The Python while Loop

The while loop is Python’s tool for indefinite iteration. Unlike a for loop, which runs a set number of times, a while loop continues to execute a block of code as long as a specified condition remains True.

You use a while loop when you don’t know in advance how many times you need to repeat an action. Think of it as telling your program, “Keep doing this until something changes.”

Basic Syntax

The structure of a while loop is simple and intuitive:

while condition:
    # Code block to execute as long as the condition is True
    # This block must be indented

• while: The keyword that starts the loop.
• condition: A Boolean expression that is checked at the beginning of each iteration. If it’s True, the loop’s body runs. If it’s False, the loop terminates.
• :: The colon marking the end of the while declaration.
• Indented Block: The code that gets repeated. Crucially, something inside this block should eventually make the condition become False.

Example: A Simple Countdown

This is the most basic form of a while loop, where a counter variable is manually updated in each iteration.

Starting countdown...
5
4
3
2
1
Liftoff!

# DANGEROUS: This will never stop
# while True:
#     print("Still going...")

Common Use Cases for while Loops

while loops excel in situations where the number of iterations is not known beforehand.

Waiting for valid input...
'0' is not between 1 and 10. Please try again.
Waiting for valid input...
'11' is not between 1 and 10. Please try again.
Waiting for valid input...
Success! You entered 5.

# Conceptual example of a game loop
game_is_running = True
player_health = 100
 
while game_is_running:
    # 1. Get user input (e.g., move left, jump)
    # get_player_action() 
    
    # 2. Update game state (e.g., move characters, check for collisions)
    # update_world()
    
    # 3. Render the screen
    # draw_screen()
    
    # 4. Check for exit conditions
    if player_health <= 0:
        print("Game Over!")
        game_is_running = False # This will make the loop condition False
    
    # if user_presses_quit_button():
    #     game_is_running = False

Controlling while Loops with break and continue

Just like for loops, while loops can be controlled with break and continue.

• break: Exits the loop immediately, regardless of the while condition.
• continue: Skips the rest of the current iteration and goes back to check the while condition again.

Example: Using break for a Sentinel Value

A “sentinel value” is a special input that signals the end of a process.

User entered: 10
User entered: 20
User entered: 5
User entered: done
The final sum is: 35

The while-else Clause

Similar to for-else, the else block in a while loop executes only if the loop terminates naturally (i.e., its condition becomes False). It does not execute if the loop is terminated by a break statement.

Example: A Risky Countdown

Let’s create a countdown that might be interrupted.

--- Natural Finish ---
3
2
1
Countdown successful!

--- Interrupted Finish ---
3
Interrupted!

Practice Problems

Problem 1: Guess the Number

Write a simple number guessing game. Keep asking the user to guess a number until they get it right.

You guessed 3...
Too low!
You guessed 8...
Too high!
You guessed 7...
You got it! The secret number was 7.