Is there any built-in way to get the length of an iterable in python?

Short of iterating through the iterable and counting the number of iterations, no. That's what makes it an iterable and not a list. This isn't really even a python-specific problem. Look at the classic linked-list data structure. Finding the length is an O(n) operation that involves iterating the whole list to find the number of elements.

As mcrute mentioned above, you can probably reduce your function to:

def count_iterable(i):
    return sum(1 for e in i)

Of course, if you're defining your own iterable object you can always implement __len__ yourself and keep an element count somewhere.

If you need a count of lines you can do this, I don't know of any better way to do it:

line_count = sum(1 for line in open("yourfile.txt"))

The cardinality package provides an efficient count() function and some related functions to count and check the size of any iterable: http://cardinality.readthedocs.org/

import cardinality

it = some_iterable(...)
print(cardinality.count(it))

Internally it uses enumerate() and collections.deque() to move all the actual looping and counting logic to the C level, resulting in a considerable speedup over for loops in Python.

I've used this redefinition for some time now:

def len(thingy):
    try:
        return thingy.__len__()
    except AttributeError:
        return sum(1 for item in iter(thingy))

It turns out there is an implemented solution for this common problem. Consider using the ilen() function from more_itertools.

more_itertools.ilen(iterable)

An example of printing a number of lines in a file (we use the with statement to safely handle closing files):

# Example
import more_itertools

with open("foo.py", "r+") as f:
    print(more_itertools.ilen(f))

# Output: 433

This example returns the same result as solutions presented earlier for totaling lines in a file:

# Equivalent code
with open("foo.py", "r+") as f:
    print(sum(1 for line in f))

# Output: 433