@@ -2108,6 +2108,15 @@ def test_mock_open_after_eof(self):
         self.assertEqual([], h.readlines())
         self.assertEqual([], h.readlines())
 
+    def test_mock_open_exit_with_contextlib_exit_stack(self):
+        # gh-150484: mock_open's __exit__ should work when called from
+        # contextlib.ExitStack, which passes (exctype, excinst, exctb).
+        from contextlib import ExitStack
+        with mock.patch('builtins.open', mock.mock_open()) as m:
+            with ExitStack() as exit_stack:
+                with exit_stack.enter_context(open('/tmp/test.txt', 'w')):
+                    pass
+
     def test_mock_parents(self):
         for Klass in Mock, MagicMock:
             m = Klass()

@@ -3010,7 +3010,7 @@ def _next_side_effect():
             return handle.readline.return_value
         return next(_state[0])
 
-    def _exit_side_effect(exctype, excinst, exctb):
+    def _exit_side_effect(*args):
         handle.close()
 
     global file_spec

diff --git a/Misc/NEWS.d/next/Library/2026-05-28-00-00-00.gh-issue-150484.XxYyZz.rst b/Misc/NEWS.d/next/Library/2026-05-28-00-00-00.gh-issue-150484.XxYyZz.rst
@@ -0,0 +1 @@
+Fix :func:`unittest.mock.mock_open` ``__exit__`` raising ``TypeError`` when used with :class:`contextlib.ExitStack`.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix :func:`unittest.mock.mock_open` ``__exit__`` raising ``TypeError`` when used with :class:`contextlib.ExitStack`.