From http://stackoverflow.com/questions/11175131/code-for-greatest-common-divisor-in-python python's built in gcd function

gcd.py

>>> from fractions import gcd
>>> gcd(20,8)
4

>>> print inspect.getsource(gcd)

def gcd(a, b):
  """Calculate the Greatest Common Divisor of a and b.

  Unless b==0, the result will have the same sign as b (so that when
  b is divided by it, the result comes out positive).
  """
  while b:
    a, b = b, a%b
    return a