[Python-Dev] On suppress()'s trail blazing (was Re: cpython: Rename contextlib.ignored() to contextlib.ignore())

[Barry Warsaw]

On Oct 17, 2013, at 08:40 PM, Xavier Morel wrote:

>I think there's already a significant split between context managers
>which handle the lifecycle of a local resource (file, transaction) and
>those which purport to locally alter global-ish state (cwd,
>decimal.localcontext, logging.captureWarnings, redirect_stdout).
>
>And the latter worries me (much more than the very localized behavior of
>suppress) because I don't see any way to implement them safely and
>correctly when mixing it with coroutines in today's Python (some of them
>aren't even thread-safe), all of that while I expect coroutines will see
>significantly more use in the very near future with yield from and
>tulip's promotion of coroutine-style async.

Although slightly orthogonal to my previous distinction, this is a great
point.  Anything affecting global state will have this problem of course.
It's definitely something to keep thinking about.

>> Just look at the implementation to see this shift in perspective.  It
>> doesn't use try/finally, it uses try/except.
>
>Transaction CMs will also use try/except:
>
>    @contextmanager
>    def transaction():
>        resource = open_tnx()
>        try:
>            yield resource
>            resource.commit()
>        except:
>            resource.rollback()
>            raise

True.  I'm sure lots of us have code almost exactly like this. ;) For me, it's
closer to the original intent though because the bare-except combined with the
re-raising of the exception feels kind of like a finally.  In both exit paths,
the resource is being released, it's just that you have to know which
"release" operation to perform.

Cheers,
-Barry