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 +          


        

Answer 1

The line:

a, b = b, a + b

is closer to:

temp_a = a
a = b
b = temp_a + b

where b is using the old value of a before a was reassigned to the value of b.

Python first evaluates the right-hand expression and stores the results on the stack, then takes those two values and assigns them to a and b. That means that a + b is calculated before a is changed.

See How does swapping of members in the python tuples (a,b)=(b,a) work internally? for the low-down on how this all works, at the bytecode level.