representation of the length should be. 81 00:05:53,570 --> 00:05:56,640 In this case we can use a special method to do that. 82 00:05:56,690 --> 00:06:00,950 We'll say the F and the method are related to the length function as underscore underscore. 83 00:06:01,170 --> 00:06:06,400 L e n underscore underscore. 84 00:06:06,620 --> 00:06:11,870 So this special length method is going to allow you to decide what you should return when you call length 85 00:06:12,050 --> 00:06:13,610 on your actual object. 86 00:06:13,730 --> 00:06:19,630 So this case let's go ahead and return the number of pages we'll say self-taught pages. 87 00:06:19,640 --> 00:06:26,120 Save that and now when I run this again it's going to return back the length of the book is 250. 88 00:06:26,150 --> 00:06:28,580 The number of pages I provided here. 89 00:06:28,580 --> 00:06:32,920 Again these are the two special methods we're going to be using throughout the course for working class. 90 00:06:33,110 --> 00:06:35,100 The string representation of an object. 91 00:06:35,130 --> 00:06:38,680 So that's going to be really useful when you want to print things out to get an idea of what they are 92 00:06:39,080 --> 00:06:44,860 and the length of an object that's going to be useful in order to interact with the length function. 93 00:06:45,020 --> 00:06:47,470 And those are really the only two special methods we need to know. 94 00:06:47,720 --> 00:06:52,790 Now you'll notice as you begin to type the F underscore underscore there's actually a ton of special 95 00:06:52,790 --> 00:06:54,290 methods available for you here. 96 00:06:54,440 --> 00:06:57,510 And if you have some sort of Python autocomplete you can get the whole list. 97 00:06:57,710 --> 00:07:00,190 But in our case we really don't need to use all. 98 00:07:00,230 --> 00:07:05,030 In fact mainly the one we're going to be using is this representation for a string to actually prints 99 00:07:05,090 --> 00:07:07,280 out our own objects. 100 00:07:07,310 --> 00:07:11,980 OK the last thing I want to mention is often in other Python courses including my own. 101 00:07:11,990 --> 00:07:19,400 You've probably seen our EPR shown as as TR So this will bouffe work for our use cases but since the 102 00:07:19,620 --> 00:07:25,670 Flast documentation tends to use our EPR for its actual string representation we're going to use that 103 00:07:25,670 --> 00:07:27,170 method throughout the course. 104 00:07:27,170 --> 00:07:30,790 Now if you're wondering what's the actual difference between SDR and our EPR. 105 00:07:30,920 --> 00:07:33,220 You can do a simple Google search for that. 106 00:07:33,380 --> 00:07:37,910 So say difference between underscore underscores TR and underscore underscore our EPR. 107 00:07:37,910 --> 00:07:42,140 This discussion is kind of advanced and it's outside the scope of this particular course. 108 00:07:42,260 --> 00:07:45,710 But you'll notice that there's actually lots and lots of posts because it's actually a really common 109 00:07:45,710 --> 00:07:46,290 question. 110 00:07:46,490 --> 00:07:52,760 But I would say for our level of discussion it's a to advance for us to really be able to see the differences 111 00:07:52,760 --> 00:07:55,940 between them and our cases we can actually use both. 112 00:07:56,030 --> 00:07:58,010 So you can check out the full discussion here. 113 00:07:58,010 --> 00:08:02,170 It's kind of a long discussion and it's also a pretty advanced discussion so just keep that in mind. 114 00:08:02,390 --> 00:08:08,570 If you're wondering what the differences for practical purposes there is none for our use cases. 115 00:08:08,630 --> 00:08:13,430 OK so those are all the special methods we need to know just how to print out our own objects and how 116 00:08:13,430 --> 00:08:15,460 to check the length of our own objects. 117 00:08:15,530 --> 00:08:19,900 Coming up next we're ancestor new skills of object oriented programming through a project. 118 00:08:19,910 --> 00:08:20,480 We'll see you there.