return index of least significant bit in Python

Question

C++ has a set of functions, ffs(), ffsl(), and ffsll(), that return the least significant bit that is set in a given binary integer.

I'm wondering if there is an equivalent function already available in Python. I don't see one described for bitarray, but perhaps there's another. I am hoping to avoid calculating the answer by looping through all possible bit masks, though of course that's an option of last resort; ffs() simply returns a single integer and I'd like to know of something comparable in Python.

Answer 1

Only in Pythons 2.7 and 3.1 and up:

def ffs(x):
    """Returns the index, counting from 0, of the
    least significant set bit in `x`.
    """
    return (x&-x).bit_length()-1

Example:

>>> ffs(136)
3