Sum even factorials.

sum_even_factorials.py

#!/usr/bin/env python3


def factorial(n):
    """
    >>> factorial(20)
    2432902008176640000
    """
    if n < 2:
        return 1
    else:
        return n*factorial(n-1)


def sum_even_factorials(r):
    """
    >>> sum_even_factorials(20)
    6423384156578667    
    """
    even_factorials = [factorial(n) for n in range(r) if n % 2 == 0]
    return sum(even_factorials)


if __name__ == '__main__':
    import doctest
    doctest.testmod(verbose=True)