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什么是状态机？ 

关键属性： 状态和转换

状态： 系统当前状态

转换：一种状态到另外一种状态的变化。

转换由触发事件或是条件启动。

状态机-状态图

状态机使用场景： 

自动售货机  电梯  交通灯   组合锁  停车计时器  

使用state_machine 模块创建状态机第一步使用@acts_as_state_machine装饰器

@acts_as_state_machine
class Process:

 initial 属性值设置为 True

created = State(initial=True)
waiting = State()
running = State()
terminated = State()
blocked = State()
swapped_out_waiting = State()
swapped_out_blocked = State()

定义转换。在 state_machine 模块中，转换是 Event 类的一个实例。我们使用
参数 from_states 和 to_state 定义可能的转换。
 

wait = Event(from_states=(created,
running,
blocked,
swapped_out_waiting),
to_state=waiting)
run = Event(from_states=waiting,
to_state=running)
terminate = Event(from_states=running,
to_state=terminated)
block = Event(from_states=(running,swapped_out_blocked),
to_state=blocked)
swap_wait = Event(from_states=waiting,
to_state=swapped_out_waiting)
swap_block = Event(from_states=blocked,
to_state=swapped_out_blocked)

 from_states 可以是单个状态，也可以是一组状态（元组）。

 state_machine 模块为我们
提供了 @before 和 @after 装饰器，二者可以分别用于在转换发生之前或之后执行操作。你可以
想象在系统中更新一些对象，或者向某人发送电子邮件或通知。在本例中，操作仅限于打印关于
进程状态更改的信息。

transition() 函数，它接受三个参数：
 process ， Process 的一个实例；
 event ， Event 的一个实例（ wait 、 run 、 terminate 等）；
 event_name ，事件的名称。

执行事件时出错，则输出事件的名称。
下面是 transition() 函数的代码：
 

def transition(process, event, event_name):
try:
event()
except InvalidStateTransition as err:
print(f'Error: transition of {process.name}
from {process.current_state} to {event_name} failed')

state_info() 函数显示进程当前（激活）状态的一些基本信息。 

def state_info(process):
print(f'state of {process.name}: {process.current_state}')

在 main() 函数的开头，我们定义了一些字符串常量，它们被作为 event_name 传递。
 

def main():
RUNNING = 'running'
WAITING = 'waiting'
BLOCKED = 'blocked'
TERMINATED = 'terminated'

创建两个 Process 实例并展示它们的初始状态信息。 

p1, p2 = Process('process1'), Process('process2')
[state_info(p) for p in (p1, p2)]

 允许的转换应该与
状态图相关。例如，应该可以从一个运行状态切换到一个阻塞状态，但是不应该从一个阻塞状态
切换到一个运行状态。

from  state_machine   import   (State,Event,acts_as_state_machine,after,before,InvalidStateTransition)@acts_as_state_machine
class Process:created=State(initial=True)  # 创建状态waiting=State() #等待状态running=State()#  运行状态terminated=State()# 停止状态blocked=State() # 阻塞swapper_out_waiting=State()#swapper_out_blocked=State()# 等待状态  转入的状态from_states ，目标状态： to_statewait=Event(from_states=(created,running,blocked,swapper_out_waiting),to_state=waiting)run=Event(from_states=waiting,to_state=running)terminate=Event(from_states=running,to_state=terminated)block=Event(from_states=(running,swapper_out_blocked),to_state=blocked)swap_wait=Event(from_states=waiting,to_state=swapper_out_waiting)swap_block=Event(from_states=blocked,to_state=swapper_out_blocked)def  __init__(self,name):self.name=name@after('wait')def wait_info(self):print(f'{self.name} entered waiting mode')@after('run')def run_info(self):print(f'{self.name} is running')@before('ternimate')def terminate_info(self):print(f"{self.name} terminated")@after('block')def block_info(self):print(f'{self.name} is blocked')@after('swap_wait')def swap_wait_info(self):print(f'{self.name} is swapped out and waiting')@after('swap_block')def swap_block_info(self):print(f'{self.name} is swapped out and blocked')@after('block')def block_info(self):print(f'{self.name} is blocked')@after('swap_wait')def swap_wait_info(self):print(f'{self.name} is swapped out and waiting')@after('swap_block')def swap_block_info(self):print(f'{self.name} is swapped out and blocked')def transition(process,event,event_name):try:event()except  InvalidStateTransition as err:print(f"Error: transaction of {process.name} from  {process.current_state}  to {event_name} failed")# 显示信息def  state_info(process):print(f'state of {process.name}:{process.current_state}')def main():RUNNING='running'WAITING='waiting'BLOCKED='blocked'TERMINATED='terminated'p1,p2=Process('process1'),Process('process2')[state_info(p)  for p in (p1,p2)]print("-------1----------")transition(p1,p1.wait,WAITING)transition(p2,p2.terminate,TERMINATED)[state_info(p) for  p in (p1,p2)]print("------2----------")transition(p1,p1.run,RUNNING)transition(p2,p2.wait,WAITING)[state_info(p) for p in (p1, p2)]print("------3----------")transition(p2, p2.run, RUNNING)[state_info(p) for p in (p1, p2)]print("------4----------")[transition(p,p.block,BLOCKED) for p in (p1,p2)][state_info(p) for p in (p1, p2)]print("------5----------")[transition(p, p.terminate, TERMINATED) for p in (p1, p2)]if __name__=='__main__':main()

state of process1:created
state of process2:created
-------1----------
process1 entered waiting mode
Error: transaction of process2 from  created  to terminated failed
state of process1:waiting
state of process2:created
------2----------
process1 is running
process2 entered waiting mode
state of process1:running
state of process2:waiting
------3----------
process2 is running
state of process1:running
state of process2:running
------4----------
process1 is blocked
process1 is blocked
process2 is blocked
process2 is blocked
state of process1:blocked
state of process2:blocked
------5----------
Error: transaction of process1 from  blocked  to terminated failed
Error: transaction of process2 from  blocked  to terminated failed