1 00:00:05,190 --> 00:00:09,840 Welcome back, everyone, to this last lecture on our discussion of object oriented programming. 2 00:00:10,080 --> 00:00:14,430 Well, we're going to be discussing what are known as special methods, also sometimes called thunder 3 00:00:14,430 --> 00:00:19,740 methods, because of the fact that they used to double underscores on each side of the method call. 4 00:00:20,130 --> 00:00:21,450 Let's explore this further. 5 00:00:21,450 --> 00:00:22,290 Are some examples. 6 00:00:22,680 --> 00:00:24,930 OK, here I am inside a Python file. 7 00:00:25,380 --> 00:00:30,510 And remember that Python itself has special, built in predefined functions. 8 00:00:30,900 --> 00:00:34,860 And a really simple example of that is the fact that you can print out objects. 9 00:00:35,010 --> 00:00:36,690 So print is a built in function. 10 00:00:37,080 --> 00:00:39,990 There's also the ability to check in the length of an object. 11 00:00:40,170 --> 00:00:42,270 That's also a special built in function. 12 00:00:42,690 --> 00:00:47,070 And you'll notice that Python objects can then interact with these built in functions. 13 00:00:47,490 --> 00:00:53,760 For example, if I create a list, my list one two three, I could say print my list. 14 00:00:54,180 --> 00:00:58,590 I could also do something like print the length of my list. 15 00:00:59,070 --> 00:01:04,620 And if I save those changes and run this file notice, I get back the list and then the fact that there's 16 00:01:04,620 --> 00:01:07,140 an integer amount of items in it, like three. 17 00:01:07,830 --> 00:01:14,340 Now the question arises, if we have created our own user defined classes or python objects, how would 18 00:01:14,340 --> 00:01:18,240 Python know what to actually print when you print a user defined object? 19 00:01:18,660 --> 00:01:21,720 How does Python know what the length of a user defined object is? 20 00:01:22,230 --> 00:01:28,170 Let's show a really simple example by creating our own class for an object we call a book. 21 00:01:28,560 --> 00:01:33,630 Maybe later on, we're developing a bookstore web application, and I've created my own custom object 22 00:01:33,630 --> 00:01:36,810 called Book in order to help organize and define what a book is. 23 00:01:37,440 --> 00:01:45,330 So upon instantiating a book, I call the init method here I pass in self, and let's imagine a book 24 00:01:45,330 --> 00:01:46,290 has a title. 25 00:01:47,650 --> 00:01:51,850 Has an author and also has a number of pages associated with it. 26 00:01:52,240 --> 00:01:58,540 And so these are attributes that when we maybe register a new book into our little Amazon online store, 27 00:01:58,750 --> 00:02:02,980 then I register a title author and pages, maybe later on or else register a price or something like 28 00:02:02,980 --> 00:02:03,250 that. 29 00:02:03,760 --> 00:02:08,020 So we're going to say self-built title is equal to the title passed in self. 30 00:02:08,050 --> 00:02:14,140 The author is equal to the author Peston and then self-taught pages is equal to the pages passed then. 31 00:02:15,100 --> 00:02:19,720 And what I'm going to do is create an instance of my book and let's create a new book. 32 00:02:20,290 --> 00:02:23,410 This book's title could be Python Rocks. 33 00:02:24,100 --> 00:02:26,030 The author could be me, Josie. 34 00:02:26,710 --> 00:02:29,680 And let's say the pages is one hundred and twenty pages long. 35 00:02:30,640 --> 00:02:34,450 Now the question is what happens if I ask Python to print out my book? 36 00:02:34,780 --> 00:02:36,370 We just saw it work with my list. 37 00:02:36,550 --> 00:02:38,200 So let's see what happens if I say prints. 38 00:02:39,180 --> 00:02:46,620 My book, so I say this and that one, I run this, I get back this kind of ugly default print, which 39 00:02:46,620 --> 00:02:49,770 is, hey, inside your main script, which is example that pie. 40 00:02:49,770 --> 00:02:53,790 Here you've created a book object at this location in memory. 