Basically _thread.RLock was added without an equivalent in _dummy_thread. Add that and it will fix the issue. It probably wouldn't be too hard since you have to implement it as if it's single-threaded and there's already a lock class in _dummy_thread to work off of. Heck, functools used to have its own dummy version in https://github.com/python/cpython/commit/a6a4dc816d68df04a7d592e0b6af8c7ecc4d4344 but it got ripped out, although it was extremely basic so I don't know if it would be the best version to put into _dummy_thread.