Why Does Multiple Assignment in Python (x, y = y, x y) Differ from Separate Assignments (x = y; y = x y)?

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:

1. y is evaluated and stored as ham.
2. x y is evaluated and stored as spam.
3. x is set to ham.
4. 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:

1. y is set to x.
2. 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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