Cannot access instance attribute through class object

[dineshbvadhia]

import threading
from typing import Any, Hashable, Type, TypeVar

T = TypeVar("T", bound="Singleton")

class Singleton:
    __instances: dict[Any, dict[Hashable, Any]] = {}        # L24
    __singleton_lock = threading.Lock()                     # L25
    __singleton_id__: Hashable

    @classmethod
    def _singleton_id(cls, *args: Any, **kwargs: Any) -> Hashable:
        """ Return a hashable object that identifies the singleton """
        return args, tuple(sorted(kwargs.items()))

    @classmethod
    def _instances(cls) -> dict[Hashable, Any]:
        """ Return all instances of this class """
        return cls.__instances.get(cls, {}).copy()          # L36

The code above runs succesfully through mypy but doesn't with mypyc:

Singleton.py:24: warning: Unsupported default attribute value
Singleton.py:25: warning: Unsupported default attribute value
Singleton.py:36: error: Cannot access instance attribute "__instances" through class object
Singleton.py:36: note: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)

Tbh, I'm lost in the woods here :(

[erictraut]

Since this issue is specific to mypyc, it's probably best to report it in the mypyc issue tracker rather than here (the general python/typing discussion forum).

[AlexWaygood]

The final line of the error message mypy gives you here says:

Singleton.py:36: note: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)

In this case, that would mean doing this:

 import threading
-from typing import Any, Hashable, Type, TypeVar
+from typing import Any, Final, Hashable, Type, TypeVar

 T = TypeVar("T", bound="Singleton")

 class Singleton:
-    __instances: dict[Any, dict[Hashable, Any]] = {}        # L24
-    __singleton_lock = threading.Lock()                     # L25
+    __instances: Final[dict[Any, dict[Hashable, Any]]] = {}        # L24
+    __singleton_lock: Final = threading.Lock()                     # L25
     __singleton_id__: Hashable

     @classmethod
     def _singleton_id(cls, *args: Any, **kwargs: Any) -> Hashable:
         """ Return a hashable object that identifies the singleton """
         return args, tuple(sorted(kwargs.items()))

     @classmethod
     def _instances(cls) -> dict[Hashable, Any]:
         """ Return all instances of this class """
         return cls.__instances.get(cls, {}).copy()          # L36

[dineshbvadhia]

Perfect - thank-you.

It is odd mypy didn't pick up this up.

I'll post future mypyc issues on their tracker. Best ...

[AlexWaygood]

Yeah, there are certain things that mypy allows even in --strict mode, but that mypyc does not allow. See also python/mypy#15757.

[dineshbvadhia]

Thanks for the link.

Some feedback: My application has 18+ program modules. mypy processed all successfully. mypyc generated 2 errors - the one above and another wrt async generators (posted earlier on the mypyc issue tracker) - which I guess is pretty good going :)