python – Breaking out of nested loops

The Question :

317 people think this question is useful

Is there an easier way to break out of nested loops than throwing an exception? (In Perl, you can give labels to each loop and at least continue an outer loop.)

for x in range(10):
    for y in range(10):
        print x*y
        if x*y > 50:
            "break both loops"

I.e., is there a nicer way than:

class BreakIt(Exception): pass

try:
    for x in range(10):
        for y in range(10):
            print x*y
            if x*y > 50:
                raise BreakIt
except BreakIt:
    pass

The Question Comments :
  • You could also import “sys” and when you want to end the program write sys.exit()
  • @LauroSkr Not if you want to do something afterward.

The Answer 1

168 people think this answer is useful

It has at least been suggested, but also rejected. I don’t think there is another way, short of repeating the test or re-organizing the code. It is sometimes a bit annoying.

In the rejection message, Mr van Rossum mentions using return, which is really sensible and something I need to remember personally. 🙂

The Answer 2

788 people think this answer is useful
for x in xrange(10):
    for y in xrange(10):
        print x*y
        if x*y > 50:
            break
    else:
        continue  # only executed if the inner loop did NOT break
    break  # only executed if the inner loop DID break

The same works for deeper loops:

for x in xrange(10):
    for y in xrange(10):
        for z in xrange(10):
            print x,y,z
            if x*y*z == 30:
                break
        else:
            continue
        break
    else:
        continue
    break

The Answer 3

74 people think this answer is useful

If you’re able to extract the loop code into a function, a return statement can be used to exit the outermost loop at any time.

def foo():
    for x in range(10):
        for y in range(10):
            print(x*y)
            if x*y > 50:
                return
foo()

If it’s hard to extract that function you could use an inner function, as @bjd2385 suggests, e.g.

def your_outer_func():
    ...
    def inner_func():
        for x in range(10):
            for y in range(10):
                print(x*y)
                if x*y > 50:
                    return
    inner_func()
    ...

The Answer 4

46 people think this answer is useful

Use itertools.product!

from itertools import product
for x, y in product(range(10), range(10)):
    #do whatever you want
    break

Here’s a link to itertools.product in the python documentation: http://docs.python.org/library/itertools.html#itertools.product

You can also loop over an array comprehension with 2 fors in it, and break whenever you want to.

>>> [(x, y) for y in ['y1', 'y2'] for x in ['x1', 'x2']]
[
    ('x1', 'y1'), ('x2', 'y1'),
    ('x1', 'y2'), ('x2', 'y2')
]

The Answer 5

36 people think this answer is useful

Sometimes I use a boolean variable. Naive, if you want, but I find it quite flexible and comfortable to read. Testing a variable may avoid testing again complex conditions and may also collect results from several tests in inner loops.

    x_loop_must_break = False
    for x in range(10):
        for y in range(10):
            print x*y
            if x*y > 50:
                x_loop_must_break = True
                break
        if x_loop_must_break: break

The Answer 6

21 people think this answer is useful

If you’re going to raise an exception, you might raise a StopIteration exception. That will at least make the intent obvious.

The Answer 7

8 people think this answer is useful

You can also refactor your code to use a generator. But this may not be a solution for all types of nested loops.

The Answer 8

3 people think this answer is useful

In this particular case, you can merge the loops with a modern python (3.0 and probably 2.6, too) by using itertools.product.

I for myself took this as a rule of thumb, if you nest too many loops (as in, more than 2), you are usually able to extract one of the loops into a different method or merge the loops into one, as in this case.

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