在Python中经过双重检查的锁定线程安全吗？

在双重检查锁定习语是不是在某些语言可靠，我想知道的Python是否是其中之一。更具体地说，是以下代码...

# Objects shared by threads:
obj = None
lock_for_obj = threading.Lock()

def get_obj():
    """Function called concurrently by threads."""
    if obj is None:
        with lock_for_obj:
            if obj is None:
                obj = factory()  # Never returns `None`
    return obj