When and Why Do Python\'s `while` Loops Have `else` Clauses?

What's the Deal with Else Clauses on Python While Statements?

In Python, it's possible to append an else clause to a while statement, a behavior that might seem puzzling to some developers.

Why Does it Work?

The else clause is not associated with the loop itself but rather with the loop's condition. It executes only when the loop condition evaluates to False. If the loop is terminated prematurely by a break statement or an exception, the else clause will not be executed.

An Analogy

To understand the concept, we can draw an analogy to an if/else construct:

if condition:
    handle_true()
else:
    handle_false()

This is equivalent to the following while loop with an else clause:

while condition:
    handle_true()
else:
    # condition is now False
    handle_false()

Practical Example

Consider the following example:

while value < threshold:
    if not process_acceptable_value(value):
        # Invalid value encountered; exit the loop immediately
        break
    value = update(value)
else:
    # Threshold reached; perform necessary actions
    handle_threshold_reached()

Here, if the value ever becomes invalid, the break statement will terminate the loop, preventing the else clause from executing. Conversely, if the loop completes without any issues, the value is guaranteed to have reached or exceeded the threshold, triggering the handle_threshold_reached() function in the else clause.

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