41 00:02:54,300 --> 00:03:00,270 This is not super useful for someone who's trying to maybe debug code or display something to a client's 42 00:03:00,270 --> 00:03:02,880 output, or they're just trying to print out my book. 43 00:03:03,330 --> 00:03:08,730 So clearly there's issues here on what Python can actually do for us just by default. 44 00:03:09,120 --> 00:03:14,010 And then if I were to do something like print out the length of my book, we definitely haven't told 45 00:03:14,010 --> 00:03:16,290 Python what the length of a book is. 46 00:03:16,770 --> 00:03:20,790 The python by itself is not going to understand that pages is probably the length. 47 00:03:21,150 --> 00:03:23,940 In fact, it's just going to give us an error here if we try to run this. 48 00:03:24,330 --> 00:03:29,520 It says, Hey, sorry, but the object of type book has no length because we haven't actually added 49 00:03:29,520 --> 00:03:33,450 in the special methods associated with these built in functions. 50 00:03:34,230 --> 00:03:40,170 You can think of special methods or thunder methods as essentially methods that are linked to built 51 00:03:40,200 --> 00:03:42,540 in functions or special calls within Python. 52 00:03:43,080 --> 00:03:49,080 So let's actually now show how we can create our own custom string representation of an object in order 53 00:03:49,080 --> 00:03:53,370 for it to be printed out, as well as our own custom length of an object for it to be printed out. 54 00:03:53,850 --> 00:03:58,530 We're going to start off with the simpler one, which is printing out an object. 55 00:03:58,950 --> 00:04:04,500 So by default, Python can print out an object by reporting it's an object at some location in memory. 56 00:04:04,920 --> 00:04:10,560 Typically, however, what we're going to want to do is have our own prints or string representation. 57 00:04:11,460 --> 00:04:17,399 So this method that's associated with print is underscore, underscore note the two sets of underscores 58 00:04:17,640 --> 00:04:20,130 and you can see there's actually a bunch of different methods here related to that. 59 00:04:20,459 --> 00:04:26,750 And then it's S.T.A.R. underscore underscore and we pass in self colon. 60 00:04:27,330 --> 00:04:33,000 And what this method does is whenever Python needs some sort of string representation of your object, 61 00:04:33,240 --> 00:04:37,650 whether it's using something like print formatting, like if string literals or is trying to print the 62 00:04:37,650 --> 00:04:39,810 object itself, it looks up. 63 00:04:40,050 --> 00:04:46,320 Does this actual object have a string representation defined by the user that it wants me to display? 64 00:04:47,100 --> 00:04:50,370 And in this case, remember, you're always going to return here. 65 00:04:50,610 --> 00:04:53,370 Don't get confused by the fact that it's connected to print. 66 00:04:53,580 --> 00:04:57,150 You do not use prints inside SDR methods. 67 00:04:57,420 --> 00:05:01,650 Instead, you're going to return the string representation you want for your book. 68 00:05:02,010 --> 00:05:04,350 And this is where you actually have a lot of flexibility. 69 00:05:04,650 --> 00:05:11,520 But maybe I just want to say the title and author for my book, so I'm going to return and f string 70 00:05:11,520 --> 00:05:12,090 literal here. 71 00:05:12,090 --> 00:05:15,810 That just says the title like self-taught. 72 00:05:17,350 --> 00:05:19,420 Title Curly Braces. 73 00:05:20,510 --> 00:05:22,550 Written by and then the author. 74 00:05:22,760 --> 00:05:25,220 Self-taught author. 75 00:05:25,850 --> 00:05:30,860 And now it's going to happen is every time Python needs a string representation of the object, it's 76 00:05:30,860 --> 00:05:37,100 going to ask for this method to be executed and return whatever that method happens to return. 