Fix detail::obj_class_name() to work correctly for meta classes. (#4436)

* Fix `detail::obj_class_name()` to work correctly for meta classes.

* Adjust expected name for PyPy

include/pybind11/pytypes.h

@@ -463,7 +463,7 @@ PYBIND11_NAMESPACE_BEGIN(detail)
 
 // Equivalent to obj.__class__.__name__ (or obj.__name__ if obj is a class).
 inline const char *obj_class_name(PyObject *obj) {
-    if (Py_TYPE(obj) == &PyType_Type) {
+    if (PyType_Check(obj)) {
         return reinterpret_cast<PyTypeObject *>(obj)->tp_name;
     }
     return Py_TYPE(obj)->tp_name;

tests/test_class.cpp

@@ -55,6 +55,8 @@ void bind_empty0(py::module_ &m) {
 } // namespace test_class
 
 TEST_SUBMODULE(class_, m) {
+    m.def("obj_class_name", [](py::handle obj) { return py::detail::obj_class_name(obj.ptr()); });
+
     // test_instance
     struct NoConstructor {
         NoConstructor() = default;

tests/test_class.py

@@ -1,10 +1,19 @@
 import pytest
 
-import env  # noqa: F401
+import env
 from pybind11_tests import ConstructorStats, UserType
 from pybind11_tests import class_ as m
 
 
+def test_obj_class_name():
+    if env.PYPY:
+        expected_name = "UserType"
+    else:
+        expected_name = "pybind11_tests.UserType"
+    assert m.obj_class_name(UserType(1)) == expected_name
+    assert m.obj_class_name(UserType) == expected_name
+
+
 def test_repr():
     assert "pybind11_type" in repr(type(UserType))
     assert "UserType" in repr(UserType)

tests/test_pytypes.cpp

@@ -99,6 +99,8 @@ void m_defs(py::module_ &m) {
 } // namespace handle_from_move_only_type_with_operator_PyObject
 
 TEST_SUBMODULE(pytypes, m) {
+    m.def("obj_class_name", [](py::handle obj) { return py::detail::obj_class_name(obj.ptr()); });
+
     handle_from_move_only_type_with_operator_PyObject::m_defs(m);
 
     // test_bool

tests/test_pytypes.py

@@ -9,6 +9,12 @@ from pybind11_tests import detailed_error_messages_enabled
 from pybind11_tests import pytypes as m
 
 
+def test_obj_class_name():
+    assert m.obj_class_name(None) == "NoneType"
+    assert m.obj_class_name(list) == "list"
+    assert m.obj_class_name([]) == "list"
+
+
 def test_handle_from_move_only_type_with_operator_PyObject():  # noqa: N802
     assert m.handle_from_move_only_type_with_operator_PyObject_ncnst()
     assert m.handle_from_move_only_type_with_operator_PyObject_const()