break vs continue vs pass in Python

All three of them are loop control statements, but they change the loop in completely different ways. break ends the loop, continue moves on to the next round, and pass does nothing at all.

for number in range(1, 8):
    if number == 3:
        continue        # skip the rest of this round
    if number == 6:
        break           # leave the loop completely
    print(number, end=' ')
print()

# pass is a placeholder that does nothing.
class Placeholder:
    pass


for character in 'abc':
    pass                # legal, but it does nothing

The output is 1 2 4 5 followed by a newline. The continue skips the round where the number is 3, so 3 is never printed, and then the loop carries on. The break at the number 6 stops the loop completely, so 6 and 7 are never reached even though the range goes all the way up to 7.

The important subtlety is that continue skips the rest of the current round only. Any code after the continue in the same loop body is skipped for that one pass, and then the loop condition is checked again. If the continue sits at the end of the body it does nothing useful, and if it sits before an important update you can end up with a loop that never ends.

pass is the odd one out here. It exists because Python has no empty block, so an if with no body would be a syntax error. pass gives you a legal empty body. It is a placeholder for code that you have not written yet, for a class that you intend to fill in later, and for the branch of an if chain that only holds a comment.

Both break and continue apply only to the innermost loop that contains them. A break inside a nested loop leaves that inner loop, and then control goes back to the outer loop, which carries on with its next round.

Take a loop over rows that contains a loop over columns, with a break once the column reaches 2. Each row prints (0, 0) and (0, 1), then leaves the inner loop and moves on to the next row. The outer loop still runs all three times, so the inner break has no effect on how many rounds the outer loop takes.

This is the classic problem in nested loop exercises, and it is the reason why Python has no labelled break. In languages such as Java or Ruby you can write break outer to escape more than one level. Python leaves that out on purpose, and the accepted alternatives are to pull the work into a helper function and return, to use a flag variable and check it in the outer loop, or to restructure the data so that one loop replaces two.

continue behaves in the same way, because it applies to the inner loop only. The subtlety there is that continue skips the rest of the inner body, so if the inner loop was the thing that advanced some state that the outer loop depends on, then that update is skipped too.

There is one more interaction to know about. Both a for loop and a while loop can carry an else clause, and that clause runs when the loop finishes without a break. A break skips the else clause completely, and that is what makes the pattern so useful for search loops. The else block is the not-found branch, and it is reached only when the loop ran all the way through without finding anything.