Recognize Type Expressions in the Context of Generics

[max-muoto]

Currently, Pyright can recognize valid type-expressions, take the following examples:

Valid:

from typing import Any

def klass_generator(klass: type[Any]) -> type[Any]:
   class Wrapper(klass):
      ...

   return Wrapper

Invalid

from typing import Any

def klass_generator(klass: str) -> type[Any]:
   # Expected type expression but received "str"
   class Wrapper(klass):
      ...

   return Wrapper

However, this doesn't apply to generics:

from typing import Any

def klass_decorator[T: type[Any]](klass: T) -> T:
    # Expected type expression...
    class Wrapped(klass):
        ...

    return Wrapped

See this Pyright Playground example.

Mostly creating this to capture the fact that this is missing, but understandable if we want to avoid implementing support until there's more real feedback around it.

[erictraut]

Yes, this should work, so I consider this a bug. The wording of the diagnostic message is also misleading because this isn't a "type expression".

[max-muoto]

Yes, this should work, so I consider this a bug. The wording of the diagnostic message is also misleading because this isn't a "type expression".

Thanks for the quick fix!

[erictraut]

BTW, I generally recommend using type[T] rather than specifying an upper bound of type for T. This tends to work more reliably across type checkers and use cases. You can use this as a workaround while you're waiting for the bug fix.

from typing import cast

def klass_decorator[T](klass: type[T]) -> type[T]:
    class Wrapped(klass):
        ...
    return cast(type[T], Wrapped)

[max-muoto]

BTW, I generally recommend using type[T] rather than specifying an upper bound of type for T. This tends to work more reliably across type checkers and use cases. You can use this as a workaround while you're waiting for the bug fix.

from typing import cast

def klass_decorator[T](klass: type[T]) -> type[T]:
    class Wrapped(klass):
        ...
    return cast(type[T], Wrapped)

Good to know, thanks!

[erictraut]

This is addressed in pyright 1.1.371.