Python的线程/进程间通讯对象分析

Python提供了一系列的对象支持线程/进程间的通讯:

1. Lock
2. RLock
3. Condition
4. Semaphone
5. BounderSemaphone
6. Event
7. Barrier

除了Lock，Rlock外，进程的相关对象只是线程相关对象的clone，而且进程的Lock，RLock也是与线程Lock，RLock相对应的对象。在这里我们只分析线程的这几种对象。
一般使用范围：

Lock用于对互斥操作(单一资源，全局变量)
RLock与Lock类似，区别仅在与RLock在同一个线程可以多次获取
Semaphone/BounderSemaphone用于对多个资源的申请使用， 如果BounderSemaphone(1)则==Lock()
Condition用于在等待某种事情发生
Event实际上是对Condition的一种操作简化包装，也更符合事件驱动的概念。
这几种对象大概使用上面这些对象所要付出的开销是不同的，我们从其原理来进行分析。

from time import sleep
from threading import Thread, Lock

class MyThread(Thread):
    def __init__(self, name, lock):
        super(MyThread, self).__init__(name=name)
        self.lock = lock

    def run(self):
        with self.lock:
            print('my name is %s, i will sleep 2 seconds' % self.name)
            sleep(2)
            print('i wake up now.')

if __name__ == '__main__':
    lock = Lock()
    mt1 = MyThread('t1', lock)
    mt2 = MyThread('t2', lock)
    mt1.start(); mt2.start()
    mt1.join(); mt2.join()
    print('main thread end!')

Lock的使用方法参考上面的代码，而RLock是跟Lock的区别是如果在本线程已经acquire，则可以多次acquire，不同线程下则跟Lock是一致的；我们来看一下RLock的实现：

class _RLock:
    def __init__(self):
        self._block = _allocate_lock()
        self._owner = None
        self._count = 0

    def acquire(self, blocking=True, timeout=-1):
        me = get_ident()   # 获取当前thread的identity
        if self._owner == me:  # 锁的所有者是自己
            self._count += 1
            return 1
        rc = self._block.acquire(blocking, timeout)   #获取锁
        if rc:
            self._owner = me
            self._count = 1
        return rc

从上面的代码可以看出，RLock实际上是使用了Lock，只是在acquire时判断了是否是本线程，如果是，则记录lock次数不做阻塞返回。

而Condition在调用wait时则新生成了一个Lock，并acquire了两次已达到阻塞的目的，而。

class Condition:
    def __init__(self, lock=None):
        .....
        self._waiters = _deque()  # 初始化waiter Lock 队列
    def wait(self, timeout=None):
        if not self._is_owned():  # 必须要先获取锁
            raise RuntimeError("cannot wait on un-acquired lock")
        waiter = _allocate_lock()
        waiter.acquire()
        self._waiters.append(waiter)  #将waiter Lock加入waiter队列
        .....
        waiter.acquire()
    def _is_owned(self):
        # Return True if lock is owned by current_thread.
        # This method is called only if _lock doesn't have _is_owned().
        if self._lock.acquire(0):
            self._lock.release()   # 在wait，notify时会释放互斥锁
            return False
        else:
            return True
    def notify(self, n=1):
        if not self._is_owned():  # 必须要先获取锁
            raise RuntimeError("cannot notify on un-acquired lock")
        all_waiters = self._waiters
        waiters_to_notify = _deque(_islice(all_waiters, n))
        if not waiters_to_notify:
            return
        for waiter in waiters_to_notify:
            waiter.release()      #根据通知数量依次释放

Lock只是一个单纯的互斥体，而Condition则可在某种条件发生后主动通知正在等待某种条件的线程。

from threading import Thread, Lock, Condition

class MyWaitThread(Thread):
    def __init__(self, name, cond):
        super(MyWaitThread, self).__init__(name=name)
        # self.name = name
        self.cond = cond

    def run(self):
        with self.cond:
            print('I am waiting something happen!')
            self.cond.wait()
            print('wait end!')

class MyNotifyThread(Thread):
    def __init__(self, name, cond):
        super(MyNotifyThread, self).__init__(name=name)
        # self.name = name
        self.cond = cond

    def run(self):
        with self.cond:
            print('I am notifying all the wait thread.')
            self.cond.notify_all()
            print('notify end.')

if __name__ == '__main__':
    cond = Condition()
    mt1 = MyWaitThread('t1', cond)
    mt2 = MyWaitThread('t2', cond)
    mt3 = MyNotifyThread('t3', cond)
    mt1.start(); mt2.start(); mt3.start()
    mt1.join(); mt2.join();mt3.join()
    print('main thread end!')

Semaphone使用Condition的wait，notify来实现，但是却可以看成一个可以在不同线程中同时获取N次的锁。