What are the causes of errors when computing the (GCD) of two numbers using Python

Titan_S

I am encountering a problem when I attempt to execute my code that is meant to calculate the greatest common divisor of two values. When I run the program, I get an error message stating that the function 'gcd' is not defined.

def GCD(12,4):
    gcd = 1
    for i in range(2, max(12,4)/2):
        if((12 % i == 0) and (4 % i == 0)):
            gcd = i
    return gcd