# Breaking out of nested loops [duplicate]

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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
``````

``````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
``````

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. ðŸ™‚

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()
...
``````

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')
]
``````

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
``````

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

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

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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