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python队列queue模块详解

作者:合肥瑶海区海美电器服务部 来源:www.an128l.com 未知发布时间:2017-09-05 12:00:34
python队列queue模块详解

队列queue 多应用在多线程应用中,多线程访问共享变量。对于多线程而言,访问共享变量时,队列queue是线程安全的。从queue队列的具体实现中,可以看出queue使用了1个线程互斥锁(pthread.Lock()),以及3个条件标量(pthread.condition()),来保证了线程安全。

queue的用法如下:

import Queque a=[1,2,3] device_que=Queque.queue() device_que.put(a) device=device_que.get() 先看看它的初始化函数__init__(self,maxsize=0): def __init__(self, maxsize=0): self.maxsize = maxsize self._init(maxsize) # mutex must be held whenever the queue is mutating. All methods # that acquire mutex must release it before returning. mutex # is shared between the three conditions, so acquiring and # releasing the conditions also acquires and releases mutex. self.mutex = _threading.Lock() # Notify not_empty whenever an item is added to the queue; a # thread waiting to get is notified then. self.not_empty = _threading.Condition(self.mutex) # Notify not_full whenever an item is removed from the queue; # a thread waiting to put is notified then. self.not_full = _threading.Condition(self.mutex) # Notify all_tasks_done whenever the number of unfinished tasks # drops to zero; thread waiting to join() is notified to resume self.all_tasks_done = _threading.Condition(self.mutex) self.unfinished_tasks = 0 定义队列时有一个默认的参数maxsize, 如果不指定队列的长度,即manxsize=0,那么队列的长度为无限长,如果定义了大于0的值,那么队列的长度就是maxsize。

self._init(maxsize):使用了python自带的双端队列deque,来存储元素。

self.mutex互斥锁:任何获取队列的状态(empty(),qsize()等),或者修改队列的内容的操作(get,put等)都必须持有该互斥锁。共有两种操作require获取锁,release释放锁。同时该互斥锁被三个共享变量同时享有,即操作conditiond时的require和release操作也就是操作了该互斥锁。

self.not_full条件变量:当队列中有元素添加后,会通知notify其他等待添加元素的线程,唤醒等待require互斥锁,或者有线程从队列中取出一个元素后,通知其它线程唤醒以等待require互斥锁。

self.not empty条件变量:线程添加数据到队列中后,会调用self.not_empty.notify()通知其它线程,唤醒等待require互斥锁后,读取队列。

self.all_tasks_done条件变量:消费者线程从队列中get到任务后,任务处理完成,当所有的队列中的任务处理完成后,会使调用queue.join()的线程返回,表示队列中任务以处理完毕。

queue.put(self, item, block=True, timeout=None)函数:

申请获得互斥锁,获得后,如果队列未满,则向队列中添加数据,并通知notify其它阻塞的某个线程,唤醒等待获取require互斥锁。如果队列已满,则会wait等待。最后处理完成后释放互斥锁。其中还有阻塞block以及非阻塞,超时等逻辑,可以自己看一下:

def put(self, item, block=True, timeout=None): """Put an item into the queue. If optional args 'block' is true and 'timeout' is None (the default), block if necessary until a free slot is available. If 'timeout' is a non-negative number, it blocks at most 'timeout' seconds and raises the Full exception if no free slot was available within that time. Otherwise ('block' is false), put an item on the queue if a free slot is immediately available, else raise the Full exception ('timeout' is ignored in that case). """ self.not_full.acquire() try: if self.maxsize > 0: if not block: if self._qsize() == self.maxsize: raise Full elif timeout is None: while self._qsize() == self.maxsize: self.not_full.wait() elif timeout < 0: raise ValueError("'timeout' must be a non-negative number") else: endtime = _time() + timeout while self._qsize() == self.maxsize: remaining = endtime - _time() if remaining <= 0.0: raise Full self.not_full.wait(remaining) self._put(item) self.unfinished_tasks += 1 self.not_empty.notify() finally: self.not_full.release()
queue.get(self, block=True, timeout=None)函数:

从队列中获取任务,并且从队列中移除此任务。首先尝试获取互斥锁,获取成功则队列中get任务,如果此时队列为空,则wait等待生产者线程添加数据。get到任务后,会调用self.not_full.notify()通知生产者线程,队列可以添加元素了。最后释放互斥锁。

def get(self, block=True, timeout=None): """Remove and return an item from the queue. If optional args 'block' is true and 'timeout' is None (the default), block if necessary until an item is available. If 'timeout' is a non-negative number, it blocks at most 'timeout' seconds and raises the Empty exception if no item was available within that time. Otherwise ('block' is false), return an item if one is immediately available, else raise the Empty exception ('timeout' is ignored in that case). """ self.not_empty.acquire() try: if not block: if not self._qsize(): raise Empty elif timeout is None: while not self._qsize(): self.not_empty.wait() elif timeout < 0: raise ValueError("'timeout' must be a non-negative number") else: endtime = _time() + timeout while not self._qsize(): remaining = endtime - _time() if remaining <= 0.0: raise Empty self.not_empty.wait(remaining) item = self._get() self.not_full.notify() return item finally: self.not_empty.release()

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