Inconsistent type inference between open() and Path().open()

[BuongiornoTexas]

I have run into an issue with inconsistent type-inferences for open() and pathlib.Path().open().

I've previously raised this as possible mypy bug #6666. In response to this gvanrossum advised:

Right now we have a plugin that special-cases builtins.open() but not Path.open(). This should be solved in typeshed by making the definition of Path.open() overloaded on various Literal types. So I am closing this as "not a mypy bug" but I recommend that you submit a PR to typeshed to add this.

So I am re-raising this issue here. The following snippet demonstrates the issue.

#!/usr/bin/env python3

from pathlib import Path
fp = open('README.rst', 'rt')
print(type(fp))
fp1 = Path('README.rst').open('rt')
print(type(fp1))

yields:

<class '_io.TextIOWrapper'>
<class '_io.TextIOWrapper'>

However, running mypy reveal_type on fp and fp1 yields:

Revealed type is 'typing.TextIO'
Revealed type is 'typing.IO[Any]'

I would expect both of these to reveal typing.TextIO. I found this issue when using a routine that switched TextIO output between sys.stdout (TextIO) and a file opened with Path().open() (IO).

This is on python 3.7.2 and mypy 0.700.

I'm happy to develop a PR on this, but it will be a while before I get started and given I've got a bit of learning curve, probably a while longer before I finish. I'm assuming this shouldn't be an issue given I couldn't find any other references to this problem in several hours of combing github issues and the web in general.

[srittau]

Literal types are not supported currently, see #2913. Using IO[Any] is currently the best we can do.

[BuongiornoTexas]

Thanks Sebastian - Should we leave this open until literal types are supported?

Regards,

Larry

[srittau]

I added the inexpressable tag. We can leave it open so it can be fixed later.

[srittau]

Literal types are now supported. Patches welcome!

[BuongiornoTexas]

I'll pull something together in a while - as I'm still a relative python novice, does the typeshed have a style guide (or ideally, something I can refer to as an acceptable style for implementing literal type lookups?).

[JelleZijlstra]

Our CONTRIBUTING.md has a couple of style guidelines. For Literals specifically, we're in mostly uncharted territory, though I think we just merged one PR adding Literals.