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chapter4/04_Handing_corotines_with_Asyncio.rst
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‎chapter4/04_Handing_corotines_with_Asyncio.rst

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使用Asyncio管理协程
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===================
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在上文提到的例子中，我们看到当一个程序变得很大而且复杂时，将其划分为子程序，每一部分实现特定的任务是个不错的方案。子程序不能单独执行，只能在主程序的请求下执行，主程序负责协调使用各个子程序。协程就是子程序的泛化。和子程序一样的事，协程只负责计算任务的一步；和子程序不一样的是，协程没有主程序来进行调度。这是因为协程通过管道连接在一起，没有监视函数负责顺序调用它们。在协程中，执行点可以被挂起，可以被从之前挂起的点恢复执行。通过协程池就可以插入到计算中：运行第一个任务，直到它返回(yield)执行权，然后运行下一个，这样顺着执行下去。
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这种插入的控制组件就是前文介绍的事件循环。它持续追踪所有的协程并执行它们。
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协程的另外一些重要特性如下：
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- 协程可以有多个入口点，并可以yield多次
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- 协程可以将执行权交给其他协程
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yield表示协程在此暂停，并且将执行权交给其他协程。因为协程可以将值与控制权一起传递给另一个协程，所以“yield一个值”就表示将值传给下一个执行的协程。
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|ready|
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-------
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使用Asyncio定义协程非常简单，只需要一个装饰器即可： ::
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import asyncio
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@asyncio.coroutine
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def coroutine_function(function_arguments):
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# DO_SOMETHING
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|how|
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-----
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在这个例子中，我们将看到如何使用Asyncio的协程来模拟有限状态机。有限状态机(finite state machine or automaton, FSA)是一个数据血腥，不仅在工程领域应用广泛，在科学领域也很著名，例如数学和计算机科学等。我们要模拟的状态机如下图所示：
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.. image:: ../images/finite-state-machine.png
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在上图中，可以看到我们的系统有 **S1**, **S2****, **S3**, **S4** 四个状态, **0** 和 **1** 是状态机可以从一个状态到另一个状态的值（这个过程叫做转换）。例如在本实验中，只有当只为1的时候， **S0** 可以转换到 **S1** ，当只为0的时候， **S0** 可以转换到 **S2** .Python代码如下，状态模拟从 **S0** 开始，叫做 **初始状态** ，最后到 **S4** ，叫做 **结束状态** 。 ::
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# Asyncio Finite State Machine
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import asyncio
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import time
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from random import randint
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@asyncio.coroutine
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def StartState():
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print("Start State called \n")
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input_value = randint(0, 1)
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time.sleep(1)
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if (input_value == 0):
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result = yield from State2(input_value)
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else:
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result = yield from State1(input_value)
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print("Resume of the Transition : \nStart State calling " + result)
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@asyncio.coroutine
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def State1(transition_value):
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outputValue = str("State 1 with transition value = %s \n" % transition_value)
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input_value = randint(0, 1)
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time.sleep(1)
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print("...Evaluating...")
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if input_value == 0:
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result = yield from State3(input_value)
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else :
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result = yield from State2(input_value)
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result = "State 1 calling " + result
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return outputValue + str(result)
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@asyncio.coroutine
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def State2(transition_value):
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outputValue = str("State 2 with transition value = %s \n" % transition_value)
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input_value = randint(0, 1)
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time.sleep(1)
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print("...Evaluating...")
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if (input_value == 0):
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result = yield from State1(input_value)
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else :
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result = yield from State3(input_value)
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result = "State 2 calling " + result
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return outputValue + str(result)
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@asyncio.coroutine
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def State3(transition_value):
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outputValue = str("State 3 with transition value = %s \n" % transition_value)
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input_value = randint(0, 1)
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time.sleep(1)
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print("...Evaluating...")
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if (input_value == 0):
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result = yield from State1(input_value)
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else :
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result = yield from EndState(input_value)
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result = "State 3 calling " + result
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return outputValue + str(result)
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@asyncio.coroutine
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def EndState(transition_value):
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outputValue = str("End State with transition value = %s \n" % transition_value)
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print("...Stop Computation...")
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return outputValue
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if __name__ == "__main__":
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print("Finite State Machine simulation with Asyncio Coroutine")
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loop = asyncio.get_event_loop()
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loop.run_until_complete(StartState())
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运行代码，我们可以看到类似以下输出（译注，运行结果随机，这里给我译者运行的三次结果）. ::
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$ python3 coroutines.py
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Finite State Machine simulation with Asyncio Coroutine
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Start State called
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Stop Computation...
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Resume of the Transition :
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Start State calling State 2 with transition value = 0
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State 2 calling State 1 with transition value = 0
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State 1 calling State 2 with transition value = 1
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State 2 calling State 1 with transition value = 0
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State 1 calling State 2 with transition value = 1
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State 2 calling State 3 with transition value = 1
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State 3 calling End State with transition value = 1
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$ python3 coroutines.py
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Finite State Machine simulation with Asyncio Coroutine
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Start State called
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...Evaluating...
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...Evaluating...
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...Stop Computation...
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Resume of the Transition :
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Start State calling State 2 with transition value = 0
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State 2 calling State 3 with transition value = 1
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State 3 calling End State with transition value = 1
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$ python3 coroutines.py
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Finite State Machine simulation with Asyncio Coroutine
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Start State called
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Evaluating...
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...Stop Computation...
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Resume of the Transition :
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Start State calling State 1 with transition value = 1
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State 1 calling State 2 with transition value = 1
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State 2 calling State 1 with transition value = 0
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State 1 calling State 3 with transition value = 0
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State 3 calling State 1 with transition value = 0
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State 1 calling State 2 with transition value = 1
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State 2 calling State 3 with transition value = 1
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State 3 calling End State with transition value = 1
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|work|
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------
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每一个状态都由装饰器装饰： ::
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@asyncio.coroutine
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例如， **S0** 的定义如下所示： ::
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@asyncio.coroutine
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def StartState():
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print("Start State called \n")
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input_value = randint(0, 1)
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time.sleep(1)
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if (input_value == 0):
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result = yield from State2(input_value)
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else:
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result = yield from State1(input_value)
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print("Resume of the Transition : \nStart State calling " + result)
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通过 ``random`` 模块的 ``randint(0, 1)`` 函数生成了 ``input_value`` 的值，决定了下一个转换状态。此函数随机生成1或0： ::
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input_value = randint(0, 1)
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得到 ``input_value`` 的值之后，通过 ``yield from`` 命令调用下一个协程。 ::
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if (input_value == 0):
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result = yield from State2(input_value)
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else:
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result = yield from State1(input_value)
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``result`` 是下一个协程返回的string，这样我们在计算的最后就可以重新构造出计算过程。
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启动事件循环的代码如下： ::
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if __name__ == "__main__":
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print("Finite State Machine simulation with Asyncio Coroutine")
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loop = asyncio.get_event_loop()
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loop.run_until_complete(StartState())

‎images/finite-state-machine.png

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