Type hints for a function (multiprocessing)

Question

The following function creates an instance of a class that can be shared between processes:

def create_shared_object (constructor: ?) -> ?:

    from multiprocessing.managers import BaseManager;

    BaseManager.register('name', constructor);

    manager = BaseManager();

    manager.start();

    return manager.name();

What are the correct type hints?

Answer 1

Such generated methods of a manager returns an instance of the BaseProxy type.

Depending on the argument to proxytype, the type might change:

def create_shared_object(...):
    BaseManager.register(..., proxytype = CustomProxy)
    ...

class C: ...

t = type(create_shared_object(C))
print(t)  # CustomProxy

When proxytype is not given or None, an AutoProxy class is dynamicaly generated. It is not importable and cannot be used as a type hint (in which case, use BaseProxy).

def create_shared_object(...):
    BaseManager.register(..., proxytype = None)

...

# multiprocessing.managers.AutoProxy[name]
print(t)

# (AutoProxy[name], BaseProxy, object)
print(t.__mro__)

As for constructor, the type BaseManager.register expects is Callable[..., object] | None. Classes are also callable, so it is fine to use this type even if you intend to pass a class.

# From typeshed
class BaseManager:
    ...
    @classmethod
    def register(
        cls,
        typeid: str,
        callable: Callable[..., object] | None = None,
        #         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        proxytype: Any = None,
        exposed: Sequence[str] | None = None,
        method_to_typeid: Mapping[str, str] | None = None,
        create_method: bool = True,
    ) -> None: ...