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A while statement is an entry-condition control flow structure that repeatedly executes a suite of statements as long as a specified expression evaluates to True in a boolean context. Because the condition is evaluated before the loop body is entered, the suite will not execute at all if the initial evaluation yields False. 
while expression:
    # suite

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Evaluation Mechanics

  1. Python evaluates the expression using standard truth value testing (bool(expression)).
  2. If the expression evaluates to True, the indented suite is executed.
  3. Upon reaching the end of the suite, execution control jumps back to step 1.
  4. If the expression evaluates to False, the loop terminates. Control flow then proceeds to the else clause suite if one is defined; otherwise, it passes to the first statement immediately following the entire while construct.
A while loop will execute indefinitely (forming an infinite loop) unless the expression eventually evaluates to False—whether through variable modifications within the suite or changes in external state (such as I/O operations or system time)—or the loop is explicitly terminated via a break statement, a return statement, or a raised exception.

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Loop Control Statements

Python provides specific keywords to alter the standard evaluation mechanics of a while loop from within its suite:
  • break: Immediately terminates the innermost enclosing while loop. Control flow jumps to the statement following the entire loop construct. The loop condition is not re-evaluated.
  • continue: Immediately terminates the current iteration. Control flow jumps back to the top of the loop, forcing a re-evaluation of the expression to determine if the next iteration should proceed.
while condition_a:
    if condition_b:
        continue  # Skips the rest of this iteration; re-evaluates condition_a
    if condition_c:
        break     # Exits the loop entirely
    # standard execution path

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The else Clause

Python supports an optional else clause associated with the while loop. This is a unique syntactic feature compared to many other C-family languages. 
while expression:
    # suite
else:
    # else-suite
The else-suite executes strictly once and only if the loop terminates normally—meaning the expression evaluated to False. Crucially, if the loop is terminated prematurely via a break statement, the else clause is bypassed entirely. If the initial evaluation of the while expression is False, the loop suite is skipped, but the else suite will execute immediately.
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