Thank you for testing that. I dug through the change log, and found
bpo-12022:
https://bugs.python.org/issue12022
It *has* been fixed in 3.11, but not mentioned in the What's New document.
Should it be?
On Sun, Sep 5, 2021, 5:49 PM Chris Angelico <ros...@gmail.com> wrote:
> On Mon, Sep 6, 2021 at 9:37 AM Finn Mason <finnjavie...@gmail.com> wrote:
> >
> > Hello all,
> >
> > In Python 3.10 and 3.11, exception tracebacks are being greatly
> improved. I noticed that there's nothing related to a fairly common (in my
> personal experience) cryptic traceback relating to the `with` statement:
> >
> > >>> with ContextManager as ctx:
> > ... # do something with `ctx`
> > ...
> > Traceback (most recent call last):
> > File "<stdin>", line 1, in <module>
> > AttributeError: __enter__
>
> I'm seeing a different message, so it looks like something HAS been
> improved:
>
> Python 3.11.0a0 (heads/main:ed524b4569, Aug 14 2021, 11:29:01) [GCC
> 8.3.0] on linux
> Type "help", "copyright", "credits" or "license" for more information.
> >>> import contextlib
> >>> with contextlib.ExitStack: ...
> ...
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: 'ABCMeta' object does not support the context manager protocol
>
>
> > This occurs when one forgets to use a instance of a context manager
> class and uses the class itself. It's obviously not a very helpful
> traceback. ("Is it not a context manager?" "Is it the wrong class?")
> Something like the following would be better.
> >
> > >>> with ContextManager as ctx:
> > ... # do something with `ctx`
> > ...
> > Traceback (most recent call last):
> > File "<stdin>", line 1, in <module>
> > AttributeError: <type Context manager> is not a context manager. Did you
> mean "with ContextManager()..."?
> >
> > The actual traceback message should probably be more specific than
> "<type Context manager> is not a context manager".
> > Thoughts?
> >
>
> The "did you mean" part depends on or assumes that it can figure out
> that calling it would have given a viable context manager. This could
> be done by probing whether thing.__enter__ exists, but I'm not sure
> that'd be entirely safe. Also, it won't catch those made from
> generators:
>
> >>> @contextlib.contextmanager
> ... def foo(): yield
> ...
> >>> with foo: ...
> ...
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: 'function' object does not support the context manager protocol
> >>> with foo(): ...
> ...
> Ellipsis
> >>> foo
> <function foo at 0x7fdeb0321bc0>
>
>
> In any case, the biggest advantage (IMO) comes from naming the type,
> which will make it easy to distinguish the object from its type.
>
> ChrisA
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