Project Euler problem #3

pe3.py

'''
The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?
'''

import math
from time import time

calcs = {}

def pfactor(n):
    if n < 2:
        return [1]
    try:
        return calcs[n]
    except:
        x = []
        for a in range(2, int(.5 + math.sqrt(n))+1):
            if n/a == int(n/a):
                x.append(a)
                x.extend(pfactor(int(n/a)))
                calcs[n] = x
                return x
        calcs[n] = [n]
        return [n]

if __name__ == "__main__":
    start = time()
    print(pfactor(600851475143)[-1], time() - start)