resetting the parameters of a for loop from inside the loop in Python3

I am working through the project euler problems in Python 3 as a fun way to learn the language, and my solution to problem number three I feel is a little funky. The problem is to find the largest prime factor of 600851475143, and I solved it in a pretty sprawling way as follows:

#!/usr/local/bin/python3
# encoding: utf-8
# what is the largest prime factor of whatever the user types


def main():
    original = int(input("Input a number: "))
    if original == isPrime(original):
        print(original, "is prime")
    else:
        print(factor(original), "is the largest prime factor of", original)

def factor(number):
    nummy=2
    for num in range(nummy, number):
        if (number%num==0 and isPrime(num)==num):
            biggest=int(num)
            number=int(number/biggest)
            nummy = int(biggest+2)
            print("so far,",biggest, "is the biggest prime factor...")
            if number < biggest:
                break
    return biggest      

def isPrime(value):
    for num in range(2, int(value**0.5)+1):
        if value%num==0:
            return int(value/num)
    return value


if __name__ == "__main__": main()

as you can see, in my "factor" function, I tried to update the upper limit of the range in my for loop, but it didn't really work like I was expecting so I had to add the if number < biggest block to get the whole thing to run in a reasonable amount of time.

Is there a way to update the parameters of a for loop from inside the loop?

I appreciate any insight on this.