Creating a program to look for the largest prime number of a number

[Wikki14]

So I'm creating a code that will take on number look for all the factors of that number and then tell me what factors are prime and which ones are not
num = (random number) <==This a preinputed number.

def print_factors(x):
    print("factors of", x,"are:")
    for i in range(1,x + 1):
        if x % i ==0:
            print(i)
print_factors(num)

I think the way to do it would be to have the program put all the factors into a list and then run a prime number program through it but I don't know how to do that and would love the help/criticism or whatever you have.
Thanks for the help, Wikki14