Multiple assignment and evaluation order in Python
In an assignment statement, the right-hand side is always evaluated fully before doing the actual setting of variables. So,
x, y = y, x + yevaluates 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 = yspam = x + yx = hamy = spamBy contrast,
x = yy = x + ysets x to y, then sets y to x (which == y) plus y, so it's equivalent to
x = yy = y + y
It is explained in the docs in the section entitled "Evaluation order":
... while evaluating an assignment, the right-hand side is evaluated before the left-hand side.
The first expression:
- Creates a temporary tuple with value
y,x+y - Assigned in to another temporary tuple
- Extract the tuple to variables
xandy
The second statement is actually two expressions, without the tuple usage.
The surprise is, the first expression is actually:
temp=xx=yy=temp+yYou can learn more about the usage of comma in "Parenthesized forms".