Merge

Lib/test/test_keywordonlyarg.py

@@ -170,6 +170,12 @@ def test_issue13343(self):
         # used to fail with a SystemError.
         lambda *, k1=unittest: None
 
+    def test_mangling(self):
+        class X:
+            def f(self, *, __a=42):
+                return __a
+        self.assertEqual(X().f(), 42)
+
 def test_main():
     run_unittest(KeywordOnlyArgTestCase)
 

Misc/NEWS

@@ -17,6 +17,8 @@ Core and Builtins
 - Issue #14592: Attempting a relative import w/o __package__ or __name__ set in
   globals raises a KeyError.
 
+- Issue #14607: Fix defaults keyword-only arguments which started with ``__``.
+
 - Issue #10854: The ImportError raised when an extension module on Windows
   fails to import now uses the new path and name attributes from
   Issue #1559549.

Python/compile.c

@@ -1409,7 +1409,11 @@ compiler_visit_kwonlydefaults(struct compiler *c, asdl_seq *kwonlyargs,
         arg_ty arg = asdl_seq_GET(kwonlyargs, i);
         expr_ty default_ = asdl_seq_GET(kw_defaults, i);
         if (default_) {
-            ADDOP_O(c, LOAD_CONST, arg->arg, consts);
+            PyObject *mangled = _Py_Mangle(c->u->u_private, arg->arg);
+            if (!mangled)
+                return -1;
+            ADDOP_O(c, LOAD_CONST, mangled, consts);
+            Py_DECREF(mangled);
             if (!compiler_visit_expr(c, default_)) {
                 return -1;
             }