When Does Python\'s `while` Loop Execute its `else` Clause?

Else Clause on Python while Statement

Python's while statement allows an optional else clause that is executed when the loop condition evaluates to False. This behavior may seem surprising to programmers coming from other languages, who are accustomed to else clauses being associated with if statements.

Why is the Else Clause Legal?

The else clause is legal in Python because it is syntactically separate from the while loop itself. It is indented at the same level as the loop, indicating that it is the next block of code to be executed after the loop terminates.

When is the Else Clause Executed?

The else clause is executed when the following two conditions are met:

• The loop condition evaluates to False.
• The loop does not prematurely terminate due to a break or exception.

Analogy to if/else Construct

The else clause can be thought of as the counterpart of the else clause in an if/else statement. The loop condition serves as the condition that determines whether the "true" (loop body) or "false" (else clause) block is executed.

Example Usage

Consider the following code:

while n != 0:
    print(n)
    n -= 1
else:
    print("what the...")

This code prints the numbers from 5 to 1, and then executes the else clause when n becomes 0. The output would be:

5
4
3
2
1
what the...

Additional Notes

• The else clause can be used to perform cleanup operations or handle special cases that occur when the loop condition becomes False.
• The else clause is not executed if an exception is raised within the loop.
• The else clause can also be used with for loops.

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