Re: Any way to "subclass" typing.Annotated?

ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:

# The factory function
def AbstractClassVariable(type_: T) -> Any:
   return Annotated[type_, abstract]

  Alright, so now I've got something here that actually compiles!

  Python

from typing import Annotated, TypeVar, Any, Generic

# This is your "abstract" flag. It's just a unique object.
abstract = object()

# A generic type variable to keep things flexible.
T = TypeVar("T")

# Define AbstractClassVariable as a class instead of a function
class AbstractClassVariable(Generic[T]):
    def __class_getitem__(cls, item):
        return Annotated[item, abstract]

class AbstractType(type):
    def __new__(cls, name, bases, namespace):
        for key, value in namespace.items():
            if isinstance(value, type(Annotated)):
                if abstract in value.__metadata__:
                    raise TypeError(f"Abstract class variable '{key}' must be defined")
       
        return super().__new__(cls, name, bases, namespace)

class Foo(object, metaclass=AbstractType):
    acv: AbstractClassVariable[int]