Cannot use None for generic return value of method

[anuraaga]

Describe the bug

I am trying to create a generic protocol that passes through any value from one method to another. This is optional, so return None aka return should be allowed - in such a case, the idea is for None to be propagated. Assuming None isn't part of the default bound of a type variable, I would at least expect bound = Any | None to work, but it seems that None is always not allowed for a T.

/Users/anuraag/git/connecpy/src/connecpy/_interceptor_sync.py
  /Users/anuraag/git/connecpy/src/connecpy/_interceptor_sync.py:80:9 - error: Type "None" is not assignable to return type "T@MetadataInterceptorSync"
    Type "None" is not assignable to type "T@MetadataInterceptorSync" (reportReturnType)

Code or Screenshots
If possible, provide a minimal, self-contained code sample (surrounded by triple back ticks) to demonstrate the issue. The code should define or import all referenced symbols.

from typing import (
    Protocol,
    TypeVar,
    runtime_checkable,
)

T = TypeVar("T", bound = Any | None) # Same without bound or bound of Any only

@runtime_checkable
class MetadataInterceptorSync(Protocol[T]):
    def on_start_sync(self, ctx: ServiceContext) -> T:
        """Called when the RPC starts. The return value will be passed to on_end as-is.
        For example, if measuring RPC invocation time, on_start may return the current
        time.
        """
        return # Type error here

    def on_end_sync(self, token: T, ctx: ServiceContext) -> None:
        """Called when the RPC ends."""
        return

If your code relies on symbols that are imported from a third-party library, include the associated import statements and specify which versions of those libraries you have installed.

VS Code extension or command-line

CLI command, v1.1.403

[erictraut]

Moving to a discussion topic since this is a question, not a bug report.

[erictraut]

You should never use Any as an upper bound to a type variable, so that immediately jumps out at me as a problem in your code.

I don't have enough context about the problem you're trying to solve to offer a definitive solution. Based on what you've shared, it sounds like you want a return type of T | None.

class ServiceContext: ...

class MetadataInterceptorSync[T]:
    def on_start_sync(self, ctx: ServiceContext) -> T | None:
        return

    def on_end_sync(self, token: T, ctx: ServiceContext) -> None:
        return

[anuraaga]

T | None doesn't work because then on_start returns a different type than on_end takes - the token can't be passed to on_end in that case. Making on_end also T | None doesn't work since the all uses of the token need to check for None in on_end even though the contract is that the same value is passed from on_start and such a check shouldn't be needed. The idea is for the user to define the type they want to propagate between the two methods, and None should be allowed as that type.

You should never use Any as an upper bound to a type variable, so that immediately jumps out at me as a problem in your code.

This mindset seems related - I can understand most use cases not wanting to use Any as a bound but propagating a user type while allowing None seems like a use case for it and I guess currently it's not supported?

[anuraaga]

Nevermind, I realized my misunderstanding - I was hoping to provide a default implementation for on_start but that's not possible in a type safe way since T isn't known in the protocol. Either that's avoided entirely, just making on_end optional, or because it can't be type-safe casting to skip checking is the only option.

Sorry for the noise.