Returning a Fibonacci list
Of course not.

If you do c, a, b = a + b, b, c, this is what you get

a, b, c = a0, b0, c0
c, a, b = a + b, b, c 
## c become a0 + b0
## a becomes b0
## b becomes c0


I'm pretty sure this is not your intention.

In tuple assignment, the right tuple is evaluated first and then assigned to the left tuple. I'm guessing you thought it's done in order. Sorry, but no.

You can instead eliminate the variable c and do this 

a, b = a0, b0
a, b = b, a + b
## a becomes b0
## b becomes a0 + b0


UPDATE

This is what is done by the machine, imagine the left values as an array of variables waiting to be assigned values that are on the left. Like this 

[c, a, b] = [a + b, b, c] 


Imagine the right values as an array of expression waiting to be evaluated. The right one gets evaluated first, then one by one the left variables takes the right values. No assignment is done until the left expressions are evaluated.