Python Event wait()用法及代碼示例

Python Event.wait() 方法

wait() 是 Python 中線程模塊的 Event 類的內置方法。

當我們想要一個線程等待一個事件時，我們可以在內部標誌設置為 false 的事件上調用 wait() 方法，從而阻塞線程，直到該事件對象的內部標誌被 set() 方法設置為 true。如果內部標誌在進入時為真，則線程不會被阻塞。線程保持阻塞狀態，直到內部標誌保持為 false 或發生超時。我們可以在 timeout 參數的幫助下為函數提供可選的超時。

模塊：

    from threading import Event

用法:

    wait(timeout=None)

參數：

• timeout: 可選參數，指定wait()方法的超時時間。一旦超過該值，等待該事件的線程就會解除阻塞。其默認值為無。

返回值：

這個方法的返回類型是<class 'Bool'>.如果線程在超時之前被釋放，則返回，否則返回 false。

範例1：

# Python program to explain the
# use of wait() method in Event() class
import threading
import time

def helper_function(event_obj, timeout,i):
  # Thread has started, but it will wait 10 seconds for the event  
  print("Thread started, for the event to set")
 
  flag = event_obj.wait(timeout)
  if flag:
    print("Event was set to true() earlier, moving ahead with the thread")
  else:
    print("Time out occured, event internal flag still false. Executing thread without waiting for event")
    print("Value to be printed=", i)
    
if __name__ == '__main__':
  # Initialising an event object
  event_obj = threading.Event()
  
  # starting the thread who will wait for the event
  thread1 = threading.Thread(target=helper_function, args=(event_obj,10,27))
  thread1.start()
  # sleeping the current thread for 5 seconds
  time.sleep(5)
  
  # generating the event
  event_obj.set()
  print("Event is set to true. Now threads can be released.")
  print()

輸出：

Thread started, for the event to set
Event is set to true. Now threads can be released.

Event was set to true() earlier, moving ahead with the thread

範例2：

# Python program to explain the
# use of wait() method in Event() class
import threading
import time

def helper_function(event_obj, timeout,i):
  # Thread has started, but it will wait 3 seconds for the event  
  print("Thread started, for the event to set")
 
  flag = event_obj.wait(timeout)
  if flag:
    print("Event was set to true() earlier, moving ahead with the thread")
  else:
    print("Time out occured, event internal flag still false. Executing thread without waiting for event")
    print("Value to be printed=", i)
    
if __name__ == '__main__':
  # Initialising an event object
  event_obj = threading.Event()
  
  # starting the thread who will wait for the event
  thread1 = threading.Thread(target=helper_function, args=(event_obj,3,27))
  thread1.start()
  # sleeping the current thread for 5 seconds
  time.sleep(5)
  
  # generating the event
  event_obj.set()
  print("Event is set to true. Now threads can be released.")
  print()

輸出：

Thread started, for the event to set
Time out occured, event internal flag still false. Executing thread without waiting for event
Value to be printed= 27
Event is set to true. Now threads can be released.