"""                                                                                       
 factorial.py       
 Looking at functions, doctests, and test-driven deveopment.
"""

def factorial(n):
    """ Return the factorial of n, i.e. n!, which is n * (n-1) * ... 2 * 1
        >>> factorial(2)                                                       
        2
        >>> factorial(5)                                                                  
        120
    """
    result = 1
    for x in range(2, n+1):
        result = result * x
    return result

def main():
    print("-- factorial --")
    n = int(input("What is n? (i.e. 5) "))
    print("{}! is {}.".format(n, factorial(n)))

if __name__ == "__main__":
    import doctest
    doctest.testmod()
    main()