Multiple assignment and evaluation order in Python

Question

What is the difference between the following Python expressions:

# First:

x,y = y,x+y

# Second:

x = y
y = x+y

First gives different results than Second.

e.g.,

First:

>>> x = 1
>>> y = 2
>>> x,y = y,x+y
>>> x
2
>>> y
3

Second:

>>> x = 1
>>> y = 2
>>> x = y
>>> y = x+y
>>> x
2
>>> y
4

y is 3 in First and 4 in Second

Answer 1

In an assignment statement, the right-hand side is always evaluated fully before doing the actual setting of variables. So,

x, y = y, x + y

evaluates y (let's call the result ham), evaluates x + y (call that spam), then sets x to ham and y to spam. I.e., it's like

ham = y
spam = x + y
x = ham
y = spam

By contrast,

x = y
y = x + y

sets x to y, then sets y to x (which == y) plus y, so it's equivalent to

x = y
y = y + y