Suppose we have:
>>> x = 1
>>> y = 2
If we try assigning both values at once like so:
>>> x, y = y, x+y
>>> x
2
>>> y
3
Then we get a different result than if we did the assignments separately:
>>> x = 1
>>> y = 2
>>> x = y
>>> y = x+y
>>> x
2
>>> y
4
Why does this happen?
See also Multiple assignment semantics regarding the effect and purpose of parentheses on the left-hand side of a multiple assignment.
See also Understand Python swapping: why is a, b = b, a not always equivalent to b, a = a, b? for more complex cases, where the order of assignment matters.
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