[Tutor] project euler prime factorization problem

[Nick]

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

What is the largest prime factor of the number 600851475143 ?

#don't forget 2,3,5,7.  this function doesn't deliver those as output.

def is_prime(b):  #checks a number greater than 7 to see if it is prime and returns if is.
    if b % 2 != 0 and b % 3 != 0 and b % 5 != 0 and b % 7 != 0:
        print b,

def factor(num):
    x = num
    b = 1
    c = num
    while b <= c:       #starts at 1 and searches all the numbers up to the number you put in
        if x % b == 0:
            is_prime(b)
            b += 1
        else:
            b += 1

print "Don't forget to consider primes 2, 3, 5, and 7\n"


#600851475143

#shows what numbers in given range are prime
'''
def is_prime(b):
    return [2,3,5,7] + [x for x in xrange(3, 10000, 2) if x % 2 != 0 and x % 3 != 0 and x % 5 != 0 and x % 7 != 0]
'''

I'm looking for some help with this problem.  I realize my code is inefficient for such a big number, and also I'm not quite sure it works perfectly in and of itself.

My current reasoning was something of this sort:  Find all the factors of a number, then reduce them to just the prime factors and you can then multiply them together to get that number thus having the prime factors.  I need a lot of help haha.  Thanks in advance everyone.  If anyone has a good resource to point me to other than the open book project and dive into python it would be much appreciated.  Would it be useful for me to buy a book, and if so what are some easily accessible ones?  I feel dive into python is just too advanced for me.  I understand a lot of the teachings, but the examples seem unwieldy and esoteric.

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