Nested iteration?

[Ian Kelly]

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).