进程之间的通行

互斥锁

• 锁：from multiprocessing import Lock,Lock即为锁

• from multiprocessing import Process
  from multiprocessing import Lock
  import os
  import time
  import random
  
  
  def task1(p,lock):
      lock.acquire()
      print(f'{p}开始打印了')
      time.sleep(random.randint(1, 4))
      print(f'{p}打印结束了')
      lock.release()
  
  
  def task2(p,lock):
      lock.acquire()
      print(f'{p}开始打印了')
      time.sleep(random.randint(1, 4))
      print(f'{p}打印结束了')
      lock.release()
  
  
  def task3(p,lock):
      lock.acquire()
      print(f'{p}开始打印了')
      time.sleep(random.randint(1, 4))
      print(f'{p}打印结束了')
      lock.release()
  
  
  if __name__ == '__main__':
      mutex = Lock()
      p1 = Process(target=task1, args=('p1', mutex))
      p2 = Process(target=task2, args=('p2', mutex))
      p3 = Process(target=task3, args=('p3', mutex))
      p1.start()
      p1.join()
      p2.start()
      p2.join()
      p3.start()
      p3.join()
  
  

• lock与join的区别

  • 共同点：都可以把并发变成串行，保证了顺序
  • 不同点：join人为设定顺序，lock让其争抢顺序，保证了公平性

进程之间的通信

1，基于文件通信

• 效率低

• 应用到了互斥锁：可以公平性的保证顺序，以数据的安全，串行

• 自己加锁麻烦且很容易出现死锁

• from multiprocessing import Process
  from multiprocessing import Lock
  import os
  import time
  import json
  
  
  def search():
      time.sleep(1)
      with open("ticket.json", 'r', encoding='utf-8') as f:
          dic = json.load(f)
          print(f'{os.getpid()}查询了票，剩余{dic["count"]}张票')
  
  
  def paid():
      with open("ticket.json", 'r', encoding='utf-8') as f:
          dic = json.load(f)
      if dic["count"] > 0:
          dic["count"] -= 1
          time.sleep(1)
          with open("ticket.json", 'w', encoding='utf-8') as f1:
              json.dump(dic, f1)
          print(f'{os.getpid()}购票成功')
  
  
  def task(lock):
      search()
      lock.acquire()
      paid()
      lock.release()
  
  
  if __name__ == '__main__':
      mutex = Lock()
      for i in range(6):
          p = Process(target=task, args=(mutex,))
          p.start()
  

2,基于队列的通行

• 进程彼此之间互相隔离，要实现进程间通信，multiprocessing模块支持两种形式：队列和管道，传递消息

• queue：创建共享的进程队列，queue是多进程安全带队列，可以使用queue实现多进程之间的数据传递

• from multiprocessing import Queue

• 队列：q = Queue()

• Queue(object)，队列是一个继承object的类，

  • 默认参数：maxsize，不传参表示零到无穷大。
  • 传参以后，put方法和get方法的最大上限就是参数的值，超过参数的值，程序进入阻塞

• 主要方法：

  • put方法

  •     def put(self, obj, block=True, timeout=None):
            assert not self._closed, "Queue {0!r} has been closed".format(self)
            if not self._sem.acquire(block, timeout):
                raise Full
    

  • get方法

  •     def get(self, block=True, timeout=None):
            if block and timeout is None:
                with self._rlock:
                    res = self._recv_bytes()
                self._sem.release()
            else:
                if block:
                    deadline = time.monotonic() + timeout
                if not self._rlock.acquire(block, timeout):
                    raise Empty
    

• 方法的默认参数

  • blocked
    • 默认为blocked = True ，主动设置为False则抛出Full\Empty异常
  • timeout
    • 默认为timeout = 0，如果指定时间，方法进入阻塞，超出时间，则会抛出Full\E异常