[Python-ideas] Yield-From: Finalization guarantees

Guido van Rossum guido at python.org
Wed Apr 1 18:20:10 CEST 2009

On Wed, Apr 1, 2009 at 4:28 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> You appear to be thinking of GeneratorExit as a way to ask a generator
> to finish normally such that it still makes sense to try to return a
> value after a GeneratorExit has been thrown in to the current frame, but
> that really isn't its role.
> Instead, it's more of an "Abandon Ship! Abandon Ship! All hands to the
> lifeboats!" indication that gives the generator a chance to release any
> resources it might be holding and bail out. The reason that close()
> accepts a StopIteration as well as a GeneratorExit is that the former
> still indicates that the generator has finalised itself, so the
> objective of calling close() has been achieved and there is no need to
> report an error.
> Any code that catches GeneratorExit without reraising it is highly
> suspect, just like code that suppresses SystemExit and KeyboardInterrupt.

Let's make that "without either returning from the generator without
yielding any more values, raising StopIteration, or re-raising
GeneratorExit." At least one example in PEP 342 catches GeneratorExit
and returns.

--Guido van Rossum (home page: http://www.python.org/~guido/)

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