77 00:05:37,340 --> 00:05:40,760 And then that's what it's going to actually print out when it prints out my book. 78 00:05:41,180 --> 00:05:47,690 So if you save these changes and now run this file instead of seeing this kind of ugly little thing 79 00:05:47,690 --> 00:05:53,240 that says, Hey, book object in this point of memory, I now get to see the title written by Jose. 80 00:05:53,450 --> 00:05:58,020 And again, you have a lot of flexibility over how you should report this or what you want to actually 81 00:05:58,070 --> 00:05:58,640 print it out. 82 00:05:58,910 --> 00:06:03,140 I just want to point out to fact that you have this string method available for you. 83 00:06:03,290 --> 00:06:08,330 Again, it's underscore, underscore, SDR, underscore, underscore, and this is your ability to return 84 00:06:08,480 --> 00:06:13,040 whatever string representation you want when you're printing out your user defined object. 85 00:06:13,310 --> 00:06:17,960 And we'll actually be using this a lot within Django because oftentimes we're going to be creating these 86 00:06:17,960 --> 00:06:23,600 unique models and databases that when we actually want to display to a user an instance of that particular 87 00:06:23,600 --> 00:06:28,520 data point, we'll probably have to customize what should actually be displayed or prints it out for 88 00:06:28,520 --> 00:06:29,030 the user. 89 00:06:29,750 --> 00:06:30,950 So that's the string method. 90 00:06:31,310 --> 00:06:35,630 The other one that's commonly used is the length method. 91 00:06:35,630 --> 00:06:41,120 So we say underscore, underscore, LPN, underscore, underscore and self. 92 00:06:41,540 --> 00:06:43,370 And this one is a little more strict. 93 00:06:43,700 --> 00:06:46,550 SDR should always be returning a string. 94 00:06:47,030 --> 00:06:49,760 LPN should always be returning an integer. 95 00:06:50,240 --> 00:06:55,100 So just keep that in mind, you can't actually return back a string or something like that has to be 96 00:06:55,100 --> 00:06:59,780 an integer, because that's the way Python thinks about the length of things as complete units. 97 00:07:00,200 --> 00:07:07,490 So what we're going to say here is I'm going to return, and if I wanted to, I could just return the 98 00:07:07,490 --> 00:07:10,850 number ninety nine whenever somebody asked me for the length of a book. 99 00:07:11,060 --> 00:07:15,230 Obviously, it's not super useful, but I do want to show you that it would technically work. 100 00:07:15,800 --> 00:07:21,230 So if I now say, Hey, print the length of my book here, I mean, certain that's a little bit so we 101 00:07:21,230 --> 00:07:21,770 can see that. 102 00:07:22,190 --> 00:07:25,790 Then it's going to say, OK, let me look up the length of this object and it's going to return back. 103 00:07:25,790 --> 00:07:26,360 Ninety nine. 104 00:07:26,930 --> 00:07:29,300 So when you run that, it returns back ninety nine. 105 00:07:29,750 --> 00:07:32,000 Clearly, we don't want this to be the same for every book. 106 00:07:32,240 --> 00:07:36,590 Probably makes more sense for it to return the number of pages, so we'll say. 107 00:07:37,770 --> 00:07:42,990 Self, that pages is going to be returned when you ask for the length of a book. 108 00:07:43,560 --> 00:07:48,870 And so now when I print the length of my book, it says, OK, it's 120, it's up to you. 109 00:07:48,870 --> 00:07:55,350 The context, the user and whoever is working with your Python file to see if just the number 120 actually 110 00:07:55,350 --> 00:07:56,130 makes sense. 111 00:07:56,430 --> 00:07:57,270 But keep that in mind. 112 00:07:57,510 --> 00:08:04,200 You have the ability to define your own string representations of objects and the length of your objects. 113 00:08:04,500 --> 00:08:08,820 And these are really the two special methods that we're going to be using quite a bit when we talk about 114 00:08:08,820 --> 00:08:11,310 creating our own databases and models within Django. 115 00:08:11,610 --> 00:08:14,790 We'll be storing things like books in our own little database. 116 00:08:14,790 --> 00:08:19,710 And then later on, maybe we want to display on the web page some string representation of the object, 117 00:08:20,010 --> 00:08:23,010 in which case it's really useful to actually show something like this. 118 00:08:23,980 --> 00:08:24,290 All right. 119 00:08:24,310 --> 00:08:25,420 That's it for special methods. 120 00:08:25,570 --> 00:08:26,590 I'll see you at the next lecture.