How Does Python's Evaluation Order Affect Simultaneous Variable Assignments?

Understanding Variable Assignment and Evaluation Order in Python

In Python, multiple assignments can be performed concisely using comma-separated variables on the left-hand side of an assignment statement. However, this behavior differs from sequential assignments, and understanding the evaluation order is crucial to avoid unexpected results.

Consider the following example:

>> x = 1
>> y = 2

Suppose we attempt to assign both values simultaneously:

>> x, y = y, x + y
>> x
2
>> y
3

The result is not what we would expect if we performed the assignments separately:

>> x = 1
>> y = 2
>> x = y
>> y = x + y
>> x
2
>> y
4

This difference in behavior arises from the order in which evaluation occurs. In Python, the right-hand side of an assignment statement is fully evaluated before performing any variable assignments.

In the first example, the expression x y is evaluated first. The result, 3, is then assigned to y. Next, y, which now holds the value 3, is assigned to x. This explains the final values of x (2) and y (3).

In contrast, in the second example, y is first assigned to x (resulting in x holding the value 2). Then, x y is evaluated, which now calculates to 4, and that result is assigned to y.

Therefore, when performing multiple assignments in Python, it is important to consider the order of evaluation to avoid unexpected results.

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