Fix _interpreters.set___main___attrs SystemError when passing non-dict

Author: brianschubert

Lib/test/test__interpreters.py

@@ -485,6 +485,10 @@ def test_signatures(self):
         msg = r'_interpreters.run_func\(\) argument 3 must be dict, not int'
         with self.assertRaisesRegex(TypeError, msg):
             _interpreters.run_func(self.id, lambda: None, shared=1)
+        # See https://github.com/python/cpython/issues/135855
+        msg = r'_interpreters.set___main___attrs\(\) argument 2 must be dict, not int'
+        with self.assertRaisesRegex(TypeError, msg):
+            _interpreters.set___main___attrs(self.id, 1)
 
     def test_invalid_shared_encoding(self):
         # See https://github.com/python/cpython/issues/127196

Misc/NEWS.d/next/Library/2025-06-23-10-19-11.gh-issue-135855.-J0AGF.rst

@@ -0,0 +1,3 @@
+Raise :exc:`TypeError` instead of :exc:`SystemError` when
+:func:`!_interpreters.set___main___attrs` is passed a non-dict object.
+Patch by Brian Schubert.

Modules/_interpretersmodule.c

@@ -1039,8 +1039,8 @@ interp_set___main___attrs(PyObject *self, PyObject *args, PyObject *kwargs)
     PyObject *id, *updates;
     int restricted = 0;
     if (!PyArg_ParseTupleAndKeywords(args, kwargs,
-                                     "OO|$p:" MODULE_NAME_STR ".set___main___attrs",
-                                     kwlist, &id, &updates, &restricted))
+                                     "OO!|$p:" MODULE_NAME_STR ".set___main___attrs",
+                                     kwlist, &id, &PyDict_Type, &updates, &restricted))
     {
         return NULL;
     }
@@ -1055,13 +1055,13 @@ interp_set___main___attrs(PyObject *self, PyObject *args, PyObject *kwargs)
 
     // Check the updates.
     if (updates != Py_None) {
-        Py_ssize_t size = PyObject_Size(updates);
+        Py_ssize_t size = PyDict_Size(updates);
         if (size < 0) {
             return NULL;
         }
         if (size == 0) {
             PyErr_SetString(PyExc_ValueError,
-                            "arg 2 must be a non-empty mapping");
+                            "arg 2 must be a non-empty dict");
             return NULL;
         }
     }