feat: Add support for paramless function variants

Author: ilotoki0804

src/fieldenum/_fieldenum.py

@@ -276,7 +276,7 @@ def __call__(self, *args, **kwargs):
 
 
 class _FunctionVariant(Variant):  # MARK: FunctionVariant
-    __slots__ = ("_func", "_signature", "_match_args", "_self_included")
+    __slots__ = ("_func", "_signature", "_match_args", "_self_included", "_paramless")
     name: str
 
     def __init__(self, func: types.FunctionType) -> None:
@@ -285,6 +285,7 @@ def __init__(self, func: types.FunctionType) -> None:
         self._func = func
         signature = inspect.signature(func)
         parameters_raw = signature.parameters
+        self._paramless = not parameters_raw or len(parameters_raw) == 1 and "self" in parameters_raw
         self._signature = signature
 
         parameters_iter = iter(parameters_raw)
@@ -326,8 +327,13 @@ def attach(
         self._base = cls
         item = self
 
+        if self._paramless:
+            meta = dict(metaclass=ParamlessSingletonMeta)
+        else:
+            meta = {}
+
         # fmt: off
-        class ConstructedVariant(cls):
+        class ConstructedVariant(cls, **meta):
             if frozen and not typing.TYPE_CHECKING:
                 __slots__ = tuple(f"__original_{name}" for name in item._slots_names)
                 for name in item._slots_names:

tests/test_fieldenum.py

@@ -60,6 +60,10 @@ def KwargsOnlyFuncVariant(*, a, b, c, d=None):
     def ParamlessFuncVariantWithBody(self):
         pass
 
+    @variant
+    def ParamlessFuncVariantWithoutBody(self):
+        pass
+
 
 def test_unreachable():
     with pytest.raises(UnreachableError, match="Unexpected type 'str' of 'hello'"):
@@ -212,6 +216,17 @@ def test_relationship():
     assert isinstance(Message.Pause(), Message)
 
 
+def test_fieldless_function():
+    # with pytest.raises(TypeError, match="unhashable type:"):
+    #     {Message.ParamlessFuncVariantWithBody()}
+
+    # with pytest.raises(TypeError, match="unhashable type:"):
+    #     {Message.ParamlessFuncVariantWithoutBody()}
+
+    assert Message.ParamlessFuncVariantWithBody() is Message.ParamlessFuncVariantWithBody()
+    assert Message.ParamlessFuncVariantWithoutBody() is Message.ParamlessFuncVariantWithoutBody()
+
+
 def test_instancing():
     type MyTypeAlias = int | str | bytes