Calculate modular inverse

modular_inverse.py

# Note: A and P must share no factor greater than 1.
def modular_inverse(A,P):
    p0, a0 = 1, 0
    p1, a1 = 0, 1
    R0, R1 = P, A%P
    while R1 != 1 and R1 != 0:
        n, R2 = divmod(R0, R1)
        p2 = p0 - n*p1 ; a2 = a0 - n*a1
        p0 = p1; a0 = a1; p1 = p2; a1 = a2
        R0 = R1; R1 = R2
    if R1 == 0:
        # return "Error: A and P share factor "+ str(R0) + ". They must share no factor greater than 1."
    return a1 % P