autodoc: Add Optional[t] to annotation of function and method

As typing.get_type_hints() doing, this adds Optional[t] to type
annotations if a default value equal to None is set.

Note: this is default behavior of inspect.signature() since Python 3.10.

CHANGES

@@ -22,6 +22,8 @@ Features added
   ``__all__`` attribute of the module should be documented or not via
   :event:`autodoc-skip-member` event
 * autodoc: Add ``Documenter.config`` as a shortcut to access the config object
+* autodoc: Add Optional[t] to annotation of function and method if a default
+  value equal to None is set.
 * #6914: Add a new event :event:`warn-missing-reference` to custom warning
   messages when failed to resolve a cross-reference
 * #6914: Emit a detailed warning when failed to resolve a ``:ref:`` reference

sphinx/util/inspect.py

@@ -470,10 +470,10 @@ def signature(subject: Callable, bound_method: bool = False, follow_wrapped: boo
             raise
 
     try:
-        # Update unresolved annotations using ``get_type_hints()``.
+        # Resolve forwared reference annotations using ``get_type_hints()`` and type_aliases.
         annotations = typing.get_type_hints(subject, None, type_aliases)
         for i, param in enumerate(parameters):
-            if isinstance(param.annotation, str) and param.name in annotations:
+            if param.name in annotations:
                 parameters[i] = param.replace(annotation=annotations[param.name])
         if 'return' in annotations:
             return_annotation = annotations['return']

tests/test_ext_autodoc_configs.py

@@ -490,7 +490,7 @@ def test_autodoc_typehints_signature(app):
         '.. py:module:: target.typehints',
         '',
         '',
-        '.. py:class:: Math(s: str, o: Any = None)',
+        '.. py:class:: Math(s: str, o: Optional[Any] = None)',
         '   :module: target.typehints',
         '',
         '',

tests/test_util_inspect.py

@@ -162,7 +162,7 @@ def test_signature_annotations():
 
     # Space around '=' for defaults
     sig = inspect.signature(f7)
-    assert stringify_signature(sig) == '(x: int = None, y: dict = {}) -> None'
+    assert stringify_signature(sig) == '(x: Optional[int] = None, y: dict = {}) -> None'
 
     # Callable types
     sig = inspect.signature(f8)
@@ -226,7 +226,7 @@ def test_signature_annotations():
         assert stringify_signature(sig) == '(self) -> List[typing_test_data.Node]'
 
     sig = inspect.signature(Node.__init__)
-    assert stringify_signature(sig) == '(self, parent: Optional[Node]) -> None'
+    assert stringify_signature(sig) == '(self, parent: Optional[typing_test_data.Node]) -> None'
 
     # show_annotation is False
     sig = inspect.signature(f7)
@@ -234,7 +234,7 @@ def test_signature_annotations():
 
     # show_return_annotation is False
     sig = inspect.signature(f7)
-    assert stringify_signature(sig, show_return_annotation=False) == '(x: int = None, y: dict = {})'
+    assert stringify_signature(sig, show_return_annotation=False) == '(x: Optional[int] = None, y: dict = {})'
 
 
 @pytest.mark.skipif(sys.version_info < (3, 8), reason='python 3.8+ is required.')