Python — While Loops

Basic While Loop

count = 0
while count < 5:
    print(count)
    count += 1
# 0, 1, 2, 3, 4

How While Loops Work

1. Check the condition
2. If True, execute the body
3. Go back to step 1
4. If False, exit the loop

# Infinite loop (be careful!)
while True:
    print("This never stops!")

While vs For

Use for when you know how many times to loop. Use while when you loop until a condition changes:

# For: known count
for i in range(10):
    print(i)

# While: unknown count
password = ""
while password != "secret":
    password = input("Enter password: ")
print("Access granted!")

break and continue

# break — exit immediately
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == "quit":
        break
    print(f"You entered: {user_input}")

# continue — skip to next iteration
num = 0
while num < 10:
    num += 1
    if num % 2 == 0:
        continue
    print(num)  # 1, 3, 5, 7, 9

while-else

The else block runs when the condition becomes False:

num = 1
while num <= 100:
    if num == 50:
        break
    num += 1
else:
    print("Loop completed normally")

# If break is hit, else doesn't run

Common Patterns

User Input Loop

name = ""
while not name:
    name = input("Enter your name: ")
print(f"Hello, {name}!")

Number Guessing Game

import random

secret = random.randint(1, 100)
guess = 0
attempts = 0

while guess != secret:
    guess = int(input("Guess a number (1-100): "))
    attempts += 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")

print(f"Correct! Took {attempts} attempts.")

Menu System

while True:
    print("\n1. Start")
    print("2. Settings")
    print("3. Quit")
    choice = input("Choose: ")
    
    if choice == "1":
        print("Starting game...")
    elif choice == "2":
        print("Opening settings...")
    elif choice == "3":
        print("Goodbye!")
        break
    else:
        print("Invalid choice")

Processing Until Empty

items = [1, 2, 3, 4, 5]
while items:
    item = items.pop()
    print(f"Processing: {item}")

Infinite Loops

# Game loop
while True:
    handle_input()
    update_game()
    render()
    if game_over:
        break

# Server loop
while True:
    request = get_request()
    process(request)

Avoiding Infinite Loops

# BAD: no exit condition
# while True:
#     print("forever")

# GOOD: always have a way out
count = 0
while count < 100:
    print(count)
    count += 1

# GOOD: use a flag
running = True
while running:
    user_input = input("Command: ")
    if user_input == "exit":
        running = False

Best Practices

• Always have a way to exit the loop
• Initialize variables before the loop
• Update the condition variable inside the loop
• Use for instead of while when the count is known
• Add a timeout or max iteration count for safety
• Use while True with break for infinite loops that need to exit

Mini Practice

1. Write a countdown from 10 to 1
2. Create a simple calculator that runs until the user types "quit"
3. Write a number guessing game
4. Create a menu system with while loop
5. Process items from a list until it's empty

Up Next

Next: Functions →