Naming Loops in Python

21
votes

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?

4
pythonfor-looplabelbreak
Check the second answer, hopefully it helps some. But from what I can see, no such system exists. - John Doe
"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
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
What's wrong with nested function definitions? Also, what about redesigning the inner loop to avoid the break? - S.Lott
I just don't like them. I guess it's a personal preference - inspectorG4dget

4 Answers

11
votes

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.

17
votes

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
14
votes

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.

1
votes

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.