Few ways to calculate factorial of the number.

some_factorials.py

"""
Few ways to calculate factorial of the number.
For my article: http://pavdmyt.com/digging-around-factorial-function/
"""
import math as m


def fact(x):
    """
    Weird factorial function implementation
    by hypothetical mathematician.
    """
    return m.exp(sum([m.log(k) for k in range(1, x+1)]))


def iterative_fact(x):
    """
    Common iterative factorial function.
    """
    res = 1
    for i in range(2, x + 1):
        res *= i
    return res


def fast_fact(x):
    """
    Calculates factorial of the number faster.
    """
    if x % 2:
        res = x
        x -= 1
    else:
        res = 1

    i = 0
    nums = range(x, 0, -2)
    for num in nums:
        i += num
        res *= i
    return res


if __name__ == '__main__':
    # Tests.
    import unittest

    class Test(unittest.TestCase):

        def test_iterative_fact(self):
            for num in range(30):
                self.assertEqual(iterative_fact(num), m.factorial(num))

        def test_fast_fact(self):
            for num in range(30):
                self.assertEqual(fast_fact(num), m.factorial(num))

    unittest.main()