


Multiple Assignment and Evaluation Order in Python
In Python, when utilizing multiple assignment, such as x, y = y, x y, it's imperative to understand the underlying order of evaluation.
Question:
When assigning multiple values at once, why does x, y = y, x y result in different values than assigning them separately, i.e. x = y; y = x y?
Answer:
In Python, the right-hand side of an assignment statement is evaluated fully before any variable setting occurs. This implies that in x, y = y, x y, the following steps take place:
- y is evaluated and stored as ham.
- x y is evaluated and stored as spam.
- x is set to ham.
- y is set to spam.
Effectively, it's equivalent to:
ham = y spam = x + y x = ham y = spam
On the other hand, in x = y; y = x y, the steps are:
- y is set to x.
- x y is evaluated as y y and set to y.
This results in x being set to the original value of y, and y being set to the sum of the original values of x and y.
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