gh-138890: Add clearer message when deleting builtins with del

Lib/test/test_scope.py

@@ -835,5 +835,23 @@ def g(self, __arg):
         with self.assertRaises(TypeError):
             closure(_MultiplyNested__arg=2)
 
+    def test_builtin_deletion_err_message(self):
+        with self.assertRaisesRegex(NameError, "cannot delete builtin 'all'"):
+            del all
+
+        def f():
+            del all
+
+        with self.assertRaisesRegex(UnboundLocalError, "cannot delete builtin 'all'"):
+            f()
+
+        def g():
+            global all
+            del all
+
+        with self.assertRaisesRegex(NameError, "cannot delete builtin 'all'"):
+            g()
+
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-09-18-01-13-22.gh-issue-138890.x2QKfy.rst

@@ -0,0 +1 @@
+Deleting built-in names with :keyword:`del` now gives a clearer error message.

Python/bytecodes.c

@@ -1577,9 +1577,13 @@ dummy_func(
             err = PyObject_DelItem(ns, name);
             // Can't use ERROR_IF here.
             if (err != 0) {
-                _PyEval_FormatExcCheckArg(tstate, PyExc_NameError,
-                                          NAME_ERROR_MSG,
-                                          name);
+                if (PyMapping_HasKeyWithError(BUILTINS(), name) == 1) {
+                    _PyEval_FormatExcCheckArg(tstate, PyExc_NameError,
+                                              CANNOT_DELETE_BUILTIN_ERROR_MSG, name);
+                } else {
+                    _PyEval_FormatExcCheckArg(tstate, PyExc_NameError,
+                                              NAME_ERROR_MSG, name);
+                }
                 ERROR_NO_POP();
             }
         }
@@ -1720,8 +1724,13 @@ dummy_func(
                 ERROR_NO_POP();
             }
             if (err == 0) {
-                _PyEval_FormatExcCheckArg(tstate, PyExc_NameError,
-                                          NAME_ERROR_MSG, name);
+                if (PyMapping_HasKeyWithError(BUILTINS(), name) == 1) {
+                    _PyEval_FormatExcCheckArg(tstate, PyExc_NameError,
+                                              CANNOT_DELETE_BUILTIN_ERROR_MSG, name);
+                } else {
+                    _PyEval_FormatExcCheckArg(tstate, PyExc_NameError,
+                                              NAME_ERROR_MSG, name);
+                }
                 ERROR_NO_POP();
             }
         }
@@ -1888,10 +1897,15 @@ dummy_func(
         inst(DELETE_FAST, (--)) {
             _PyStackRef v = GETLOCAL(oparg);
             if (PyStackRef_IsNull(v)) {
-                _PyEval_FormatExcCheckArg(tstate, PyExc_UnboundLocalError,
-                    UNBOUNDLOCAL_ERROR_MSG,
-                    PyTuple_GetItem(_PyFrame_GetCode(frame)->co_localsplusnames, oparg)
-                );
+                PyObject *localsplusnames = _PyFrame_GetCode(frame)->co_localsplusnames;
+                PyObject *name = PyTuple_GetItem(localsplusnames, oparg);
+                if (PyMapping_HasKeyWithError(BUILTINS(), name) == 1) {
+                    _PyEval_FormatExcCheckArg(tstate, PyExc_UnboundLocalError,
+                                              CANNOT_DELETE_BUILTIN_ERROR_MSG, name);
+                } else {
+                    _PyEval_FormatExcCheckArg(tstate, PyExc_UnboundLocalError,
+                                              UNBOUNDLOCAL_ERROR_MSG, name);
+                }
                 ERROR_IF(true);
             }
             _PyStackRef tmp = GETLOCAL(oparg);

Python/ceval_macros.h

@@ -303,6 +303,7 @@ GETITEM(PyObject *v, Py_ssize_t i) {
     "cannot access free variable '%s' where it is not associated with a value" \
     " in enclosing scope"
 #define NAME_ERROR_MSG "name '%.200s' is not defined"
+#define CANNOT_DELETE_BUILTIN_ERROR_MSG "cannot delete builtin '%.200s'"
 
 // If a trace function sets a new f_lineno and
 // *then* raises, we use the destination when searching