How Does Python's Multiple Assignment Affect Variable Evaluation Order?

Multiple Assignment and Evaluation Order in Python

In Python, multiple assignments are a common practice, where multiple variables are assigned values simultaneously. However, the order of evaluation in such assignments can have unexpected consequences.

The Problem

Consider the following code:

>>> x = 1
>>> y = 2
>>> x, y = y, x + y  # Multiple assignment

After this assignment, the values of x and y are:

>>> x
2
>>> y
3

This differs from what one might expect if the assignments were done separately:

>>> x = 1
>>> y = 2
>>> x = y  # Assign x to the current value of y
>>> y = x + y  # Assign y to the sum of x and y
>>> x
2
>>> y
4

The Explanation

The difference arises due to the order of evaluation in assignment statements. In Python, the right-hand side of an assignment is evaluated first, before the actual assignment to the variables takes place.

In the first example, when we assign x, y = y, x y, the right-hand side is evaluated as follows:

ham = y  # Where ham = 2
spam = x + y  # Where spam = 1 + 2 = 3

Then, x is assigned the value of ham (2), and y is assigned the value of spam (3). This is why we get the result x = 2 and y = 3.

In contrast, in the second example, x is first assigned the value of y, which is 2. Then, y is assigned the sum of x (2) and y (2), resulting in 4.

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