Why Does Python\'s While Loop Have an Else Clause?

Indentation Anomaly: The Else Clause in Python While Statements

In Python, an else clause can be appended to a while statement. However, unlike traditional programming conventions, the else clause is not directly related to the while loop's condition. Instead, it executes only when the condition becomes false.

Why Is It Legal?

Python allows the use of an else clause with while loops to provide an alternative flow of execution when the condition no longer holds true. This is distinct from if/else blocks, where the else clause is paired with a specific if condition.

Execution Logic

The else clause in a while loop behaves as follows:

• If the while condition continues to be true, the loop body continues to execute iteratively.
• When the condition becomes false, the loop body is exited, and the execution proceeds to the else clause.

Analogy to if/else Constructs

One can visualize the while loop with an else clause as an if/else construct with respect to the condition:

if condition:
    handle_true()
else:
    handle_false()

is analogous to:

while condition:
    handle_true()
else:
    # condition is false now, handle and go on with the rest of the program
    handle_false()

Practical Example

For instance:

while value < threshold:
    if not process_acceptable_value(value):
        # something went wrong, exit the loop; don't pass go, don't collect 200
        break
    value = update(value)
else:
    # value >= threshold; pass go, collect 200
    handle_threshold_reached()

In this code, the else clause executes if and only if the value meets or exceeds the threshold.

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