Make singleton class instance dict unique per subclass (#318)

Co-authored-by: Bernát Gábor <gaborjbernat@gmail.com>

src/filelock/_api.py

@@ -8,7 +8,7 @@
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
 from threading import local
-from typing import TYPE_CHECKING, Any, ClassVar
+from typing import TYPE_CHECKING, Any
 from weakref import WeakValueDictionary
 
 from ._error import Timeout
@@ -77,7 +77,7 @@ class ThreadLocalFileContext(FileLockContext, local):
 class BaseFileLock(ABC, contextlib.ContextDecorator):
     """Abstract base class for a file lock object."""
 
-    _instances: ClassVar[WeakValueDictionary[str, BaseFileLock]] = WeakValueDictionary()
+    _instances: WeakValueDictionary[str, BaseFileLock]
 
     def __new__(  # noqa: PLR0913
         cls,
@@ -100,6 +100,11 @@ def __new__(  # noqa: PLR0913
 
         return instance  # type: ignore[return-value] # https://github.com/python/mypy/issues/15322
 
+    def __init_subclass__(cls, **kwargs: dict[str, Any]) -> None:
+        """Setup unique state for lock subclasses."""
+        super().__init_subclass__(**kwargs)
+        cls._instances = WeakValueDictionary()
+
     def __init__(  # noqa: PLR0913
         self,
         lock_file: str | os.PathLike[str],

tests/test_filelock.py

@@ -14,6 +14,7 @@
 from types import TracebackType
 from typing import TYPE_CHECKING, Any, Callable, Iterator, Tuple, Type, Union
 from uuid import uuid4
+from weakref import WeakValueDictionary
 
 import pytest
 
@@ -687,3 +688,17 @@ def test_singleton_locks_are_deleted_when_no_external_references_exist(
     assert lock_type._instances == {str(lock_path): lock}  # noqa: SLF001
     del lock
     assert lock_type._instances == {}  # noqa: SLF001
+
+
+@pytest.mark.skipif(hasattr(sys, "pypy_version_info"), reason="del() does not trigger GC in PyPy")
+@pytest.mark.parametrize("lock_type", [FileLock, SoftFileLock])
+def test_singleton_instance_tracking_is_unique_per_subclass(lock_type: type[BaseFileLock]) -> None:
+    class Lock1(lock_type):  # type: ignore[valid-type, misc]
+        pass
+
+    class Lock2(lock_type):  # type: ignore[valid-type, misc]
+        pass
+
+    assert isinstance(Lock1._instances, WeakValueDictionary)  # noqa: SLF001
+    assert isinstance(Lock2._instances, WeakValueDictionary)  # noqa: SLF001
+    assert Lock1._instances is not Lock2._instances  # noqa: SLF001