while, break and the Loop That Never Ends

Sometimes you cannot know in advance how many times to repeat. Keep asking until the answer is a number. Keep adding until the savings reach the target. A while loop repeats its block as long as a condition stays True, which also means that, if the condition never changes, it repeats forever.

Think of stirring a sauce until it thickens. You do not count the stirs; you check after each one. Forget to check, and you would stir all night. A while loop is that instruction, with the checking built in.

while condition

A while line has a condition and a colon, like an if. Before each round, Python checks the condition. If it is True, the block runs, and then Python checks again. The first time it is False, the loop ends and the program carries on. In the program below, the savings grow by 150 a month until they reach 1000.

savings = 0
months = 0
while savings < 1000:
    savings = savings + 150
    months = months + 1
print(f"{months} months to save {savings}")

7 months to save 1050

The Loop That Never Ends

The condition must depend on something that changes inside the loop. Leave out the line that changes it and the condition stays True forever: the program keeps going and never finishes. Nothing is damaged; your program is just busy. Press Control and C in the terminal to stop it. Python stops at once and reports a KeyboardInterrupt, which only means "stopped by the keyboard". Exactly where the two characters ^C appear, and which line the report names, depends on the moment you pressed the keys.

savings = 0
while savings < 1000:
    print("Saving...")

Saving...
Saving...
Saving...
^CTraceback (most recent call last):
  File "/Users/you/python-course/forever.py", line 3, in <module>
    print("Saving...")
    ~~~~~^^^^^^^^^^^^^
KeyboardInterrupt

break and continue

Two words change a loop's flow from inside. break leaves the loop immediately, whatever the condition says. continue skips the rest of this round and goes straight to the next check. A common shape is while True with a break: loop forever, and leave the moment the job is done.

Ask Until the Answer Is Valid

Chapter 2's tip calculator crashed when someone typed a word. With a loop it can ask again. The isdigit method answers True when a string is made only of digits. The loop below keeps asking until it is, then breaks out and converts the answer safely. Chapter 8 shows a more complete way to handle bad input; this is the first step.

while True:
    answer = input("Tip percent: ")
    if answer.isdigit():
        break
    print("Please type a whole number, like 15.")

percent = int(answer)
print(f"Tip set to {percent} percent")

Tip percent: forty
Please type a whole number, like 15.
Tip percent: 15
Tip set to 15 percent