handle arg=None in stubgenc (#18768)

resolves #18757

Author: asottile

mypy/stubgenc.py

@@ -6,6 +6,7 @@
 
 from __future__ import annotations
 
+import enum
 import glob
 import importlib
 import inspect
@@ -211,6 +212,9 @@ def __get__(self) -> None:  # noqa: PLE0302
         pass
 
 
+_Missing = enum.Enum("_Missing", "VALUE")
+
+
 class InspectionStubGenerator(BaseStubGenerator):
     """Stub generator that does not parse code.
 
@@ -310,12 +314,12 @@ def get_annotation(key: str) -> str | None:
 
         # Add the arguments to the signature
         def add_args(
-            args: list[str], get_default_value: Callable[[int, str], object | None]
+            args: list[str], get_default_value: Callable[[int, str], object | _Missing]
         ) -> None:
             for i, arg in enumerate(args):
                 # Check if the argument has a default value
                 default_value = get_default_value(i, arg)
-                if default_value is not None:
+                if default_value is not _Missing.VALUE:
                     if arg in annotations:
                         argtype = annotations[arg]
                     else:
@@ -330,11 +334,11 @@ def add_args(
                 else:
                     arglist.append(ArgSig(arg, get_annotation(arg), default=False))
 
-        def get_pos_default(i: int, _arg: str) -> Any | None:
+        def get_pos_default(i: int, _arg: str) -> Any | _Missing:
             if defaults and i >= len(args) - len(defaults):
                 return defaults[i - (len(args) - len(defaults))]
             else:
-                return None
+                return _Missing.VALUE
 
         add_args(args, get_pos_default)
 
@@ -345,11 +349,11 @@ def get_pos_default(i: int, _arg: str) -> Any | None:
         elif kwonlyargs:
             arglist.append(ArgSig("*"))
 
-        def get_kw_default(_i: int, arg: str) -> Any | None:
-            if kwonlydefaults:
-                return kwonlydefaults.get(arg)
+        def get_kw_default(_i: int, arg: str) -> Any | _Missing:
+            if kwonlydefaults and arg in kwonlydefaults:
+                return kwonlydefaults[arg]
             else:
-                return None
+                return _Missing.VALUE
 
         add_args(kwonlyargs, get_kw_default)
 

mypy/test/teststubgen.py

@@ -856,6 +856,30 @@ class TestClassVariableCls:
         assert_equal(gen.get_imports().splitlines(), ["from typing import ClassVar"])
         assert_equal(output, ["class C:", "    x: ClassVar[int] = ..."])
 
+    def test_generate_c_type_none_default(self) -> None:
+        class TestClass:
+            def test(self, arg0=1, arg1=None) -> None:  # type: ignore[no-untyped-def]
+                pass
+
+        output: list[str] = []
+        mod = ModuleType(TestClass.__module__, "")
+        gen = InspectionStubGenerator(mod.__name__, known_modules=[mod.__name__], module=mod)
+        gen.is_c_module = False
+        gen.generate_function_stub(
+            "test",
+            TestClass.test,
+            output=output,
+            class_info=ClassInfo(
+                self_var="self",
+                cls=TestClass,
+                name="TestClass",
+                docstring=getattr(TestClass, "__doc__", None),
+            ),
+        )
+        assert_equal(
+            output, ["def test(self, arg0: int = ..., arg1: Incomplete | None = ...) -> None: ..."]
+        )
+
     def test_non_c_generate_signature_with_kw_only_args(self) -> None:
         class TestClass:
             def test(