gh-82062: Fix support of parameter defaults on methods in extension modules (GH-115270)

Now inspect.signature() supports references to the module globals in
parameter defaults on methods in extension modules.  Previously it was
only supported in functions.  The workaround was to specify the fully
qualified name, including the module name.

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 @@ class TestSignatureObject(unittest.TestCase):
                 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;
 }