gh-152083: Fix crash clearing a managed dict under low memory

Author: deadlovelll

Lib/test/test_class.py

@@ -1040,5 +1040,35 @@ def __init__(self):
         self.assertFalse(out, msg=out.decode('utf-8'))
         self.assertFalse(err, msg=err.decode('utf-8'))
 
+    @support.nomemtest
+    def test_clear_managed_dict_no_memory_keeps_exception(self):
+        # gh-152083: an exception may already be set when the managed dict is
+        # cleared under low memory. PyErr_FormatUnraisable() must not clear it.
+        code = """if 1:
+            import _testcapi
+
+            class A:
+                def __init__(self):
+                    self.a = 1
+                    self.b = 2
+
+            def f():
+                a = A()
+                a.__dict__
+                return [None] * 1000
+
+            for start in range(120):
+                _testcapi.set_nomemory(start)
+                try:
+                    f()
+                except BaseException:
+                    pass
+            _testcapi.remove_mem_hooks()
+        """
+        rc, out, err = script_helper.assert_python_ok("-c", code)
+        self.assertEqual(rc, 0)
+        self.assertFalse(out, msg=out.decode('utf-8'))
+        self.assertFalse(err, msg=err.decode('utf-8'))
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-07-03-22-32-58.gh-issue-152083.nSAuhO.rst

@@ -0,0 +1,2 @@
+Fix a crash when an object's managed dictionary is cleared under low memory
+while an exception is set.

Objects/dictobject.c

@@ -7944,6 +7944,9 @@ PyObject_ClearManagedDict(PyObject *obj)
             // values. We need to materialize the keys. Nothing can modify
             // this object, but we need to lock the dictionary.
             int err;
+            // gh-152083: an exception may already be set, so keep it.
+            // The OOM path below reports via PyErr_FormatUnraisable() which clears it.
+            PyObject *exc = PyErr_GetRaisedException();
             Py_BEGIN_CRITICAL_SECTION(dict);
             err = detach_dict_from_object(dict, obj);
             Py_END_CRITICAL_SECTION();
@@ -7963,6 +7966,7 @@ PyObject_ClearManagedDict(PyObject *obj)
                 clear_inline_values(_PyObject_InlineValues(obj));
                 Py_END_CRITICAL_SECTION();
             }
+            PyErr_SetRaisedException(exc);
         }
     }
     Py_CLEAR(_PyObject_ManagedDictPointer(obj)->dict);