Question

Might be a weird question. But I have been scratching my head over why if you want to get the length of a list you can’t simply say list.len() and you have to pass the list to len() to get its size? And where this len() is actually coming from?

Answer 1

You can get the length of a lot of items. Lists, dicts, sets, other collections. So the builtin len() that calls type(obj).__len__(obj) internally gives you a standard API to get the length.

If all those collection types had a len() method that was called directly there would be nothing to prevent someone from creating a custom collection class that uses e.g. .length() or a .length property.

Here’s the explanation from Guido van Rossum, the creator of Python:

First of all, I chose len(x) over x.len() for HCI reasons (def __len__() came much later). There are two intertwined reasons actually, both HCI:

(a) For some operations, prefix notation just reads better than
postfix — prefix (and infix!) operations have a long tradition in
mathematics which likes notations where the visuals help the
mathematician thinking about a problem. Compare the easy with which we
rewrite a formula like x*(a+b) into x*a + x*b to the clumsiness of
doing the same thing using a raw OO notation.

(b) When I read code that says len(x) I know that it is asking for
the length of something. This tells me two things: the result is an
integer, and the argument is some kind of container. To the contrary,
when I read x.len(), I have to already know that x is some kind of
container implementing an interface or inheriting from a class that
has a standard len(). Witness the confusion we occasionally have when
a class that is not implementing a mapping has a get() or keys()
method, or something that isn’t a file has a write() method.

Saying the same thing in another way, I see ‘len’ as a built-in
operation. I’d hate to lose that. I can’t say for sure whether you meant that or not, but ‘def len(self): …’ certainly sounds like you
want to demote it to an ordinary method. I’m strongly -1 on that.