The math module docs state

Except when explicitly noted otherwise, all return values are floats.

But math.factorial isn't what I would call explicit about returning int:

math.factorial(x)
    Return x factorial. Raises ValueError if x is not integral or is negative.

At minimum, shouldn't the first sentence be "Return x factorial as an int."? I haven't tested on all Python versions, but math.factorial on 2.7 and 3.2 definitely return int (or long in Python 2 when necessary).