Euclidean Algorithm / GCD in Python

Question

I'm trying to write the Euclidean Algorithm in Python. It's to find the GCD of two really large numbers. The formula is a = bq + r where a and b are your two numbers, q is the number of times b divides a evenly, and r is the remainder.

I can write the code to find that, however if it the original numbers don't produce a remainder (r) of zero then the algorithm goes to step 2 => b = rx + y. (same as the first step but simply subbing b for a, and r for b) the two steps repeat until r divides both a and b evenly.

This is my code, I haven't yet figured out how to do the subbing of values and create a loop until the GCD is found.

a = int(input("What's the first number? "))
b = int(input("What's the second number? ")) 
r = int(a - (b)*int(a/b))

if r == 0:
  print("The GCD of the two choosen numbers is " + str(b))

elif r != 0:
  return b and r
  (b == a) and (r == b)

print("The GCD of the two numbers is " + str(r))

Answer 1

a = int(input("What's the first number? "))
b = int(input("What's the second number? ")) 
r=a%b
while r:
    a=b
    b=r
    r=a%b
print('GCD is:', b)

or use break in loop:

a = int(input("What's the first number? "))
b = int(input("What's the second number? ")) 
while 1:
    r=a%b
    if not r:
        break
    a=b
    b=r
print('GCD is:', b)