Deriving from multiprocessing.get_context().Process #9860

Activity

1. AlexWaygood commented on Mar 9, 2023

   

   AlexWaygood

   on Mar 9, 2023

   Member

   More actions

   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 :/
2. AlexWaygood commented on Mar 9, 2023

   

   AlexWaygood

   on Mar 9, 2023

   Member

   More actions

   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
3. AlexWaygood

   closed this as not plannedon Mar 9, 2023
4. marcelm commented on Mar 9, 2023

   

   marcelm

   on Mar 9, 2023

   Last edited by marcelm

   Author

   More actions

   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.

   ❤️React with ❤️1Reacted by Alex Waygood
5. AlexWaygood commented on Mar 9, 2023

   

   AlexWaygood

   on Mar 9, 2023

   Member

   More actions

   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)

   👍React with 👍1Reacted by Marcel Martin