unable to understand comma separated variable in python

Question

if

a, b = 0, 1

means

a = 0 
b = 1 

then

a, b = b , a + b

should be

a = b 
b = a + b 

but if I use that in fib function at (https://docs.python.org/3/tutorial/controlflow.html#defining-functions) it does not work. can some one help me understand how it work ?

so output of

def fib(n):
    a, b = 0, 1
    while a < n:
        print(a, end = ' ')
        a = b 
        b = a + b 
    print()

fib(50)

is 0 1 2 4 8 16 32