Raise error on incompatible singleton timeout and mode args

src/filelock/_api.py

@@ -82,8 +82,8 @@ class BaseFileLock(ABC, contextlib.ContextDecorator):
     def __new__(  # noqa: PLR0913
         cls,
         lock_file: str | os.PathLike[str],
-        timeout: float = -1,  # noqa: ARG003
-        mode: int = 0o644,  # noqa: ARG003
+        timeout: float = -1,
+        mode: int = 0o644,
         thread_local: bool = True,  # noqa: ARG003, FBT001, FBT002
         *,
         is_singleton: bool = False,
@@ -97,6 +97,9 @@ def __new__(  # noqa: PLR0913
         if not instance:
             instance = super().__new__(cls)
             cls._instances[str(lock_file)] = instance
+        elif timeout != instance.timeout or mode != instance.mode:
+            msg = "Singleton lock instances cannot be initialized with differing arguments"
+            raise ValueError(msg)
 
         return instance  # type: ignore[return-value] # https://github.com/python/mypy/issues/15322
 
@@ -174,6 +177,11 @@ def timeout(self, value: float | str) -> None:
         """
         self._context.timeout = float(value)
 
+    @property
+    def mode(self) -> int:
+        """:return: the file permissions for the lockfile"""
+        return self._context.mode
+
     @abstractmethod
     def _acquire(self) -> None:
         """If the file lock could be acquired, self._context.lock_file_fd holds the file descriptor of the lock file."""

tests/test_filelock.py

@@ -676,6 +676,17 @@ def test_singleton_locks_are_distinct_per_lock_file(lock_type: type[BaseFileLock
     assert lock_1 is not lock_2
 
 
+@pytest.mark.parametrize("lock_type", [FileLock, SoftFileLock])
+def test_singleton_locks_must_be_initialized_with_the_same_args(lock_type: type[BaseFileLock], tmp_path: Path) -> None:
+    lock_path = tmp_path / "a"
+    lock = lock_type(str(lock_path), is_singleton=True)  # noqa: F841
+
+    with pytest.raises(ValueError, match="Singleton lock instances cannot be initialized with differing arguments"):
+        lock_type(str(lock_path), timeout=10, is_singleton=True)
+    with pytest.raises(ValueError, match="Singleton lock instances cannot be initialized with differing arguments"):
+        lock_type(str(lock_path), mode=0, is_singleton=True)
+
+
 @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_locks_are_deleted_when_no_external_references_exist(