gh-130821: Add type info to error messages for __len__ and __sizeof__

Misc/NEWS.d/next/Core_and_Builtins/2026-06-22-10-39-56.gh-issue-130821.b5e011.rst

@@ -0,0 +1,2 @@
+Add type information to error messages for ``__len__()`` and ``__sizeof__()``.
+Patch by tangyuan0821.

Objects/typeobject.c

@@ -10625,8 +10625,8 @@ slot_sq_length(PyObject *self)
     assert(PyLong_Check(res));
     if (_PyLong_IsNegative((PyLongObject *)res)) {
         Py_DECREF(res);
-        PyErr_SetString(PyExc_ValueError,
-                        "__len__() should return >= 0");
+        PyErr_Format(PyExc_ValueError,
+                        "%T.__len__() must return >= 0", self);
         return -1;
     }
 

Python/sysmodule.c

@@ -1933,8 +1933,8 @@ _PySys_GetSizeOf(PyObject *o)
         return (size_t)-1;
 
     if (size < 0) {
-        _PyErr_SetString(tstate, PyExc_ValueError,
-                          "__sizeof__() should return >= 0");
+        _PyErr_Format(tstate, PyExc_ValueError,
+                          "%T.__sizeof__() must return >= 0", o);
         return (size_t)-1;
     }