Implementing Shor's algorithm in Python

Now, let's implement Shor's algorithm in Python. Recall that % is the mod operator in Python, and to check if an integer is even, we check if the integer mod 2 is equal to zero. The following code is Shor's algorithm in Python:

import math
import random
def shors_algorithm_classical(N):
    assert(N>0)
    assert(int(N)==N)
    while True:
        a=random.randint(0,N-1)
        g=math.gcd(a,N)
        if g!=1 or N==1:
            first_factor=g
            second_factor=int(N/g)
            return first_factor,second_factor
        else:
            r=period_finding_classical(a,N) 
            if r % 2 != 0:
                continue
            elif a**(int(r/2)) % N == -1 % N:
                continue
            else:
                first_factor=math.gcd(a**int(r/2)+1,N)
                second_factor=math.gcd(a**int(r/2)-1,N)
                if first_factor==N or second_factor==N:
                    continue
                return first_factor,second_factor