Length of generator output [duplicate]

Answer 1

def count(iter):
    return sum(1 for _ in iter)

Or better yet:

def count(iter):
    try:
        return len(iter)
    except TypeError:
        return sum(1 for _ in iter)

If it's not iterable, it will throw a TypeError.

Or, if you want to count something specific in the generator:

def count(iter, key=None):
    if key:
        if callable(key):
            return sum(bool(key(x)) for x in iter)
        return sum(x == key for x in iter)
    try:
        return len(iter)
    except TypeError:
        return sum(1 for _ in iter)