3.5. Loop Control Keywords

For extra control over how a loop operates, we have the break and continue keywords.

3.5.1. Exiting a Loop with break

Sometimes you don’t know it’s time to end a loop until you get half way through the body. In that case you can write an infinite loop on purpose and then use the break statement to jump out of the loop.

This loop is an infinite loop because the logical expression on the while statement is simply the logical constant True:

n = 10
while True:
    print(n)
    n = n - 1
print('Done!')

This program will run forever, or at least until your computer loses power, because the loop’s condition is always true by virtue of the fact that the expression is the constant value True.

While this is a dysfunctional infinite loop, we can still use this pattern to build useful loops as long as we carefully add code to the body of the loop to explicitly exit the loop using break when we have reached the exit condition.

For example, suppose you want to take input from the user until they type done. You could write:

while True: line = input("Input (type 'done' to stop): ") if line == 'done': break print(line) print('Done!')

The loop condition is True, which is always true, so the loop always repeats when it reaches the end of its body, but if it hits the break statement it leaves the loop immediately.

Each time through the loop, it prompts the user. If the user types done, then the break statement is reached, which exits the loop. Otherwise the program echoes whatever the user types and goes back to the top of the loop. Try it out!

This way of writing while loops is common because you can check the condition anywhere in the loop (not just at the top) and you can express the stop condition affirmatively (“stop when this is true”) rather than negatively (“keep going until this is not true.”).

3.5.2. Skipping to the Next Iteration with continue

Sometimes you are in an iteration of a loop and want to finish the current iteration and immediately jump to the next iteration. In that case you can use the continue statement to skip to the next iteration without finishing the body of the loop for the current iteration.

Here is an example of a loop that copies its input until the user types “done”, but treats lines that start with the hash character as lines not to be printed (kind of like Python comments).

while True: line = input("Input (start with '#' to skip, type 'done' to stop): ") if line.startswith('#'): continue if line == 'done': break print(line) print('Done!')

Here, all lines input by the user are printed except any that start with the hash sign, because when the continue is executed, it ends the current iteration and jumps back to the while statement to check the condition and start the next iteration, thus skipping the print statement.