I used the order I did because it's idiomatic to return the value the user
needs followed by the value the user wants.
On Tue, 13 Jul 2021, 20:24 Serhiy Storchaka, <storch...@gmail.com> wrote:
> 13.07.21 18:59, Thomas Grainger пише:
> > given that it's hard to implement this correctly I think there should be
> a builtins.enter and builtins.aenter for use like this:
> >
> >
> > ```
> > @dataclasses.dataclass
> > class WrapCmgr:
> > _cmgr: ContextManager[T]
> >
> > def __enter__(self) -> Wrapped[T]:
> > exit, value = enter(self._cmgr)
> > self.__exit = exit
> > return wrap(value)
> >
> > def __exit__(self, \, t: Type[BaseException] | None, v:
> BaseException, tb: types.TracebackType) -> bool:
> > return self.__exit(t, v, tb)
> > ```
>
> I considered similar idea when worked on issue12022 and issue44471. The
> only difference is that I added the helper in the operator module, and
> my operator.enter() returned the value and the callback in the different
> order.
>
> def enter(cm):
> # We look up the special methods on the type to match the with
> # statement.
> cls = type(cm)
> try:
> enter = cls.__enter__
> exit = cls.__exit__
> except AttributeError:
> raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object
> does "
> f"not support the context manager protocol")
> from None
> callback = _MethodType(exit, cm)
> return enter(cm), callback
>
> I still investigate possible consequences and alternatives.
>
> Since we need to save the callback as an object attribute in any case,
> one of alternatives is using ExitStack instead of introducing new API.
> It is more heavyweight, but if implemented in C the difference can be
> reduced.
>
> Other alternative is to expose _PyObject_LookupSpecial() at Python
> level. It would be useful not only for __enter__ and __exit__, but for
> other special methods.
>
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