gh-82062: correctly set module for built-in instance methods in inspect.signature()

Lib/inspect.py

@@ -2285,7 +2285,12 @@ def _signature_fromstr(cls, obj, s, skip_bound_arg=True):
 
     module = None
     module_dict = {}
+
     module_name = getattr(obj, '__module__', None)
+    if not module_name:
+        objclass = getattr(obj, '__objclass__', None)
+        module_name = getattr(objclass, '__module__', None)
+
     if module_name:
         module = sys.modules.get(module_name, None)
         if module:

Lib/test/test_inspect/test_inspect.py

@@ -3069,6 +3069,13 @@ def test_signature_on_builtins_no_signature(self):
                 self.assertEqual(inspect.signature(builtin),
                                  inspect.signature(template))
 
+    @unittest.skipIf(MISSING_C_DOCSTRINGS,
+                     "Signature information for builtins requires docstrings")
+    def test_signature_parsing_with_defaults(self):
+        _testcapi = import_helper.import_module("_testcapi")
+        meth = _testcapi.DocStringUnrepresentableSignatureTest.with_default
+        self.assertEqual(str(inspect.signature(meth)), '(self, /, x=1)')
+
     def test_signature_on_non_function(self):
         with self.assertRaisesRegex(TypeError, 'is not a callable object'):
             inspect.signature(42)

Misc/NEWS.d/next/Library/2024-02-11-07-31-43.gh-issue-82062.eeS6w7.rst

@@ -0,0 +1,3 @@
+Fix :func:`inspect.signature()` to correctly handle parameter defaults
+on methods in extension modules that use names defined in the module
+namespace.

Modules/_testcapi/docstring.c

@@ -169,6 +169,13 @@ static PyMethodDef DocStringUnrepresentableSignatureTest_methods[] = {
             "--\n\n"
             "This docstring has a signature with unrepresentable default."
         )},
+    {"with_default",
+        (PyCFunction)test_with_docstring, METH_VARARGS,
+        PyDoc_STR(
+            "with_default($self, /, x=ONE)\n"
+            "--\n\n"
+            "This instance method has a default parameter value from the module scope."
+        )},
     {NULL},
 };
 
@@ -193,5 +200,8 @@ _PyTestCapi_Init_Docstring(PyObject *mod)
     if (PyModule_AddType(mod, &DocStringUnrepresentableSignatureTest) < 0) {
         return -1;
     }
+    if (PyModule_AddObject(mod, "ONE", PyLong_FromLong(1)) < 0) {
+        return -1;
+    }
     return 0;
 }