[Python-Dev] pre-PEP: Resource-Release Support for Generators

[Samuele Pedroni]

At 14:46 13.09.2003 +0100, Armin Rigo wrote:
>Hello,
>
>(My reply didn't seem to have reached python-dev... second try.)
>
>On Tue, Aug 26, 2003 at 01:50:52PM -0700, Guido van Rossum wrote:
> > Another comment on Samuele's PEP: It is sort of sad that the *user* of
> > a generator has to know that the generator's close() must be called.
> > Normally, the beauty of using a try/finally for cleanup is that your
> > callers don't need to know about it.  But I see no way around this.
>
>What about letting the 'for' handle this? It's the most common way generators
>are used. When a 'for' loop on a generator-iterator finishes it would call the
>close() method of the iterator, which on generators would (say) simulate a
>return from the latest 'yield'.
>
>Well, close() might not be such a good name because it would probably break
>exsiting code (e.g. closing files unexpectedly), but __exit__() might do. In
>other words we could import some of the proposed functionality of the 'with'
>keyword (PEP 310) into 'for'. I think it makes sense because 'for' is already
>defined in term of implicit calls to next(), the only method of iterators; so
>if a second method is added, 'for' can be taught about it too.

I expect generators to grow also a __exit__ if they grow a close and PEP 
310 is accepted,
this idiom will then be common

with g = gen()
  for v in g:
   ...

but it will be common for files and file-like too. It can be reasonable to 
discuss
whether we want a special syntax then that conflates 'with' and 'for' behavior.
But we can experiment a while before seeing whether writing the idiom is 
unbearably
tedious ;).

OTOH conflating 'with' and 'for' just for generators seems a rather ad-hoc 
breaking of
orthoganility of the two, you could not write anymore code like this:

g = gen()
for v in g:
   ... do something up to a point ...
...
for v in g:
   ...

now this is rare but still breaking orthoganility of primitives is 
something I would think twice about.

regards.