Nested iteration?

[Ian Kelly]

On Tue, Apr 23, 2013 at 10:30 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Tue, Apr 23, 2013 at 10:21 AM, Chris Angelico <rosuav at 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.