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

Multiple Assignment and Evaluation Order in Python Unveiled

In Python, multiple assignment provides a convenient way to assign values to multiple variables simultaneously. However, understanding the evaluation order is crucial to avoid unexpected results.

Consider the following code snippet:

>>> x = 1
>>> y = 2
>>> x, y = y, x + y
>>> x
2
>>> y
3

While the result may seem intuitive, it differs from what you might expect. The key lies in Python's evaluation order.

In an assignment statement, the right-hand side is evaluated before setting the values of the variables on the left-hand side. Thus, in the given code:

• y is evaluated first, giving the value ham.
• Then, x y is evaluated, resulting in spam.
• Finally, x is assigned ham and y is assigned spam.

This evaluation order is analogous to the following steps:

ham = y
spam = x + y
x = ham
y = spam

Contrast this behavior with the following separate assignments:

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

Here, x is assigned y, and then y is assigned x plus y. This is equivalent to:

>>> x = y
>>> y = y + y

Understanding the evaluation order is essential for both single and multiple assignments. By considering the order in which expressions are evaluated, you can anticipate the results and avoid potential pitfalls.

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