Fix #8872: autodoc: stacked singledispatches are wrongly rendered

When multiple singledispatch decorators are stacked, the first typehints
are copied to the subsequent definitions unexpectedly.

Now autodoc generates a dummy function not to affect typehints to
subsequent functions.

CHANGES

@@ -25,6 +25,8 @@ Features added
 Bugs fixed
 ----------
 
+* #8872: autodoc: stacked singledispatches are wrongly rendered
+
 Testing
 --------
 

sphinx/ext/autodoc/__init__.py

@@ -1328,12 +1328,12 @@ class FunctionDocumenter(DocstringSignatureMixin, ModuleLevelDocumenter):  # typ
                 if typ is object:
                     pass  # default implementation. skipped.
                 else:
-                    self.annotate_to_first_argument(func, typ)
+                    dispatchfunc = self.annotate_to_first_argument(func, typ)
-
+                    if dispatchfunc:
-                    documenter = FunctionDocumenter(self.directive, '')
+                        documenter = FunctionDocumenter(self.directive, '')
-                    documenter.object = func
+                        documenter.object = dispatchfunc
-                    documenter.objpath = [None]
+                        documenter.objpath = [None]
-                    sigs.append(documenter.format_signature())
+                        sigs.append(documenter.format_signature())
         if overloaded:
             actual = inspect.signature(self.object,
                                        type_aliases=self.config.autodoc_type_aliases)
@@ -1358,28 +1358,34 @@ class FunctionDocumenter(DocstringSignatureMixin, ModuleLevelDocumenter):  # typ
 
         return overload.replace(parameters=parameters)
 
-    def annotate_to_first_argument(self, func: Callable, typ: Type) -> None:
+    def annotate_to_first_argument(self, func: Callable, typ: Type) -> Optional[Callable]:
         """Annotate type hint to the first argument of function if needed."""
         try:
             sig = inspect.signature(func, type_aliases=self.config.autodoc_type_aliases)
         except TypeError as exc:
             logger.warning(__("Failed to get a function signature for %s: %s"),
                            self.fullname, exc)
-            return
+            return None
         except ValueError:
-            return
+            return None
 
         if len(sig.parameters) == 0:
-            return
+            return None
+
+        def dummy():
+            pass
 
         params = list(sig.parameters.values())
         if params[0].annotation is Parameter.empty:
             params[0] = params[0].replace(annotation=typ)
             try:
-                func.__signature__ = sig.replace(parameters=params)  # type: ignore
+                dummy.__signature__ = sig.replace(parameters=params)  # type: ignore
+                return dummy
             except (AttributeError, TypeError):
                 # failed to update signature (ex. built-in or extension types)
-                return
+                return None
+        else:
+            return None
 
 
 class DecoratorDocumenter(FunctionDocumenter):
@@ -2118,13 +2124,13 @@ class MethodDocumenter(DocstringSignatureMixin, ClassLevelDocumenter):  # type:
                 if typ is object:
                     pass  # default implementation. skipped.
                 else:
-                    self.annotate_to_first_argument(func, typ)
+                    dispatchmeth = self.annotate_to_first_argument(func, typ)
-
+                    if dispatchmeth:
-                    documenter = MethodDocumenter(self.directive, '')
+                        documenter = MethodDocumenter(self.directive, '')
-                    documenter.parent = self.parent
+                        documenter.parent = self.parent
-                    documenter.object = func
+                        documenter.object = dispatchmeth
-                    documenter.objpath = [None]
+                        documenter.objpath = [None]
-                    sigs.append(documenter.format_signature())
+                        sigs.append(documenter.format_signature())
         if overloaded:
             if inspect.isstaticmethod(self.object, cls=self.parent, name=self.object_name):
                 actual = inspect.signature(self.object, bound_method=False,
@@ -2158,27 +2164,34 @@ class MethodDocumenter(DocstringSignatureMixin, ClassLevelDocumenter):  # type:
 
         return overload.replace(parameters=parameters)
 
-    def annotate_to_first_argument(self, func: Callable, typ: Type) -> None:
+    def annotate_to_first_argument(self, func: Callable, typ: Type) -> Optional[Callable]:
         """Annotate type hint to the first argument of function if needed."""
         try:
             sig = inspect.signature(func, type_aliases=self.config.autodoc_type_aliases)
         except TypeError as exc:
             logger.warning(__("Failed to get a method signature for %s: %s"),
                            self.fullname, exc)
-            return
+            return None
         except ValueError:
-            return
+            return None
+
         if len(sig.parameters) == 1:
-            return
+            return None
+
+        def dummy():
+            pass
 
         params = list(sig.parameters.values())
         if params[1].annotation is Parameter.empty:
             params[1] = params[1].replace(annotation=typ)
             try:
-                func.__signature__ = sig.replace(parameters=params)  # type: ignore
+                dummy.__signature__ = sig.replace(parameters=params)  # type: ignore
+                return dummy
             except (AttributeError, TypeError):
                 # failed to update signature (ex. built-in or extension types)
-                return
+                return None
+        else:
+            return None
 
 
 class NonDataDescriptorMixin(DataDocumenterMixinBase):

tests/roots/test-ext-autodoc/target/singledispatch.py

@@ -15,6 +15,7 @@ def func(arg, kwarg=None):
 
 
 @func.register(int)
+@func.register(float)
 def _func_int(arg, kwarg=None):
     """A function for int."""
     pass

tests/roots/test-ext-autodoc/target/singledispatchmethod.py

@@ -10,6 +10,7 @@ class Foo:
         pass
 
     @meth.register(int)
+    @meth.register(float)
     def _meth_int(self, arg, kwarg=None):
         """A method for int."""
         pass

tests/test_ext_autodoc.py

@@ -2080,6 +2080,7 @@ def test_singledispatch(app):
         '',
         '',
         '.. py:function:: func(arg, kwarg=None)',
+        '                 func(arg: float, kwarg=None)',
         '                 func(arg: int, kwarg=None)',
         '                 func(arg: str, kwarg=None)',
         '   :module: target.singledispatch',
@@ -2107,6 +2108,7 @@ def test_singledispatchmethod(app):
         '',
         '',
         '   .. py:method:: Foo.meth(arg, kwarg=None)',
+        '                  Foo.meth(arg: float, kwarg=None)',
         '                  Foo.meth(arg: int, kwarg=None)',
         '                  Foo.meth(arg: str, kwarg=None)',
         '      :module: target.singledispatchmethod',
@@ -2125,6 +2127,7 @@ def test_singledispatchmethod_automethod(app):
     assert list(actual) == [
         '',
         '.. py:method:: Foo.meth(arg, kwarg=None)',
+        '               Foo.meth(arg: float, kwarg=None)',
         '               Foo.meth(arg: int, kwarg=None)',
         '               Foo.meth(arg: str, kwarg=None)',
         '   :module: target.singledispatchmethod',

tests/test_ext_autodoc_autofunction.py

@@ -119,6 +119,7 @@ def test_singledispatch(app):
     assert list(actual) == [
         '',
         '.. py:function:: func(arg, kwarg=None)',
+        '                 func(arg: float, kwarg=None)',
         '                 func(arg: int, kwarg=None)',
         '                 func(arg: str, kwarg=None)',
         '   :module: target.singledispatch',