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I recently read this question which had a solution about labeling loops in Java.

I am wondering if such a loop-naming system exists in Python. I have been in a situation multiple times where I do need to break out of an outer for loop from an inner for loop. Usually, I solve this problem by putting the inner loop in a function that returns (among others) a boolean which is used as a breaking condition. But labeling loops for breaking seems a lot simpler and I would like to try that, if such functionality exists in python.

Does anyone know if it does?

Benjamin Loison
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inspectorG4dget
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  • Check the [second answer](http://stackoverflow.com/questions/653509/breaking-out-of-nested-loops), hopefully it helps some. But from what I can see, no such system exists. – John Doe Dec 07 '11 at 17:37
  • "But labeling loops for breaking seems a lot simpler "? Simpler than proper functions? How so? Can you provide some evidence of how this would be "simpler"? – S.Lott Dec 07 '11 at 18:38
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    I don't have an example handy, but it would be a situation where I would have to create a function which will be used only in that one spot. Perhaps "simpler" was the wrong word. What I meant was that I wouldn't have to define a new function just for use in that ONE spot. – inspectorG4dget Dec 07 '11 at 18:46
  • What's wrong with nested function definitions? Also, what about redesigning the inner loop to avoid the `break`? – S.Lott Dec 07 '11 at 19:01
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    I just don't like them. I guess it's a personal preference – inspectorG4dget Dec 07 '11 at 20:43

4 Answers4

19

Here's a way to break out of multiple, nested blocks using a context manager:

import contextlib

@contextlib.contextmanager
def escapable():
    class Escape(RuntimeError): pass
    class Unblock(object):
        def escape(self):
            raise Escape()

    try:
        yield Unblock()
    except Escape:
        pass

You can use it to break out of multiple loops:

with escapable() as a:
    for i in xrange(30):
        for j in xrange(30):
            if i * j > 6:
                a.escape()

And you can even nest them:

with escapable() as a:
    for i in xrange(30):
        with escapable() as b:
            for j in xrange(30):
                if i * j == 12:
                    b.escape()  # Break partway out
                if i * j == 40:
                    a.escape()  # Break all the way out
Stu Gla
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    yeah, that is extremely more readable than simply doing a loop with; `break 2` (php) or `continue loop1;` (javascript).. also, how pythonic is this indent hell spaghetti code really? Using `raise StopIteration` seems to be the cleanest, most readable, pythonic solution – Stof Aug 08 '21 at 07:08
19

There was a proposal to include named loops in python PEP3136, however, it was rejected with an explanation here. The rejection was mostly due to the rare number of circumstances where code readability would be improved by including this construct.

GWW
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16

Though there are reasons to include named looped in language construct you can easily avoid it in python without loss of readability. An implementation of the referred example in python

>>> try:
    for i in xrange(0,5):
        for j in xrange(0,6):
            if i*j > 6:
                print "Breaking"
                raise StopIteration
            print i," ",j
except StopIteration:
    print "Done"


0   0
0   1
0   2
0   3
0   4
0   5
1   0
1   1
1   2
1   3
1   4
1   5
2   0
2   1
2   2
2   3
Breaking
Done
>>>

I solve this problem by putting the inner loop in a function that returns (among others) a boolean which is used as a breaking condition.

I think you should try this. This is very pythonic, simple and readable.

Abhijit
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1

Nope.

Depending on what you are doing, there is good chance you can use something from itertools to flatten your nested for loops into a single for loop.

Winston Ewert
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