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[Python-Dev] Inconsistency between PEP492, documentation and behaviour of "async with"

Nick Coghlan ncoghlan at gmail.com
Mon Jun 19 08:27:40 EDT 2017 
On 19 June 2017 at 16:35, Damien George <damien.p.george at gmail.com> wrote:
> Hi all,
>
> Regarding the behaviour of the "async with" statement: it seems that the
> description of it in PEP492, and the language documentation, do not match
> the behaviour of CPython (v3.6.1).
>
> The PEP and the docs here:
> https://www.python.org/dev/peps/pep-0492/#asynchronous-context-managers-and-async-with
> https://docs.python.org/3/reference/compound_stmts.html#async-with
> say that "async with" is equivalent to a particular use of try/except/else.
>
> But the implementation seems more like a try/except/finally, because the
> __aexit__ is always executed, even if a return statement is in the try block
> ("else" won't be executed if there's a "return" in the "try").  Also, as per
> normal "with", the implementation is a bit more complex than
> try/except/finally because you don't want to execute the __aexit__ method
> twice if there is an exception in the try.
>
> Can someone please clarify the exact behaviour of "async with"?

"async with" is expected to behave essentially the same way that
normal "with" does as far as return, break, and continue are concerned
(i.e. calling __aexit__ without an exception set, so it's more like
try/finally than it is try/else).

Would you mind filing a documentation bug for that? We clearly missed
that the semantics described in the new documentation didn't actually
match the original with statement semantics (even though matching
those semantics is the intended behaviour).

Regards,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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