gh-107810: Improve DeprecationWarning for metaclasses with custom tp_new (GH-107834)

Co-authored-by: Kirill Podoprigora <[email protected]>

Author: cdce8p

Lib/test/test_capi/test_misc.py

@@ -611,7 +611,7 @@ class Base(metaclass=metaclass):
 
         # Class creation from C
         with warnings_helper.check_warnings(
-                ('.*custom tp_new.*in Python 3.14.*', DeprecationWarning),
+                ('.* _testcapi.Subclass .* custom tp_new.*in Python 3.14.*', DeprecationWarning),
                 ):
             sub = _testcapi.make_type_with_base(Base)
         self.assertTrue(issubclass(sub, Base))

Misc/NEWS.d/next/C API/2023-08-10-11-12-25.gh-issue-107810.oJ40Qx.rst

@@ -0,0 +1 @@
+Improve :exc:`DeprecationWarning` for uses of :c:type:`PyType_Spec` with metaclasses that have custom ``tp_new``.

Objects/typeobject.c

@@ -4264,9 +4264,9 @@ _PyType_FromMetaclass_impl(
         if (_allow_tp_new) {
             if (PyErr_WarnFormat(
                     PyExc_DeprecationWarning, 1,
-                    "Using PyType_Spec with metaclasses that have custom "
-                    "tp_new is deprecated and will no longer be allowed in "
-                    "Python 3.14.") < 0) {
+                    "Type %s uses PyType_Spec with a metaclass that has custom "
+                    "tp_new. This is deprecated and will no longer be allowed in "
+                    "Python 3.14.", spec->name) < 0) {
                 goto finally;
             }
         }