Fibonacci sequence python

Question

I am trying to understand Python, but I still don't get it. I am new to the language, and wants to understand it properly. This is a line from a Fibonacci sequence using loops. Please explain the meaning of this code. I am trying to get the pattern by hand. I got the pattern up to 3, but after 3 I am not getting the answer.

a, b = 0, 1
while b < 50:
    print(b)
    a, b = b, a + b

Answer 1

a, b = b, a + b

This is known as multiple assignment. It's basically an atomic version of:

a = b
b = a + b

By atomic, I mean everything on the right is calculated before pacing it into the variables on the left. So a becomes b and b becomes the old version of a plus b, equivalent to the non-atomic:

old_a = a
a = b
b = old_a + b

So, in terms of what you see:

        a                        b               output
================    =========================    ======
(initial values)        (initial values)
        0                        1                  1
(becomes prev b)    (becomes sum of prev a,b)
        1                        1                  1
        1                        2                  2
        2                        3                  3
        3                        5                  5
        5                        8                  8

That exact code (along with the explanation of multiple assignment) can be found here in the tutorial.