Dict .get Type Checking

[ztnel]

Hey guys,

I reviewed my code and am pretty confident my typing implementation is correct. I have a dictionary with keys bound to type str | int and values bound to a generic type var _S which is further bound to 3 python object models. I defined a getter for getting an element from the dictionary and typed the return as follows:

def get(self, _id: KeyTypes) -> Optional[_S]:
    return self.registry.get(_id)

However pylance yields the following error on the return line:

Expression of type "_S@Registry | None" cannot be assigned to return type "_S@Registry | None"
  Type "_S@Registry | None" cannot be assigned to type "_S@Registry | None"
    Type "Tower*" cannot be assigned to type "_S@Registry | None"
      Type "Tower*" cannot be assigned to type "_S@Registry"
      Type cannot be assigned to type "None"
    Type "Protocol*" cannot be assigned to type "_S@Registry | None"
      Type "Protocol*" cannot be assigned to type "_S@Registry"
      Type cannot be assigned to type "None"
    Type "User*" cannot be assigned to type "_S@Registry | None"

See the full code snippet at the bottom of the issue. Cheers.

Environment data

• Language Server version: Pylance language server 2021.10.0 (pyright d5878534)
• OS and version: MacOS Big Sur 11.2.3 (20D91)
• Python version (& distribution if applicable, e.g. Anaconda): python3.9

Expected behaviour

Should produce no error

Actual behaviour

yields the following error:

Logs

XXX

Code Snippet / Additional information

KeyTypes = Union[str, int]
RegistryTypes = Union[Tower, Protocol, User]
_S = TypeVar('_S', bound=RegistryTypes)

class Registry(Generic[_S]):

    def __init__(self) -> None:
        self.registry = {}

    @property
    def registry(self) -> Dict[KeyTypes, _S]:
        return self.__registry

    @registry.setter
    def registry(self, registry: Dict[KeyTypes, _S]) -> None:
        self.__registry = registry
        self.initialized = True

    def get(self, _id: KeyTypes) -> Optional[_S]:
        return self.registry.get(_id)

[erictraut]

Thanks for the bug report. I agree this is a bug in the type checker. I'll create a tracking bug in the pyright repo and investigate further when I have a chance.

[erictraut]

This will be fixed in the next release.

[bschnurr]

This issue has been fixed in version 2021.10.1, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#2021101-14-october-2021