set – 判断是否为空 states = set(["mt", "wa", "or", "id", "nv", "ut", "ca", "az"]) # 传入的数组被转换为集合 while states: # OK # while len(states) > 0: # OK # while len(states) is not None: # Error!!! 会进入死循环!!! # while states is not None: # Error!!! 会进入死循环!!! ...