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Python Condition wait() Method with Example

Python Condition.wait() Method: In this tutorial, we will learn about the wait() method of Condition class in Python with its usage, syntax, and examples. By Hritika Rajput Last updated : April 25, 2023

Python Condition.wait() Method

The Condition.wait() is an inbuilt method of the Condition class of the threading module. The Condition.wait() method is used to block the thread and wait until some other thread notifies it by calling the notify() or notify_all() method or if the timeout occurs. The method returns a True Boolean value if it is released by notify() or notify_all() method otherwise a timeout happens, in that case, it returns False. If you wait on an unacquired lock, a Runtime Error is raised.

Read about Producer-Consumer here: Condition.acquire() Method

Module

The following module is required to use wait() method:

import threading

Class

The following class is required to use wait() method:

from threading import Condition

Syntax

The following is the syntax of wait() method:

wait(timeout=None)

Parameter(s)

The following are the parameter(s):

• timeout: It is an optional parameter, which specifies the time for which the thread will wait for a notify call. Its default value is None.

Return Value

The return type of this method is <class 'bool'>. It returns True if it gets notified by the notify() method within given time. In case of a timeout, it returns False.

Example of Condition.wait() Method in Python

# 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")

Output

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