Is Python multiple assignment really right-to-left?

Question

x = 1
y = 2

x, y = y, x + y
print(x, y)

The above piece prints me: 2, 3

The Python documentation and previous stackoverflow question answers regarding this topic tells that assignment goes from right to left.

So in my case y = x + y goes first -> y = 3, then x = y goes second -> x = 3, and the output should be: 3, 3

If assignment goes from left-to-right, then the resulting output should be: 2, 4

So both left-to-right and right-to-left doesn't work here, seems like it is done simultaneously. My interpreter version is 3.8

Help me please. Thanks!

The operations are done before assignment I believe, as in right-to-left, assignment is done last. — Paddy Harrison, Apr 28 '20 at 18:24
There are no "multiple assignments" per-se here. `x + y` is done first, then just a single tuple assignment. — DeepSpace, Apr 28 '20 at 18:25
In *iterable unpacking*, the expression on the right-hand side is fully evaluated *first* (note, you are essentially creating a tuple object) — juanpa.arrivillaga, Apr 28 '20 at 18:27

score 1 · Accepted Answer · answered Apr 28 '20 at 18:24

The right hand side of the assignment is computed all before the assignment happens.

So first you compute (y, x+y) which is (2, 3) and then you unpack it into x and y.

See also what happens if on the left you have more than identifiers but also expressions:

>>> a = [1, 2]
>>> i = 1
>>> a[i], i = 3, 4
>>> a, i
([1, 3], 4)

1 Answers1