Leaving early: break and continue

Sometimes a loop doesn’t need to go all the way round. If you’re searching a shelf of books for one title, you stop as soon as you find it. If you’re counting the red cars in a car park, you glance at a blue one and move straight on to the next. Python has two small words for exactly these situations: break and continue.

break: stop the loop now

break ends the loop immediately. Python jumps to the first line after the loop, even if there were more iterations to go.

for n in range(1, 10):
    if n == 5:
        break
    print(n)
print("Loop finished")

It prints 1, 2, 3, 4, then Loop finished. When n reaches 5, the if is True, break runs, and the loop is over. The print(n) for 5 never happens, and 6, 7, 8 and 9 are never even looked at.

continue: skip to the next one

continue ends only the current iteration. Python skips the rest of the block and goes straight back to the top of the loop for the next value.

for n in range(1, 10):
    if n % 2 == 0:
        continue
    print(n)

This prints only the odd numbers. For even numbers, continue skips the print; for odd numbers the if is False, so the print runs.

Watch them work

The visualiser shows each iteration as a tile. A tile marked break is where the loop stopped; tiles after it are never reached. A tile marked continue was visited, but the rest of its block was skipped. Try the dropdowns to add a break, a continue, or both, and step through.

Notice the order of the checks in the code: the break check comes first, then the continue check, then the body. Order matters here, just as it did with elif.

Searching with break

break is perfect for searching: look at candidates one by one and stop at the first one that fits. What’s the first number above 100 that divides exactly by both 7 and 9?

for n in range(101, 1000):
    if n % 7 == 0 and n % 9 == 0:
        print("Found it:", n)
        break

Without the break, the loop would carry on and print every match up to 1000. Try deleting it to see.

Filtering with continue

continue is handy for skipping items you don’t care about, so the main work isn’t buried inside an if. Add up the numbers from 1 to 20, but skip multiples of 3:

total = 0
for n in range(1, 21):
    if n % 3 == 0:
        continue
    total += n
print(total)

You could write the same thing without continue, by putting total += n inside if n % 3 != 0:. Both are correct. continue reads well when there are several reasons to skip an item, each on its own line at the top of the loop.

while True … break

Sometimes the natural place to decide whether to stop is in the middle of the loop, not at the top. A common pattern is a loop that would run forever, while True:, with a break inside it:

balance = 100
months = 0
while True:
    balance = balance * 1.05
    months += 1
    if balance >= 150:
        break
print(f"After {months} months the balance is {balance:.2f}")

while True on its own would be an infinite loop, because True is always true. The break is the way out. When you use this pattern, make sure the break can actually be reached.

break only leaves one loop

If loops are nested, break and continue affect only the innermost loop they’re in:

for row in range(1, 4):
    for col in range(1, 4):
        if col > row:
            break
        print(row, col)

The break ends the inner col loop each time, but the outer row loop carries on.

Your turn

answer = None
checked = 0
for n in range(1, 1000):
    checked += 1
    # if n fits every rule, store it in answer and stop the loop

print(answer, "found after checking", checked, "numbers")

Check yourself