Python Condition wait()用法及代码示例

Python Condition.wait() 方法

wait() 是 Python 中线程模块的 Condition 类的内置方法。

条件类实现条件变量对象。条件变量允许一个或多个线程等待，直到它们被另一个线程通知。 wait() 方法用于阻塞线程并等待其他线程通过调用 notify() 或 notify_all() 方法通知它或者是否发生超时。如果它被 notify() 或 notify_all() 方法释放，则该方法返回一个 True 布尔值，否则会发生超时，在这种情况下，它返回 False。如果您等待未获得的锁，则会引发运行时错误。

在此处阅读有关 Producer-Consumer 的信息：Condition.acquire() 方法

模块：

    from threading import Condition

用法:

    wait(timeout=None)

参数：

• timeout:它是一个可选参数，它指定线程等待通知调用的时间。其默认值为无。

返回值：

这个方法的返回类型是<class 'bool'>.如果在给定时间内收到 notify() 方法的通知，则返回 True。如果超时，则返回 False。

例：

# Python program to explain the
# use of wait() method for Condition object

import threading
import time
import random

class subclass:
  # Initialising the shared resources
  def __init__(self):
    self.x = []
  
  # Add an item for the producer
  def produce_item(self, x_item):
    print("Producer adding an item to the list")
    self.x.append(x_item)
    
  # Consume an item for the consumer
  def consume_item(self):
    print("Consuming from the list")
    consumed_item = self.x[0]
    print("Consumed item:", consumed_item)
    self.x.remove(consumed_item)

def producer(subclass_obj, condition_obj):
    # Selecting a random number from the 1 to 3
    r = random.randint(1,3)
    print("Random number selected was:", r)
    
    # Creting r number of items by the producer
    for i in range(1, r):
      print("Producing an item, time it will take(seconds):" + str(i))
      time.sleep(i)
      
      print("Producer acquiring the lock")
      condition_obj.acquire()
      try:
        # Produce an item
        subclass_obj.produce_item(i)
        # Notify that an item  has been produced
        condition_obj.notify()
      finally:
        # Releasing the lock after producing
        condition_obj.release()
      
def consumer(subclass_obj, condition_obj):
    condition_obj.acquire()
    while True:
      try:
        # Consume the item 
        subclass_obj.consume_item()
      except:
        print("No item to consume, list empty")
        print("Waiting for 10 seconds")
        # wait with a maximum timeout of 10 sec
        value = condition_obj.wait(10)
        if value:
          print("Item produced notified")
          continue
        else:
          print("Waiting timeout")
          break
        
    # Releasig the lock after consuming
    condition_obj.release()
    
if __name__=='__main__':
    
  # Initialising a condition class object
  condition_obj = threading.Condition()
  # subclass object
  subclass_obj = subclass()
  
  # Producer thread
  pro = threading.Thread(target=producer, args=(subclass_obj,condition_obj,))
  pro.start()
  
  # consumer thread
  con = threading.Thread(target=consumer, args=(subclass_obj,condition_obj,))
  con.start()

  pro.join()
  con.join()
  print("Producer Consumer code executed")

输出：

Random number selected was:3
Producing an item, time it will take(seconds):1
Consuming from the list
No item to consume, list empty
Waiting for 10 seconds
Producer acquiring the lock
Producer adding an item to the list
Item produced notified
Consuming from the list
Consumed item: 1
Consuming from the list
No item to consume, list empty
Waiting for 10 seconds
Producing an item, time it will take(seconds):2
Producer acquiring the lock
Producer adding an item to the list
Item produced notified
Consuming from the list
Consumed item: 2
Consuming from the list
No item to consume, list empty
Waiting for 10 seconds
Waiting timeout
Producer Consumer code executed