Python while loop. What is this construct called?

Question

On the Literate Programs site, I was looking at the Python code for the GCD algorithm.

def gcd(a,b):
        """ the euclidean algorithm """
        while a:
                a, b = b%a, a
        return b

What is going on in the body? Expression evaluation? Is it a compressed form of another structure?

Answer 1

There are two things going on here:

            a, b = b%a, a

First, a tuple is created with the contents (b%a, a) . Then the contents of that tuple are unpacked and assigned to the names a and b .

Answer 2

Looks like shorthand for:

while a > 0:
    temp = a
    a = b%a
    b = temp
return b

Answer 3

a is receiving the result of b%a while b is receiving the value of a

Works the same as:

while a > 0:
    tmp = a
    a = b%a
    b = tmp
return b

See this post for more information on switching variables: Is there a standardized method to swap two variables in Python?