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Inconsistent type inference between open() and Path().open() #2911
Description
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.
Activity
srittau commentedon Apr 12, 2019
Literal types are not supported currently, see #2913. Using
IO[Any]is currently the best we can do.BuongiornoTexas commentedon Apr 15, 2019
Thanks Sebastian - Should we leave this open until literal types are supported?
Regards,
Larry
srittau commentedon Apr 15, 2019
I added the inexpressable tag. We can leave it open so it can be fixed later.
srittau commentedon Jun 14, 2019
Literal types are now supported. Patches welcome!
BuongiornoTexas commentedon Jun 18, 2019
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 commentedon Jun 18, 2019
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.