实现有状态对象或状态机

想实现一个状态机或可以在许多不同状态下运行的对象，但又不想在代码中添加很多条件。
一般通过添加不同状态符号来运行不同状态下的代码，如下：

class Connection:
    def __init__(self):
        self.state = 'CLOSED'

    def read(self):
        if self.state != 'OPEN':
            raise RuntimeError('Not open')
        print('reading')

    def write(self, data):
        if self.state != 'OPEN':
            raise RuntimeError('Not open')
        print('writing')

    def open(self):
        if self.state == 'OPEN':
            raise RuntimeError('Already open')
        self.state = 'OPEN'

    def close(self):
        if self.state == 'CLOSED':
            raise RuntimeError('Already closed')
        self.state = 'CLOSED'

该实现存在两个困难。 首先，由于引入了许多状态条件检查，因此代码变得很复杂。 其次，由于通用操作（例如read（）和write（））总是在进行操作之前检查状态，因此性能会降低。

一种更优雅的方法是将每个操作状态编码为一个单独的类，并安排Connection类委派给该状态类。如下：

class Connection:
    def __init__(self):
        self.new_state(ClosedConnectionState)

    def new_state(self, newstate):
        self._state = newstate

    # Delegate to the state class
    def read(self):
        return self._state.read(self)

    def write(self, data):
        return self._state.write(self, data)

    def open(self):
        return self._state.open(self)

    def close(self):
        return self._state.close(self)


# Connection state base class
class ConnectionState:
    @staticmethod
    def read(conn):
        raise NotImplementedError()

    @staticmethod
    def write(conn, data):
        raise NotImplementedError()

    @staticmethod
    def open(conn):
        raise NotImplementedError()

    @staticmethod
    def close(conn):
        raise NotImplementedError()


# Implementation of different states
class ClosedConnectionState(ConnectionState):
    @staticmethod
    def read(conn):
        raise RuntimeError('Not open')

    @staticmethod
    def write(conn, data):
        raise RuntimeError('Not open')

    @staticmethod
    def open(conn):
        conn.new_state(OpenConnectionState)

    @staticmethod
    def close(conn):
        raise RuntimeError('Already closed')


class OpenConnectionState(ConnectionState):
    @staticmethod
    def read(conn):
        print('reading')
    
    @staticmethod
    def write(conn, data):
        print('writing')

    @staticmethod
    def open(conn):
        raise RuntimeError('Already open')

    @staticmethod
    def close(conn):
        conn.new_state(ClosedConnectionState)


c = Connection()
print(c._state)
c.open()
c.read()
print(c._state)
c.read()
c.write('hello')
c.close()
print(c._state)
------------------------------------------
<class '__main__.ClosedConnectionState'>
reading
<class '__main__.OpenConnectionState'>
reading
writing
<class '__main__.ClosedConnectionState'>

另一种实现技术涉及对实例的__class__属性的直接操作：

class Connection:
    def __init__(self):
        self.new_state(ClosedConnection)

    def new_state(self, newstate):
        self.__class__ = newstate

    def read(self):
        raise NotImplementedError()

    def write(self, data):
        raise NotImplementedError()

    def open(self):
        raise NotImplementedError()

    def close(self):
        raise NotImplementedError()


class ClosedConnection(Connection):
    def read(self):
        raise RuntimeError('Not open')

    def write(self, data):
        raise RuntimeError('Not open')

    def open(self):
        self.new_state(OpenConnection)

    def close(self):
        raise RuntimeError('Already closed')


class OpenConnection(Connection):
    def read(self):
        print('reading')

    def write(self, data):
        print('writing')

    def open(self):
        raise RuntimeError('Already open')

    def close(self):
        self.new_state(ClosedConnection)


c = Connection()
print(c)
c.open()
print(c)
c.close()
print(c)
------------------------------------------
<__main__.ClosedConnection object at 0x7fd57c2603d0>
<__main__.OpenConnection object at 0x7fd57c2603d0>
<__main__.ClosedConnection object at 0x7fd57c2603d0>

这种方案修改类实例的__class__属性，看似违背了面向对象的基本原则，但从技术上是可行的。此外，由于Connection上的所有方法都不再涉及额外的委派步骤，因此这可能会导致代码速度更快。