CIMultiDict has incompatible type Dict[istr, str]

[rugleb]

Long story short

The mypy checks fail if the dictionary keys passed to the constructor are of the istr type.

Expected behaviour

The mypy checks have passed.

Actual behaviour

Error from mypy:

headers.py:5: error: Argument 1 to "CIMultiDict" has incompatible type "Dict[istr, str]"; expected "Union[Mapping[Union[str, istr], Any], Dict[Union[str, istr], Any], MultiMapping[Any], Iterable[Tuple[Union[str, istr], Any]]]"

Steps to reproduce

Run mypy on this script:

from aiohttp import hdrs
from multidict import CIMultiDict, CIMultiDictProxy

headers = CIMultiDictProxy(
    CIMultiDict({
        hdrs.CONTENT_LENGTH: "40",
        hdrs.CONTENT_TYPE: "text/xml",
    })
)

Your environment

Python v3.9.4
aiohttp v3.7.4.post0
multidict v5.1.0
mypy v0.910

[asvetlov]

Would you make a Pull Request with fix?

[rugleb]

Yes!

[rugleb]

I don't quite understand how to fix it.
I thought that i should fix the _Arg type, but it already support Mapping[_S, _T] where _S = Union[str, istr]. So, I'm confused.

[asvetlov]

The PR breaks aiohttp and almost every project that depends on multidict.
Should revert it, sorry.