Improved error message for metaclass incompatibility

Include/internal/pycore_object.h

@@ -878,7 +878,7 @@ static inline int _PyType_SUPPORTS_WEAKREFS(PyTypeObject *type) {
 extern PyObject* _PyType_AllocNoTrack(PyTypeObject *type, Py_ssize_t nitems);
 PyAPI_FUNC(PyObject *) _PyType_NewManagedObject(PyTypeObject *type);
 
-extern PyTypeObject* _PyType_CalculateMetaclass(PyTypeObject *, PyObject *);
+extern PyTypeObject* _PyType_CalculateMetaclass(PyTypeObject *, PyObject *, PyObject *);
 extern PyObject* _PyType_GetDocFromInternalDoc(const char *, const char *);
 extern PyObject* _PyType_GetTextSignatureFromInternalDoc(const char *, const char *, int);
 extern int _PyObject_SetAttributeErrorContext(PyObject *v, PyObject* name);

Lib/test/test_types.py

@@ -2562,5 +2562,19 @@ def collate_results(raw):
         self.assertEqual(interp_results, {})
 
 
+class TestMetaclassConflict(unittest.TestCase):
+    def test_metaclass_conflict_message(self):
+        class MetaFoo(type): pass
+        class MetaBar(type): pass
+        class Bar(metaclass=MetaBar): pass
+        with self.assertRaises(TypeError) as cm:
+            class Foo(Bar, metaclass=MetaFoo): pass
+        msg = str(cm.exception)
+        self.assertIn("Metaclass conflict", msg)
+        self.assertIn("MetaFoo", msg)
+        self.assertIn("MetaBar", msg)
+        self.assertIn("Bar", msg)
+
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-05-28-20-47-06.gh-issue-skip.CO_d25.rst

@@ -0,0 +1 @@
+Metaclass incompatibility error message now more clearly explains the issue including the name of the offending baseclass and the conflicting metaclasses.

Misc/NEWS.d/next/Core_and_Builtins/2025-05-28-metaclass-error.rst

@@ -0,0 +1,2 @@
+Metaclass incompatibility error message now more clearly explains the issue
+including the name of the offending baseclass and the conflicting metaclasses.

Objects/typeobject.c

@@ -4105,13 +4105,14 @@ PyType_SUPPORTS_WEAKREFS(PyTypeObject *type)
 
 /* Determine the most derived metatype. */
 PyTypeObject *
-_PyType_CalculateMetaclass(PyTypeObject *metatype, PyObject *bases)
+_PyType_CalculateMetaclass(PyTypeObject *metatype,
+                            PyObject *bases,
+                            PyObject *name)
 {
     Py_ssize_t i, nbases;
     PyTypeObject *winner;
     PyObject *tmp;
     PyTypeObject *tmptype;
-
     /* Determine the proper metatype to deal with this,
        and check for metatype conflicts while we're at it.
        Note that if some other metatype wins to contract,
@@ -4129,11 +4130,43 @@ _PyType_CalculateMetaclass(PyTypeObject *metatype, PyObject *bases)
             continue;
         }
         /* else: */
-        PyErr_SetString(PyExc_TypeError,
-                        "metaclass conflict: "
-                        "the metaclass of a derived class "
-                        "must be a (non-strict) subclass "
-                        "of the metaclasses of all its bases");
+        PyObject *declared_meta_name = NULL;
+        PyObject *base_class_name = NULL;
+        PyObject *base_meta_name = NULL;
+        PyObject *format_result = NULL;
+
+        declared_meta_name = PyObject_GetAttrString((PyObject *)winner, "__name__");
+        base_class_name = PyObject_GetAttrString(tmp, "__name__");
+        base_meta_name = PyObject_GetAttrString((PyObject *)tmptype, "__name__");
+
+        if (declared_meta_name && base_class_name && base_meta_name) {
+            format_result = PyUnicode_FromFormat(
+                "Metaclass conflict while defining class: '%U'\n"
+                "- Declared metaclass: '%U'\n"
+                "- Incompatible base class: '%U'\n"
+                "- That base is derived from metaclass: '%U'\n"
+                "All base classes must be based on classes derived from the same "
+                "metaclass or a subclass thereof.",
+                name,
+                declared_meta_name,
+                base_class_name,
+                base_meta_name
+            );
+            if (format_result) {
+                PyErr_SetObject(PyExc_TypeError, format_result);
+                Py_DECREF(format_result);
+            }
+        } else {
+            /* fallback */
+            PyErr_SetString(PyExc_TypeError,
+                "metaclass conflict: "
+                "the metaclass of a derived class must be a (non-strict) "
+                "subclass of the metaclasses of all its bases");
+        }
+
+        Py_XDECREF(declared_meta_name);
+        Py_XDECREF(base_class_name);
+        Py_XDECREF(base_meta_name);
         return NULL;
     }
     return winner;
@@ -4917,7 +4950,7 @@ type_new_get_bases(type_new_ctx *ctx, PyObject **type)
 
     // Search the bases for the proper metatype to deal with this
     PyTypeObject *winner;
-    winner = _PyType_CalculateMetaclass(ctx->metatype, ctx->bases);
+    winner = _PyType_CalculateMetaclass(ctx->metatype, ctx->bases, ctx->name);
     if (winner == NULL) {
         return -1;
     }
@@ -5333,10 +5366,19 @@ PyType_FromMetaclass(
     if (!metaclass) {
         metaclass = &PyType_Type;
     }
-    metaclass = _PyType_CalculateMetaclass(metaclass, bases);
+    /* Get resolve the name from 'spec' */
+    PyObject *name = NULL;
+    if (spec && spec->name) {
+        name = PyUnicode_FromString(spec->name);
+    }
+
+    metaclass = _PyType_CalculateMetaclass(metaclass, bases, name);
     if (metaclass == NULL) {
         goto finally;
     }
+    /* Remove name again */
+    Py_XDECREF(name);
+
     if (!PyType_Check(metaclass)) {
         PyErr_Format(PyExc_TypeError,
                      "Metaclass '%R' is not a subclass of 'type'.",

Python/bltinmodule.c

@@ -169,7 +169,7 @@ builtin___build_class__(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
         /* meta is really a class, so check for a more derived
            metaclass, or possible metaclass conflicts: */
         winner = (PyObject *)_PyType_CalculateMetaclass((PyTypeObject *)meta,
-                                                        bases);
+                                                        bases,name);
         if (winner == NULL) {
             goto error;
         }