Subclassing Built-ins

I think we're going to leave our role playing game characters behind. 0:00 They were really handy for exploring brand new classes but a lot of the power in 0:02 object oriented design comes from taking advantage of the classes and 0:06 objects provided by your language. 0:09 Python gives us tons of objects that we can use as the basis of our 0:11 own custom ones. 0:14 We've been really heavy into magic methods in this stage. 0:15 That's not gonna change now. 0:18 When you're dealing with custom versions of built-ins, 0:19 there are two magic methods that you'll want to know about above all others. 0:21 Init and new. 0:25 You've already seen init, 0:27 we use it when we want to customize how a new instance of a class is created. 0:28 New works very similarly but 0:32 has some special magic that we're not quite ready for it just yet. 0:34 The major difference between the two though is when you use them. 0:37 If you're customizing a mutable data type like list, you override init. 0:39 If you're customizing an immutable type, you use new. 0:44 Let's go to work spaces to try these new methods out. 0:47 All right, so let's start with an immutable class first. 0:52 How about we make a version of string, of STR that is always reversed. 0:54 I know, I know, you just thought of a million uses for such a thing. 1:00 How it's not already built into the library, 1:03 I don't know, we'll just have to submit I guess. 1:05 [LAUGH] First things first, I have a file named reversedstr.py, 1:07 and we need to go ahead and create our class. 1:12 So I'm gonna call it ReversedStr, because that's what it is, right? 1:14 I don't wanna confuse anyone. 1:19 I don't want them thinking this is a normal string. 1:21 And instead of going straight to the colon, 1:23 I'm going to extend it with str or I'm going to make it extend str. 1:26 So then I'm just going to say pass. 1:31 And if I use the class right now, it's a string. 1:34 Now let's just go ahead and do it. 1:36 So Python from reversesdtr import ReversedStr. 1:42 rs = ReversedStr('hello'), 1:48 and rs is hello, right? 1:52 And len(rs) I get 5. 1:56 So yep, that's a string. 1:59 Okay, so we want to change what happens when the string is created. 2:02 Since strings are immutable, 2:06 the only place we should change them is at creation time. 2:08 We do that in new for immutable objects. 2:13 So we're gonna say def __new__( And 2:16 this is going to take whatever args and kwargs come in, just in case. 2:19 And then, we're gonna say, self =, and self here is just a variable name, 2:25 we don't have to call it self, but self = str._ 2:28 _new_ _ (args, kwargs). 2:33 And then we're gonna say self = self reversed, ::-1. 2:38 And then we're going to return self. 2:43 Unlike init, new does a return. 2:46 Now did you notice that new doesn't take self? 2:49 That's because new is a special kind of method known as a class method. 2:51 We'll talk more about them in a later video, but for now, 2:55 just now that they're methods that operate on a class, and not on an instance. 2:57 And inside of here, why aren't we using super? 3:02 Well, with immutable types like this, 3:05 it can actually be unsafe to use super inside of new. 3:06 It's safer and easier to explicitly use the new method of the class that 3:10 you're extending directly, which is why we did the str.__new__. 3:14 And then of course, we're gonna go ahead and reverse our string, and return it. 3:19 So does this work? 3:23 Let's find out. 3:24 Python, let's just import reversedstr, that's a little bit easier. 3:28 And then let's say rs = reversedstr.ReversedStr( And 3:33 I'm gonna say hello again. 3:38 And now let's look at rs. 3:41 I got olleh, ole. 3:44 It does, all right. 3:46 Well, great. 3:48 That was easier than I expected. 3:50 What if we wanted to make a sub-class of list, though? 3:52 Now, since list is mutable, we'll have to override init instead of new. 3:56 We should come up with something to make first, though. 4:01 Let's borrow something from the data science world, at least partially, very, 4:04 very partially. 4:08 And build a list that can be prefilled with a certain number of numbers. 4:09 So I'm gonna make a New File for this, I'm gonna call it filledlist.py 4:14 And then inside of here I'm actually going to import a library called copy, 4:20 and then I'll make my class, FilledList. 4:25 And this is going to extend list, and I need to override init. 4:28 And I know init takes self. 4:33 All right, so I'm gonna take args and 4:35 kwargs because If anybody passes me an iterable or 4:38 something like that that they want made into a list, I want to accept that. 4:43 But basically I'm going to ignore it. 4:47 I want two other arguments. 4:49 I want a count argument, and 4:51 a value argument, so how many do you want, and what do you want, right? 