[Python-Dev] PEP 479: Change StopIteration handling inside generators

[Guido van Rossum]

Can you summarize that in a self-contained form for inclusion in the PEP?

(That was a rhetorical question. :-)

On Wed, Nov 26, 2014 at 12:17 PM, Isaac Schwabacher <ischwabacher at wisc.edu>
wrote:

> On 14-11-26, Guido van Rossum  wrote:
> > On Wed, Nov 26, 2014 at 8:54 AM, Paul Moore wrote:
> >
> > > On 26 November 2014 at 16:24, Isaac Schwabacher wrote:
> > > > This actually leads to a good example of why the PEP is necessary:
> > > [...]
> > >
> > > Oh! If that's the current behaviour, then it probably needs to go into
> > > the PEP as a motivating example. It's far more convincing than most of
> > > the other arguments I've seen. Just one proviso - is it fixable in
> > > contextlib *without* a language change? If so, then it loses a lot of
> > > its value.
> >
> > It's hard to use as an example because the behavior of contextlib is an
> integral part of it -- currently for me the example boils down to "there is
> a bug in contextlib". Maybe it would have been caught earlier with the
> change in the PEP, but when using it as a motivating example you have to
> show the code containing the bug, not just a demonstration.
>
> How is this a bug in contextlib? The example behaves the way it does
> because gen.throw(StopIteration) behaves differently depending on whether
> gen is paused at a yield or a yield from. What *should*
> contextlib.contextmanager do in this instance? It has faithfully forwarded
> the StopIteration raised in the protected block to the generator, and the
> generator has forwarded this to the subgenerator, which has elected to fail
> and report success. The bug is in the subgenerator, because it fails to
> treat StopIteration as an error. But the subgenerator can't in general be
> converted to treat StopIteration as an error, because clearly it's used in
> other places than as a nested context manager (otherwise, it would itself
> be decorated with @contextlib.contextmanager and accessed as such, instead
> of yielded from). And in those places, perhaps it needs to simply allow
> StopIteration to bubble up. And can we factor out the error checking so
> that we don't have to duplicate subgenerator? Well... yes, but it's tricky
> because we'll introduce an extra yield from in the process, so we have to
> put the handling in the subgenerator itself and wrap the
> *non*-context-manager uses.
>
> ijs
>
> > If you want to try though, I'm happy to entertain a pull request for the
> PEP.
>



-- 
--Guido van Rossum (python.org/~guido)
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