On 25 October 2016 at 03:32, Chris Barker <chris.bar...@noaa.gov> wrote: > On Sat, Oct 22, 2016 at 8:22 PM, Nick Coghlan <ncogh...@gmail.com> wrote: >> The big refactoring benefit that this feature would offer over with >> statements is that it doesn't require a structural change to the code >> - it's just wrapping an existing expression in a new function call >> that says "clean this up promptly when the function terminates, even >> if it's still part of a reference cycle, or we're not using a >> reference counting GC". > > hmm -- that would be simpler in one sense, but wouldn't it require a new > function to be defined for everything you might want to do this with? rather > than the same "with" syntax for everything?
Nope, hence the references to contextlib.ExitStack: https://docs.python.org/3/library/contextlib.html#contextlib.ExitStack That's a tool for dynamic manipulation of context managers, so even today you can already write code like this: >>> @with_resource_manager ... def example(rm, *, msg=None, exc=None): ... rm.enter_context(cm()) ... rm.callback(print, "Deferred callback") ... if msg is not None: print(msg) ... if exc is not None: raise exc ... >>> example(msg="Normal return") Enter CM Normal return Deferred callback Exit CM >>> example(exc=RuntimeError("Exception thrown")) Enter CM Deferred callback Exit CM Traceback (most recent call last): ... RuntimeError: Exception thrown The setup code to support it is just a few lines of code: >>> import functools >>> from contextlib import ExitStack >>> def with_resource_manager(f): ... @functools.wraps(f) ... def wrapper(*args, **kwds): ... with ExitStack() as rm: ... return f(rm, *args, **kwds) ... return wrapper ... Plus the example context manager definition: >>> from contextlib import contextmanager >>> @contextmanager ... def cm(): ... print("Enter CM") ... try: ... yield ... finally: ... print("Exit CM") ... So the gist of my proposal (from an implementation perspective) is that if we give frame objects an ExitStack instance (or an operational equivalent) that can be created on demand and will be cleaned up when the frame exits (regardless of how that happens), then we can define an API for adding "at frame termination" callbacks (including making it easy to dynamically add context managers to that stack) without needing to define your own scaffolding for that feature - it would just be a natural part of the way frame objects work. Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
