python version checks don't seem to work with 0.780

[samuelcolvin]

Upgrading to mypy 0.780 from 0.770, I can't find a way to get the following lines to pass with both python 3.6 and 3.7.

If I try ignoring different lines with # type: ignore I get either errors or unused 'type: ignore' comment from one version of python or the other.

if sys.version_info < (3, 7):
    from typing import _ForwardRef as ForwardRef
else:
    from typing import ForwardRef

Is this a problem with 0.780 or am I doing something wrong.

[srittau]

This is a typeshed issue. ForwardRef is only defined for Python 3.7+, since it was added then under that name. The previous _ForwardRef name is not represented. PR for typeshed welcome that adds it.

[samuelcolvin]

okay, but is there any way around this using # type: ignore for both python 3.6 and 3.7?

I don't understand why the errors are different between python versions.

There seems no way to specify a "type ignore" that only applies to one python version.

Linked PR on pydantic pydantic/pydantic#1598

[hauntsaninja]

Basically a dupe of #8823 (typeshed-specific details aside)

You should be able to work around with something like:

from typing import Any, TYPE_CHECKING

if TYPE_CHECKING:
    ForwardRef: Any
else:
    from typing import ...

which isn't ideal, since mypy will now treat ForwardRef as Any.

Edit: Actually, you could do the following:

~/tmp λ cat test1.py                      
import sys
if sys.version_info >= (3, 7):
    from typing import ForwardRef
else:
    import typing
    from typing import cast, Any
    ForwardRef = cast(Any, typing)._ForwardRef

reveal_type(ForwardRef)
~/tmp λ mypy --python-version 3.8 test1.py
test1.py:9: note: Revealed type is 'def (arg: builtins.str, is_argument: builtins.bool =) -> typing.ForwardRef'
~/tmp λ mypy --python-version 3.6 test1.py
test1.py:9: note: Revealed type is 'Any'

[ilevkivskyi]

It doesn't look like there is a (new) action item for mypy here, so closing.

[samuelcolvin]

@hauntsaninja thanks for pointing to the other issue. Unfortunately you ForwardRef = cast(Any, typing)._ForwardRef solution doesn't work - this is currently preventing pydantic from upgrading latest mypy (I've found another work around).