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

Damien George damien.p.george at gmail.com
Mon Jun 19 02:35:45 EDT 2017 
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"?

Background: in implementing "async with" in MicroPython, we went by the
PEP/docs, and now our behaviour doesn't match that of CPython.

Cheers,
Damien.
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