Looking at Lib/threading.py and Modules/_threadmodule.c, it actually appears that RLock.locked() is only implemented by the C implementation, but not the Python implementation which is used as a backup.

If we're going to make the locked() method more visible by documenting it, we should expose such a method on the Python implementation, so that threading.RLock will indeed always have a locked() method.

From a quick look, it seems that the Python RLock's _is_owned() method might be just what we need, but that should be validated.