[Python-Dev] pre-PEP: Resource-Release Support for Generators (original) (raw)

Samuele Pedroni pedronis at bluewin.ch
Sat Sep 13 10🔞51 EDT 2003

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.

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