gh-106236: Replace assert with raise RuntimeError in threading.py (#106237)

Replace `assert` with `raise ` in `threading.py` so that -OO does not alter _DummyThread behavior.

Lib/test/test_threading.py

@@ -251,6 +251,14 @@ class ThreadTests(BaseTestCase):
         #Issue 29376
         self.assertTrue(threading._active[tid].is_alive())
         self.assertRegex(repr(threading._active[tid]), '_DummyThread')
+
+        # Issue gh-106236:
+        with self.assertRaises(RuntimeError):
+            threading._active[tid].join()
+        threading._active[tid]._started.clear()
+        with self.assertRaises(RuntimeError):
+            threading._active[tid].is_alive()
+
         del threading._active[tid]
 
     # PyThreadState_SetAsyncExc() is a CPython-only gimmick, not (currently)

Lib/threading.py

@@ -1451,11 +1451,12 @@ class _DummyThread(Thread):
         pass
 
     def is_alive(self):
-        assert not self._is_stopped and self._started.is_set()
+        if not self._is_stopped and self._started.is_set():
             return True
+        raise RuntimeError("thread is not alive")
 
     def join(self, timeout=None):
-        assert False, "cannot join a dummy thread"
+        raise RuntimeError("cannot join a dummy thread")
 
 
 # Global API functions

Misc/NEWS.d/next/Library/2023-06-29-15-10-44.gh-issue-106236.EAIX4l.rst

@@ -0,0 +1,2 @@
+Replace ``assert`` statements with ``raise RuntimeError`` in
+:mod:`threading`, so that ``_DummyThread`` cannot be joined even with ``-OO``.