Right now _DummyThread claims to not allow join:

But, it can be easily changed with python -OO mode, which strips assert statements.

The easiest way to check this is:

# ex.py
import threading, _thread

def f(mutex):
    threading.current_thread()
    mutex.release()

mutex = threading.Lock()
mutex.acquire()

tid = _thread.start_new_thread(f, (mutex,))
mutex.acquire()
threading._active[tid].join()
print('done')

python ex.py results in:

Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/ex.py", line 12, in <module>
    threading._active[tid].join()
  File "/Users/sobolev/Desktop/cpython/Lib/threading.py", line 1458, in join
    assert False, "cannot join a dummy thread"
AssertionError: cannot join a dummy thread

But, python -OO ex.py results in:

done

This looks like an important behavior change. I propose to use explicit AssertionError / RuntimeError instead.

I have a PR ready.

Linked PRs

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