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

Xavier Morel python-dev at masklinn.net
Thu Oct 17 20:40:56 CEST 2013

On 2013-10-17, at 18:06 , Barry Warsaw wrote:

On Oct 18, 2013, at 01:26 AM, Nick Coghlan wrote:

By contrast, suppress() and redirectstdout() are the first general purpose context managers added to contextlib since its incarnation in Python 2.5 (although there have been many various domain specific context manager additions elsewhere in the standard library). There's a fundamental conceptual shift here that's worth exploring more, and which I think was first identified by RDM. Until now, context managers were at their heart (at least IMHO) about managing "resources". A general resource might be an open file, or it might be a database transaction, or even the current working directory. Context managers (as expressed elegantly by the with statement) are used to ensure that a resource acquired for some limited operation is - to Python's best ability - "released" at the end of that operation, no matter what happens. E.g. the file is closed even if there's a write error, or the current working directory is restored to its original location.

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.

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

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