Value returned by enter_context method of contextlib.ExitStack appears as __NoneType__

[willfrey]

I'm having an issue with the returned value of the enter_context(...) instance method of contextlib.ExitStack.

If you enter the following snippet into VSCode and mouse-over the file variable, the tool-tip says that the inferred type is __NoneType__ but it should be TextIOWrapper.

import contextlib

with contextlib.ExitStack() as stack:
    file = stack.enter_context(open("temporary-file.txt", mode="w"))
    # reveal_type(file)  # Revealed type is 'typing.TextIO*'

You can uncomment the reveal_type(file) line with mypy enabled and it will tell you that the revealed type is typing.TextIO*, which is fine for this case.

I've also observed this behavior with tempfile.NamedTemporaryFile.

Any idea what's going on here?

Thanks!

[CTrando]

After looking at contextlib.pyi, we haven't specialized ContextManager, which is part of typing. That's ongoing in #535.

Your issue in particular however is from my PR #1463. What happened is that according to the typing module in cpython, the default parameter for TypeVar.bound is None, so I used that as the upper bound for the type. It should more realistically be object, or even perhaps just return the type variable itself instead of using None. I'll have a fix out soon, sorry for the error.

[willfrey]

Thanks for your response and pending fix!

[CTrando]

I've made a PR which will fix returning None; however it will take much longer to specialize ContextManager, hopefully this is enough for now.