Python a, b = b, a +b

Question

This is my first question and I started to learn Python. Is there a difference between:

a, b = b, a + b

and

a = b
b = a + b

When you write it in below example it shows different results.

def fib(n):
    a, b = 0, 1
    while a < n:
        print(a, end=' ')
        a, b = b, a + b
    print()
fib(1000)

and

def fib(n):
    a, b = 0, 1
    while a < n:
        print(a, end=' ')
        a = b
        b = a + b
    print()
fib(1000)

Answer 1

In a, b = b, a + b, the expressions on the right hand side are evaluated before being assigned to the left hand side. So it is equivalent to:

c = a + b
a = b
b = c

In the second example, the value of a has already been changed by the time b = a + b is run. Hence, the result is different.