这篇文章主要介绍了Python 使用生成器代替线程的方法，文中讲解非常细致，代码帮助大家更好的理解和学习，感兴趣的朋友可以了解下
问题
你想使用生成器（协程）替代系统线程来实现并发。这个有时又被称为用户级线程或绿色线程。
解决方案
要使用生成器实现自己的并发，你首先要对生成器函数和 yield 语句有深刻理解。 yield 语句会让一个生成器挂起它的执行，这样就可以编写一个调度器， 将生成器当做某种“任务”并使用任务协作切换来替换它们的执行。 要演示这种思想，考虑下面两个使用简单的 yield 语句的生成器函数：
# Two simple generator functions def countdown(n): while n > 0: print('T-minus', n) yield n -= 1 print('Blastoff!') def countup(n): x = 0 while x <n: print('counting up', x) yield x +=1
这些函数在内部使用yield语句，下面是一个实现了简单任务调度器的代码：
from collections import deque class TaskScheduler: def __init__(self): self._task_queue = deque() def new_task(self, task): ''' Admit a newly started task to the scheduler ''' self._task_queue.append(task) def run(self): ''' Run until there are no more tasks ''' while self._task_queue: task = self._task_queue.popleft() try: # Run until the next yield statement next(task) self._task_queue.append(task) except StopIteration: # Generator is no longer executing pass # Example use sched = TaskScheduler() sched.new_task(countdown(10)) sched.new_task(countdown(5)) sched.new_task(countup(15)) sched.run()
TaskScheduler 类在一个循环中运行生成器集合――每个都运行到碰到yield语句为止。 运行这个例子，输出如下：
T-minus 10
T-minus 5
Counting up 0
T-minus 9
T-minus 4
Counting up 1
T-minus 8
T-minus 3
Counting up 2
T-minus 7
T-minus 2
…
到此为止，我们实际上已经实现了一个“操作系统”的最小核心部分。来源gao.dai.ma.com搞@代*码网 生成器函数就是任务，而yield语句是任务挂起的信号。 调度器循环检查任务列表直到没有任务要执行为止。
实际上，你可能想要使用生成器来实现简单的并发。 那么，在实现actor或网络服务器的时候你可以使用生成器来替代线程的使用。
下面的代码演示了使用生成器来实现一个不依赖线程的actor：
from collections import deque class ActorScheduler: def __init__(self): self._actors = {} # Mapping of names to actors self._msg_queue = deque() # Message queue def new_actor(self, name, actor): ''' Admit a newly started actor to the scheduler and give it a name ''' self._msg_queue.append((actor,None)) self._actors[name] = actor def send(self, name, msg): ''' Send a message to a named actor ''' actor = self._actors.get(name) if actor: self._msg_queue.append((actor,msg)) def run(self): ''' Run as long as there are pending messages. ''' while self._msg_queue: actor, msg = self._msg_queue.popleft() try: actor.send(msg) except StopIteration: pass # Example use if __name__ == '__main__': def printer(): while True: msg = yield print('Got:', msg) def counter(sched): while True: # Receive the current count n = yield if n == 0: break # Send to the printer task sched.send('printer', n) # Send the next count to the counter task (recursive) sched.send('counter', n-1) sched = ActorScheduler() # Create the initial actors sched.new_actor('printer', printer()) sched.new_actor('counter', counter(sched)) # Send an initial message to the counter to initiate sched.send('counter', 10000) sched.run()
完全弄懂这段代码需要更深入的学习，但是关键点在于收集消息的队列。 本质上，调度器在有需要发送的消息时会一直运行着。 计数生成器会给自己发送消息并在一个递归循环中结束。
下面是一个更加高级的例子，演示了使用生成器来实现一个并发网络应用程序：
from collections import deque from select import select # This class represents a generic yield event in the scheduler class YieldEvent: def handle_yield(self, sched, task): pass def handle_resume(self, sched, task): pass # Task Scheduler class Scheduler: def __init__(self): self._numtasks = 0 # Total num of tasks self._ready = deque() # Tasks ready to run self._read_waiting = {} # Tasks waiting to read self._write_waiting = {} # Tasks waiting to write # Poll for I/O events and restart waiting tasks def _iopoll(self): rset,wset,eset = select(self._read_waiting, self._write_waiting,[]) for r in rset: evt, task = self._read_waiting.pop(r) evt.handle_resume(self, task) for w in wset: evt, task = self._write_waiting.pop(w) evt.handle_resume(self, task) def new(self,task): ''' Add a newly started task to the scheduler ''' self._ready.append((task, None)) self._numtasks += 1 def add_ready(self, task, msg=None): ''' Append an already started task to the ready queue. msg is what to send into the task when it resumes. ''' self._ready.append((task, msg)) # Add a task to the reading set def _read_wait(self, fileno, evt, task): self._read_waiting[fileno] = (evt, task) # Add a task to the write set def _write_wait(self, fileno, evt, task): self._write_waiting[fileno] = (evt, task) def run(self): ''' Run the task scheduler until there are no tasks ''' while self._numtasks: if not self._ready: self._iopoll() task, msg = self._ready.popleft() try: # Run the coroutine to the next yield r = task.send(msg) if isinstance(r, YieldEvent): r.handle_yield(self, task) else: raise RuntimeError('unrecognized yield event') except StopIteration: self._numtasks -= 1 # Example implementation of coroutine-based socket I/O class ReadSocket(YieldEvent): def __init__(self, sock, nbytes): self.sock = sock self.nbytes = nbytes def handle_yield(self, sched, task): sched.
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