Someone

def is_prime(n):
    
    if n < 3:
        return False #too small/negative
    
    if n%2 == 0:
        return False #even
    
    for i in range(3, int(n**0.5)+1, 2):
    
        if n%i == 0:
            return False
        
    return True
        
def check_prime_factors(n, prime_list):
    for prime in prime_list:
        if prime < n and n%prime == 0:
            return prime
         
prime_list = []
for n in range(1000000):
    if is_prime(n):
        prime_list.append(n)

prime_list.reverse()
print "done making list"

print check_prime_factors(600851475143, prime_list)