Break and Continue

Leave a loop early, or skip the rest of one pass.

break leaves the loop

Python

for n in [1, 2, 3, 4, 5]:
    if n == 3:
        break
    print(n)

print("out")

Output

1
2
out

The loop stops the moment break runs. Nothing after it in that pass happens, and there are no more passes.

The usual reason is that you have found what you were looking for:

Python

names = ["Ada", "Grace", "Katherine"]

for name in names:
    if name.startswith("G"):
        print("first G name:", name)
        break

Output

first G name: Grace

continue skips the rest of this pass

Python

for n in range(6):
    if n % 2 == 0:
        continue
    print(n)

Output

1
3
5

continue is a guard clause for loops. It lets you deal with the cases you do not care about and keep the real work unindented:

Python

rows = ["", "ada,36", "# a comment", "grace,45"]

for row in rows:
    if not row:
        continue
    if row.startswith("#"):
        continue
    name, age = row.split(",")
    print(name, age)

Output

ada 36
grace 45

Written with nested if statements instead, that body would be two levels deeper and harder to follow.

They only affect the nearest loop

In nested loops, break leaves the inner one only:

Python

for row in range(3):
    for col in range(3):
        if col == 1:
            break
        print(row, col)

Output

0 0
1 0
2 0

To leave both, the usual options are a flag, a function with return, or the for...else construction:

Python

def find(grid, target):
    for r, row in enumerate(grid):
        for c, value in enumerate(row):
            if value == target:
                return r, c
    return None

grid = [[1, 2], [3, 4]]
print(find(grid, 4))
print(find(grid, 9))

Output

(1, 1)
None

Putting the search in a function is usually the clearest of the three, because return leaves everything at once.

break and the else clause

A loop's else runs only if no break happened. That pairing is what makes for...else useful:

Python

def has_factor(n):
    for d in range(2, n):
        if n % d == 0:
            print(n, "divides by", d)
            break
    else:
        print(n, "is prime")

has_factor(9)
has_factor(7)

Output

9 divides by 3
7 is prime

continue in a while loop

The trap worth repeating: continue jumps straight back to the condition, so anything that advances the counter must come before it.

Python

n = 0
while n < 5:
    n += 1
    if n == 3:
        continue
    print(n)

Output

Move n += 1 below the continue and the loop never ends.

pass is not the same thing

pass does nothing and carries on with the rest of the body. continue skips the rest of the body:

Python

for n in range(3):
    if n == 1:
        pass
    print(n)

Output

0
1
2

Python

for n in range(3):
    if n == 1:
        continue
    print(n)

Output

0
2

Test yourself

2 questions

In nested loops, what does break leave?

Show the answer

The innermost loop only — To leave both, use a flag, or put the search in a function and return.

Why is 'continue' dangerous in a while loop?

Show the answer

It jumps back to the condition, so it can skip the line that advances the counter — Put the increment before the continue and the problem disappears.

Next chapter

Functions

Name a piece of work once and run it whenever you need it.