while loops — repeat until done

The shape of a while loop

balance = 100
months = 0
while balance < 200:
    balance = balance * 1.05   # 5% per month
    months += 1
print(f"Took {months} months")   # 15

"Keep going while the condition is true." When the condition flips false, the loop exits.

The classic infinite loop

x = 0
while x < 10:
    print(x)
    # forgot to change x — runs forever

If you ever fire one off by accident, hit the ⬛ stop button in Jupyter, or Ctrl+C in the terminal.

while True + break — the "until" pattern

while True:
    answer = input("Type 'quit' to stop: ")
    if answer == "quit":
        break
    print(f"You typed: {answer}")

When to use while vs for