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Re: Nested iteration?

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On Tue, Apr 23, 2013 at 10:30 AM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Tue, Apr 23, 2013 at 10:21 AM, Chris Angelico <rosuav@gmail.com> wrote:
>> The definition of the for loop is sufficiently simple that this is
>> safe, with the caveat already mentioned (that __iter__ is just
>> returning self). And calling next() inside the loop will simply
>> terminate the loop if there's nothing there, so I'd not have a problem
>> with code like that - for instance, if I wanted to iterate over pairs
>> of lines, I'd happily do this:
>>
>> for line1 in f:
>>   line2=next(f)
>>   print(line2)
>>   print(line1)
>>
>> That'll happily swap pairs, ignoring any stray line at the end of the
>> file. Why bother catching StopIteration just to break?
>
> The next() there will *not* "simply terminate the loop" if it raises a
> StopIteration; for loops do not catch StopIteration exceptions that
> are raised from the body of the loop.  The StopIteration will continue
> to propagate until it is caught or it reaches the sys.excepthook.  In
> unusual circumstances, it is even possible that it could cause some
> *other* loop higher in the stack to break (i.e. if the current code is
> being run as a result of the next() method being called by the looping
> construct).

To expand on this, the prevailing wisdom here is that calls to next()
should always be guarded with a StopIteration exception handler.  The
one exception to this is when the next() call is inside the body of a
generator function, and the exception handler would cause the
generator to exit anyway; in that case there is little difference
between "except StopIteration: return" and letting the StopIteration
propagate to the generator object.

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Nested iteration? roy@panix.com (Roy Smith) - 2013-04-23 11:40 -0400
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 17:05 +0100
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:05 -0600
  Re: Nested iteration? Peter Otten <__peter__@web.de> - 2013-04-23 18:15 +0200
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:21 +1000
  Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:35 +0000
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:30 -0600
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:39 -0600
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:42 +1000
    Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:53 +0000
  Re: Nested iteration? Terry Jan Reedy <tjreedy@udel.edu> - 2013-04-23 16:49 -0400
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:14 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 22:29 +0100
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:41 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 23:42 +0100

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