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

[Guido van Rossum]

On Wed, Nov 26, 2014 at 3:15 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

>
> On 27 Nov 2014 03:58, "Paul Moore" <p.f.moore at gmail.com> wrote:
> >
> > On 26 November 2014 at 17:19, Guido van Rossum <guido at python.org> wrote:
> > > 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"
> >
> > Hmm, fair point. I was assuming that the bug in contextlib can't be
> > fixed with the current language behaviour (and I'd personally be OK
> > with the example simply adding a comment "this can't be fixed without
> > changing Python as proposed in the PEP"). But I'm not sure how true
> > that is, so maybe it's not quite as compelling as it seemed to me at
> > first.
>
> The "contextlib only" change would be to map StopIteration in the body of
> the with statement to gen.close() on the underlying generator rather than
> gen.throw(StopIteration). (That's backwards incompatible in its own way,
> since it means you *can't* suppress StopIteration via a generator based
> context manager any more)
>
> This is actually the second iteration of this bug: the original
> implementation *always* suppressed StopIteration. PJE caught that one
> before Python 2.5 was released, but we didn't notice that 3.3 had brought
> it back in a new, more subtle form :(
>
> It's worth noting that my "allow_implicit_stop" idea in the other thread
> wouldn't affect subgenerators - those would still convert StopIteration to
> RuntimeError unless explicitly silenced.
>
You've lost me in this subthread. Am I right to conclude that the PEP
change doesn't cause problems for contextlib(*), but that the PEP change
also probably wouldn't have helped diagnose any contextlib bugs?

(*) Except perhaps that some old 3rd party copy of contextlib may
eventually break if it's not updated.

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