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

Barry Warsaw barry at python.org
Thu Oct 17 20:48:59 CEST 2013

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, redirectstdout).

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 = opentnx() 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

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