3rd problem from project Euler

[Richard_SS]

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

What is the largest prime factor of the number 600851475143 ?
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I have written a programm for solving this problem, it easily displays prime factors of 13195, but once I insert a bigger number (like 600851475143), a programm goes into an infinite loop, it returns neither error, nor answer. Please help, where is the problem in code?

My code:

import math
a = []
b = range(1,600851475143)

def is_prime(n):
    if n == 2:
        return True
    if n % 2 == 0 or n <= 1:
        return False

    sqr = int(math.sqrt(n)) + 1

    for divisor in range(3, sqr, 2):
        if n % divisor == 0:
            return False
    return True

b = (filter(is_prime,b))

for i in b:
    if 600851475143 % i == 0:
        a.append(i)
    else:
        pass
print (a)
print("The largest prime factory is",a[-1::])