Python While Loops

Python While Loops: Iteration, Guards, & Safe Loops

A while loop executes code repeatedly as long as its target boolean condition remains True. while loops are ideal when the number of required iterations is unknown before loop entry.

Execution Flow & Else Clause

flowchart TD
    A["Loop Entry"] --> B{"Condition Evaluates True?"}
    B -- "Yes" --> C["Execute Loop Body"]
    C --> D{"Break Encountered?"}
    D -- "Yes" --> E["Exit Loop Immediately"]
    D -- "No" --> B
    B -- "No" --> F["Execute while...else Block (If no break occurred)"]
    F --> G["Resume Execution"]
    E --> G

Loop Control Keywords

• break: Terminate the loop immediately.
• continue: Skip the rest of the current iteration and re-evaluate condition.
• else: Executes once when the loop condition turns False (skipped if exited via break).

Practical Code Example

import time

def retry_connection(max_attempts: int = 3) -> bool:
    """Simulate exponential backoff connection retries using a while loop."""
    attempt = 1
    connected = False

    while attempt <= max_attempts:
        print(f"Connection attempt {attempt} of {max_attempts}...")
        
        # Simulated connection condition (succeeds on attempt 3)
        if attempt == 3:
            connected = True
            print("Successfully established database connection!")
            break
            
        attempt += 1
        time.sleep(0.1)
    else:
        # Executes only if loop finishes naturally without 'break'
        print("Failed to connect after maximum attempts.")

    return connected

if __name__ == "__main__":
    retry_connection(max_attempts=4)

Best Practices & Gotchas

• Prevent Infinite Loops: Always ensure loop state variables advance toward the termination condition within the loop body.
• while True with Explicit Break: For event loops or interactive interfaces, use while True: combined with explicit if exit_condition: break.
• The while...else Semantic: Remember that the else block runs only when the condition evaluates to False, NOT when the loop is terminated by break.

Self-Check Challenge

Write a while loop that calculates the factorial of a given integer $N$ ($N! = N imes (N-1) imes \dots imes 1$) and prints the result.