#!/usr/bin/env python3

class 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()