Making Your Class Iterable

00:00 You’ve implemented membership checking with __contains__, but then you know this, this doesn’t support iteration because those are two different concepts.

00:07 So there is actually a fallback that if you do implement __iter__, which is primarily making your class iterable, you also get membership checking for free basically.

00:17 So the simplest way to get both of those functionalities is to implement instead of __contains__, just implement __iter__. Let’s try that out. So I go ahead and actually remove __contains__, and call it __iter__.

00:33 You’re not going to need to pass an item

00:35 and instead of doing a membership check in here, you just pass self.wares to the built-in iter() function. And with this, you get both. Let’s try it out.

00:47 I’m going to start a new Python REPL, and then again from storehouse import Storehouse,

00:56 paste the Storehouse object, and then try again. Is rose in storehouse? True. Alright, so membership checking still works.

01:08 And now you can also do for ware in storehouse:

01:12 print(ware) and this works as well.

01:16 So you may wonder why do you want to ever implement __contains__ instead of just doing __iter__ where you get more functionality?

01:24 The answer to that is that with __iter__, Python just falls back to iteration to do membership checking, which means it may be slow depending on what you’re working with, right?

01:33 So if you care for the growth of your merchant friend’s business and they implement their inventory in some sort of a hash data structure, like a dictionary or a set, then you wouldn’t want to just implement __iter__ because that’ll then default to iterating to check for membership, but then instead you want to implement both.

01:52 You want to do __contains__ for membership checking and a separate __iter__ for iteration.

01:58 And you have a similar situation with __getitem__, which I’ve also mentioned here. That’s another way you could also just implement __getitem__ and get access to membership checking.

02:08 But the primary special method that Python uses for membership checking is __contains__.

02:15 I’ve also mentioned that you can do it using __getitem__. So this is what an implementation with __getitem__ would look like.

02:23 And if you implement just __getitem__, then you get membership checks, iteration, and also indexing. So additionally to what you get with implementing __iter__, you also can use square brackets and access items by their index.

02:39 And why you wouldn’t want to just do that is the same as with __iter__, that it defaults to sequentially processing items, which means that you have less control over how membership checks work and how iteration works, and it’s generally less efficient.

02:53 So generators, custom classes, what would Shakespeare say to all of that? Maybe, “Enough, no more; ‘Tis not so sweet now as it was before.” Or hopefully, maybe just “Enough, no more.” I’ll stick with that and call it a course.

03:08 In the next lesson, you’ll go over what you learned, and I’ll give you a couple of other resources that you can continue learning with.