Question

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?

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_{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.}

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