4:53 So I want five zeros. 4:57 So, first things first, let's go ahead and super, and then init. 5:00 And we're going to ignore everything that was passed in. 5:07 I don't care what they gave me. 5:10 All I want is an empty list when I get to the end of this, okay? 5:12 So at this point, 5:16 right now, I have a brand new, empty list instance, stored inside of self. 5:17 That I can just do whatever I want with. 5:22 So now, I use a for loop that'll go through the count and we'll do something. 5:24 Now this underscore here ignores 5:32 the number that would come out of range, right? 5:35 So, range would normally be zero, one, two, three, four, five, 5:37 whatever I don't care about that, so I'm going to ignore it. 5:41 And that's what underscore let's me do. 5:44 I could just use like x and then never use x there. 5:46 But underscore is cleaner and implies that you don't care about this value. 5:49 All right, so then I'm going to append a copy 5:53 of whatever it is that they gave me, whatever the value is that they gave me. 5:57 So I'm gonna use copy.copy. 6:01 And then value. 6:03 So I will append five copies or ten copies or whatever. 6:05 Now why am I using copy.copy? 6:08 Well, if they send in something mutable, like say, another list, 6:10 each member in that filled list or each member in our filled list 6:14 that was a copy of that list would be the same member if we weren't using copy.copy. 6:19 If you changed one of them it would change all of the others too 6:24 because we're just putting in references to that list. 6:26 Okay, let's try this one out. 6:30 With copy.copy though we're not getting references. 6:33 We're just getting brand new versions of those lists. 6:35 So let's say import filledlist and we'll say fl = filledlist.FilledList. 6:40 And I want this to have 9 copies of the number 2. 6:48 So let's do len(fl), it's 9, cool. 6:51 Now let's do fl and there's a whole bunch of 2s, nice. 6:56 Now let's make sure that the copy part is working, right? 7:00 So let's do fl2 = filledlist.FilledList. 7:04 And I want two copies of the list 1, 2, 3. 7:09 Okay, so if I look at fl, sorry, if I look at fl2, I get 1, 2, 3, 1, 2, 3, great. 7:14 Let's change, on fl2, the first item's second item to be a 5. 7:21 So we should have 1, 5, 3, 1, 2, 3. 7:27 So if I look at fl2, that's what I have, 1, 5, 3, 1, 2, 3. 7:30 So excellent, that's working too. 7:35 Now we could go another way, too, and just change some of the abilities of a class. 7:37 We don't have to totally change the class, and just destroy everything. 7:42 So let's make a new file. 7:48 And let's call this one javascriptobject.py. 7:50 Maybe you can guess what I'm gonna do here. 7:55 Let's make a dictionary that sorta kind acts like a JavaScript object. 7:57 Or rather, it has one thing that we want, 8:03 which is the ability to look up keys with a dot. 8:05 So there's a matching method called getAttribute. 8:08 And that method is use whenever we ask for an attribute using dot notation. 8:12 So let's try to use that to give us a key. 8:16 So well call us class JavaScriptObject. 8:19 This is going to extend dict. 8:23 And then we're gonna find getattribute__(self, item). 8:24 And we're gonna try, return self[item], except KeyError, so 8:32 if that key doesn't exist, if item isn't a valid key, 8:38 then return super().__getattribute__(item). 8:44 So if the key does exist, then we'll get that off of the dict. 8:50 If the key doesn't exist though, we fall back to the dict version 8:55 of getattribute, which actually looks to see if there's an attribute, 9:00 an actual real attribute with that name, okay. 9:04 So let's give that one a try now. 9:08 We're just making weirder and weirder things? 9:10 Okay, so lets say import javascriptobject. 9:14 Okay, and lets call this jso = javascriptobject.JavaScriptObject, 9:21 and we're gonna give this one key, which is a (name). 9:28 And I always like to use my name, feel free to use your own. 9:32 Let's set an actual attribute on this, so 9:36 jso.language and will set that language to python, okay. 9:38 So, jso.name is Kenneth it went locked up the key as an attribute, cool. 9:42 And if I do jso.language I get Python, and if I do jso.fake, so 9:48 that means it doesn't exist at all, I get an AttributeError. 9:53 And now an interesting thing here is, if we scroll up just a little bit, 9:58 we see that our traceback, we got this KeyError, which that's fine, right? 10:03 That's the KeyError that happened here. 10:08 And then while that KeyError happened, it came in here into this attribute, and 10:10 it couldn't find that attribute. 10:15 So it threw a second error, which was this AttributeError. 10:16 Hopefully this gives you some ideas, of how you can customize the building classes 10:19 to do what you need, with writing minimal code of your own. 10:23