gh-144412: Require tuple args in pickle REDUCE

Lib/pickle.py

@@ -1724,6 +1724,8 @@ def load_reduce(self):
         stack = self.stack
         args = stack.pop()
         func = stack[-1]
+        if not isinstance(args, tuple):
+            raise TypeError("argument list must be a tuple")
         stack[-1] = func(*args)
     dispatch[REDUCE[0]] = load_reduce
 

Lib/test/pickletester.py

@@ -1435,6 +1435,7 @@ def test_bad_reduce(self):
         self.assertEqual(self.loads(b'cbuiltins\nint\n)R.'), 0)
         self.check_unpickling_error(TypeError, b'N)R.')
         self.check_unpickling_error(TypeError, b'cbuiltins\nint\nNR.')
+        self.check_unpickling_error(TypeError, b'cbuiltins\nint\nNR.')
 
     def test_bad_newobj(self):
         error = (pickle.UnpicklingError, TypeError)

Misc/NEWS.d/next/Library/2026-02-03-12-00-00.gh-issue-144412.q7KpXg.rst

@@ -0,0 +1,2 @@
+``pickle.load_reduce()`` now requires a tuple for ``REDUCE`` arguments to
+match ``_pickle``; non-tuple argument lists now raise ``TypeError``.