python使用多线程和Lock实现单例时报错AttributeError: __enter__

初学python的多线程，在尝试使用单例，请问为什么会出现下面的情况？

import threading

class Index:
    _lock = threading.Lock
    _index = None

    @staticmethod
    def index():
        if Index._index is None:
            with Index._lock:
                if Index._index is None:
                    Index._index = dict()
        return Index._index


if __name__ == '__main__':
    l = [threading.Thread(target=Index.index()) for i in range(15)]
    for thread in l:
        thread.start()

_lock = threading.Lock()
你少打了括号