Deriving from multiprocessing.get_context().Process

[marcelm]

This code:

import multiprocessing

context = multiprocessing.get_context()

class MyProcess(context.Process):
    pass

fails to typecheck with the message error: Name "context.Process" is not defined [name-defined].

I tried to see whether I could contribute a PR, but have to admit I don’t even understand the error message. According to reveal_type, the context variable is of type multiprocessing.context.DefaultContext, but that class appears to have a Process attribute:

typeshed/stdlib/multiprocessing/context.pyi

Lines 149 to 150 in a47dd76

	class DefaultContext(BaseContext):
	Process: ClassVar[type[Process]]

[AlexWaygood]

Mypy's error message here is terrible, but it has a legitimate complaint: you're using an object annotated as a dynamic variable (Process) as a base class. That's problematic from a type-checking point of view.

This is the same issue as:

• Bad error message: mypy reports 'Name is not defined' when using an object member as super class of another class mypy#8603
• Cannot access class defined type aliases from self or cls [name-defined] mypy#14022

I'll check later to see if there's anything we can do to improve things, but I believe the stubs are probably correct here, in which case there's probably not much we can do here :/

[AlexWaygood]

Yes, there's nothing typeshed can do here to fix the error mypy emits on your current code :/

I wondered if changing the annotation here to Process: TypeAlias = Process would help us at all, but it doesn't look like it:

typeshed/stdlib/multiprocessing/context.pyi

Line 150 in a47dd76

Process: ClassVar[type[Process]]

The mypy error message is terrible (and the error itself is probably a pain to work around), but those are issues for mypy, not typeshed.

I guess my best suggestion is to do an if TYPE_CHECKING hack, something like the below:

import multiprocessing
import multiprocessing.context
import typing

context = multiprocessing.get_context()

if typing.TYPE_CHECKING:
    class MyProcess(multiprocessing.context.Process):
        pass
else:
    class MyProcess(context.Process):
        pass

[marcelm]

Thanks! I understand now where the error message comes from. I’ll add a comment to the linked MyPy issue. Naively, this appears to be something that should work (considering this is code that works and that is even in the stdlib).

Thanks also for the workaround; I would not have come up with it myself so quickly.

[AlexWaygood]

I gave a little bit more context here FYI, if that's helpful for understanding why mypy emits this error here!

pallets-eco/flask-sqlalchemy#1112 (comment)