Is double-checked locking thread-safe in Python?
Have a look at PEP 583 - Concurrency memory model for Python, which was withdrawn.
I guess the reason it was withdrawn (though I can't find enough information on that) is since there are different implementations of Python, and it's difficult to enforce a standard like this on every one of them.
Conclusion: this code might be safe when using CPython implementation on a single processor, or it might be safe to use it in general using Jython implementation, but there's no guarantee.