#!/usr/bin/env python3class Connection(object):    def __init__(self):        self.change_state(ClosedConnection)    def change_state(self,new_state):        self.__class__ = new_state    def read(self):        raise NotImplementedError("未实现")    def write(self):        raise NotImplementedError("未实现")    def open(self):        raise NotImplementedError("未实现")    def close(self):        raise NotImplementedError("未实现")class OpenedConnection(Connection):    def read(self):        print("read")    def write(self):        print("write")    def open(self):        raise RuntimeError("连接已经打开")    def close(self):        self.change_state(ClosedConnection)class ClosedConnection(Connection):    def read(self):        raise RuntimeError("连接没有打开")    def write(self):        raise RuntimeError("连接没有打开")    def open(self):        self.change_state(OpenedConnection)    def close(self):        raise RuntimeError("连接已经关闭") if __name__=="__main__":    conn = Connection()    conn.open()    conn.write()