Some factorial implementations in Python

fact.py

import operator

# naive recursive solution
def fact1(n):
    if n <= 1:
        return 1
    return n * fact1(n - 1)

fact2 = lambda n: reduce(operator.mul, xrange(1, n + 1))

# for loop solution
def fact3(n):
    res = 1
    for i in range(1, n+1):
        res = res * i
    return res

# same as fact2, but uses a lambda instead of operator.mul
# fact4 = lambda n: reduce(lambda x, y: x * y, xrange(1, n + 1))

# same as fact1, but uses an accumulator for possible TCO
def fact5(n):
    def fact(n, acc):
        if n<= 1:
            return acc
        return fact(n - 1, n * acc)
    return fact(n, 1)

if __name__ == '__main__':
    from timeit import timeit
    
    print "Naive recursive solution => ",
    print timeit(lambda: fact1(100))

    print "Reduce solution => ",
    print timeit(lambda: fact2(100))

    print "For loop solution => ",
    print timeit(lambda: fact3(100))

    print "Tail recursive solution => ",
    print timeit(lambda: fact5(100))


# PyPy
# Naive recursive solution =>  28.4803950787
# Reduce solution =>  13.8635640144
# For loop solution =>  13.879529953
# Tail recursive solution =>  32.0280330181

# CPython
# Naive recursive solution =>  47.6345591545
# Reduce solution =>  22.992249012
# For loop solution =>  24.9572100639
# Tail recursive solution =>  60.3095831871