bpo-29376: Fix assertion error in threading._DummyThread.is_alive() (GH-330)

Lib/test/test_threading.py

@@ -170,6 +170,9 @@ class ThreadTests(BaseTestCase):
         mutex.acquire()
         self.assertIn(tid, threading._active)
         self.assertIsInstance(threading._active[tid], threading._DummyThread)
+        #Issue 29376
+        self.assertTrue(threading._active[tid].is_alive())
+        self.assertRegex(repr(threading._active[tid]), '_DummyThread')
         del threading._active[tid]
 
     # PyThreadState_SetAsyncExc() is a CPython-only gimmick, not (currently)

Lib/threading.py

@@ -1217,6 +1217,10 @@ class _DummyThread(Thread):
     def _stop(self):
         pass
 
+    def is_alive(self):
+        assert not self._is_stopped and self._started.is_set()
+        return True
+
     def join(self, timeout=None):
         assert False, "cannot join a dummy thread"
 

Misc/NEWS

@@ -69,6 +69,8 @@ Extension Modules
 Library
 -------
 
+- bpo-29376: Fix assertion error in threading._DummyThread.is_alive().
+
 - bpo-28624: Add a test that checks that cwd parameter of Popen() accepts
   PathLike objects.  Patch by Sayan Chowdhury.