Python中的Euler Project#3

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

What is the largest prime factor of the number 600851475143 ?

我知道我的程序效率低且超大,但我只是想知道它为什么不起作用？
这是代码：

def check(z):

# checks if the given integer is prime

    for i in range(2,z):
        if z % i == 0:
            return False
            break
        i = i+1
    return True

def primegen(y):
# generates a list of prime integers in the given range

    tab = []
    while y >= 2:
        if check(y) == True:
            tab.append(y)
        i = i-1


def mainfuntion(x):
# main function; prints the largest prime factor of the given integer

    primegen(x)
    for i in range(len(tab)):
        if x % tab[i] == 0:
            print tab[i]
            break

mainfuntion(600851475143)

这是错误：

for i in range(2,z):
OverflowError: range() result has too many items