Python break and continue

3Python break & continue: explained step by step

Most loops run from the first item to the last, or until their condition turns False. break and continue let you change that from inside the body: one ends the loop early, the other jumps ahead to the next round. Used well, they remove awkward nesting. Used carelessly, they make a loop hard to follow, so this lesson also covers when each one fits.

break: stop the whole loop

break means "I have what I need, leave the loop now". The moment it runs, Python jumps to the first line after the loop, and the items you have not reached yet are never visited. That is exactly what you want for a search-and-stop: once you have found the answer, looking at the rest is wasted work.

inbox = ["Newsletter", "Invoice #204", "Lunch?", "Invoice #205"]
for subject in inbox:
    if subject.startswith("Invoice"):
        print("First invoice:", subject)
        break
print("Search finished, subject is still", subject)

Notice the last line. A loop variable is not erased when the loop ends, so after a break it still holds the item you stopped on. Only trust that if you know the loop really did stop on a match.

continue: skip this item and carry on

continue means "not this one": the rest of the body is skipped for the current item, and the loop moves straight on to the next item. Nothing is stopped, the loop just does less work for this round.

entries = ["42", "n/a", "17", "", "8"]
total = 0
for text in entries:
    if not text.isdigit():
        continue
    total += int(text)
print("Total:", total)

Putting the rejection first and then calling continue keeps the normal case at a single level of indentation. The alternative is to wrap the whole body in if text.isdigit():, which works but pushes the real work further to the right with every extra rule you add.

Which one do I need?

Ask yourself: if this item is bad or uninteresting, should the remaining items still be processed? If yes, use continue. If the answer is "no, we are done", use break. Mixing the two up is the most common slip, and it rarely produces an error message, only a wrong result.

In while loops

Both keywords work in while loops, with one twist. continue jumps back to the condition check, so everything after it in the body is skipped, including the line that moves the loop toward its end. If that line sits after the continue, the loop can get stuck:

# Do not run this: it never ends
n = 0
while n < 5:
    if n == 2:
        continue      # n is still 2, so the check passes again, forever
    print(n)
    n += 1

It prints 0 and 1, then spins silently, because n stays at 2. The rule: do the update before any continue, or take items from a queue so that every round moves forward on its own.

while True: combined with break is the usual way to work through a queue of unknown length. The loop looks endless, but each exit is spelled out inside it:

jobs = ["resize logo", "", "send email", "STOP", "backup db"]
while True:
    if not jobs:
        print("Queue empty")
        break
    job = jobs.pop(0)
    if job == "":
        continue
    if job == "STOP":
        print("Stop signal received,", len(jobs), "job(s) left")
        break
    print("Running:", job)

Here pop(0) removes the first job, so every round makes progress and the continue on a blank entry is safe.

break only leaves the innermost loop

When one loop sits inside another, break ends only the loop it is written in. The outer loop carries on with its next item.

teams = [["Asha", "Ben"], ["Chen", "Dina", "Eli"], ["Fay"]]
for team in teams:
    for member in team:
        if member == "Dina":
            print("Found Dina, leaving the inner loop")
            break
        print("Checked", member)
    print("Done with team")

Eli is never checked, but the outer loop still visits Fay's team. If you want to stop everything once something is found, you need to tell the outer loop too.

A flag variable as an alternative

A flag is a variable, usually True or False, that records whether something happened. Set it right before the inner break, then check it in the outer loop and break there as well. A flag is also handy in a single loop when you need to know afterwards whether the search succeeded.

teams = [["Asha", "Ben"], ["Chen", "Dina", "Eli"], ["Fay"]]
found = False
for team in teams:
    for member in team:
        print("Checking", member)
        if member == "Dina":
            found = True
            break
    if found:
        break
print("Found Dina:", found)

A last word of advice: one or two break or continue lines in a loop read well. If you need several, step back and see whether the loop can be simplified.