bpo-42308: Add threading.__excepthook__ (GH-23218)

Add threading.__excepthook__ to allow retrieving the original value
of threading.excepthook in case it is set to a broken or a different
value.

1 files changed, 21 insertions, 0 deletions

diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index e0e5406..db440d4 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -1352,6 +1352,27 @@ class ExceptHookTests(BaseTestCase):
                          'Exception in threading.excepthook:\n')
         self.assertEqual(err_str, 'threading_hook failed')
 
+    def test_original_excepthook(self):
+        def run_thread():
+            with support.captured_output("stderr") as output:
+                thread = ThreadRunFail(name="excepthook thread")
+                thread.start()
+                thread.join()
+            return output.getvalue()
+
+        def threading_hook(args):
+            print("Running a thread failed", file=sys.stderr)
+
+        default_output = run_thread()
+        with support.swap_attr(threading, 'excepthook', threading_hook):
+            custom_hook_output = run_thread()
+            threading.excepthook = threading.__excepthook__
+            recovered_output = run_thread()
+
+        self.assertEqual(default_output, recovered_output)
+        self.assertNotEqual(default_output, custom_hook_output)
+        self.assertEqual(custom_hook_output, "Running a thread failed\n")
+
 
 class TimerTests(BaseTestCase):