想實現乙個狀態機或可以在許多不同狀態下執行的物件，但又不想在**中新增很多條件。
一般通過新增不同狀態符號來執行不同狀態下的**，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'
一種更優雅的方法是將每個操作狀態編碼為乙個單獨的類，並安排connection類委派給該狀態類。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)
------------------------------------------
reading
reading
writing
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>
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