Inconsistency between PEP492, documentation and behaviour of "async with"
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-... 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.
On 19 June 2017 at 16:35, Damien George
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-... 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@gmail.com | Brisbane, Australia
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).
Ok, bug filed at: http://bugs.python.org/issue30707
participants (2)
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Damien George -
Nick Coghlan