Why does len() not support iterators?

Question

Many of Python's built-in functions (any(), all(), sum() to name some) take iterables but why does len() not?

One could always use sum(1 for i in iterable) as an equivalent, but why is it len() does not take iterables in the first place?

Answer 1

Many iterables are defined by generator expressions which don't have a well defined len. Take the following which iterates forever:

def sequence(i=0):
    while True:
        i+=1
        yield i

Basically, to have a well defined length, you need to know the entire object up front. Contrast that to a function like sum. You don't need to know the entire object at once to sum it -- Just take one element at a time and add it to what you've already summed.

Be careful with idioms like sum(1 for i in iterable), often it will just exhaust iterable so you can't use it anymore. Or, it could be slow to get the i'th element if there is a lot of computation involved. It might be worth asking yourself why you need to know the length a-priori. This might give you some insight into what type of data-structure to use (frequently list and tuple work just fine) -- or you may be able to perform your operation without needing calling len.