feat: allow DenoKvError to have no message argument

It used to always require a message string, which made it awkward to
subclass for errors that don't want to define a message at init.

Author: h4l

src/denokv/errors.py

@@ -1,20 +1,22 @@
 from dataclasses import dataclass
 
-from denokv._pycompat.typing import cast
+from denokv._pycompat.typing import TYPE_CHECKING
 
 
 @dataclass(init=False)
 class DenoKvError(Exception):
-    message: str
+    # Define message for dataclass field metadata only, not type annotation.
+    if not TYPE_CHECKING:
+        message: str
 
-    def __init__(self, message: str, *args: object) -> None:
-        super().__init__(message, *args)
-        if not isinstance(message, str):
-            raise TypeError(f"first argument must be a str message: {message!r}")
+    def __init__(self, *args: object) -> None:
+        super(DenoKvError, self).__init__(*args)
 
-    @property  # type: ignore[no-redef]
+    @property
     def message(self) -> str:
-        return cast(str, self.args[0])
+        if args := self.args:
+            return str(args[0])
+        return type(self).__name__
 
 
 class DenoKvValidationError(ValueError, DenoKvError):

test/test_errors.py

@@ -7,3 +7,14 @@ def test_errors_are_regular_exceptions() -> None:
     """Errors must be caught by generic Exception handlers — not BaseException."""
     with pytest.raises(Exception):  # noqa: B017
         raise DenoKvError("error")
+
+
+def test_DenoKvError_message() -> None:
+    assert DenoKvError().message == "DenoKvError"
+    assert DenoKvError("Foo bar").message == "Foo bar"
+
+    class CustomError(DenoKvError):
+        pass
+
+    assert CustomError().message == "CustomError"
+    assert CustomError("Bar baz").message == "Bar baz"