[python] Getting number of elements in an iterator in Python

You cannot (except the type of a particular iterator implements some specific methods that make it possible).

Generally, you may count iterator items only by consuming the iterator. One of probably the most efficient ways:

import itertools
from collections import deque

def count_iter_items(iterable):
    """
    Consume an iterable not reading it into memory; return the number of items.
    """
    counter = itertools.count()
    deque(itertools.izip(iterable, counter), maxlen=0)  # (consume at C speed)
    return next(counter)

(For Python 3.x replace itertools.izip with zip).