Python Multithreading

In this Python tutorial, let us discuss Python Multithreading with suitable examples.

Introduction of Python Multithreading

A thread has a beginning, an execution sequence, and a conclusion. It has an instruction pointer that keeps track of where within its context it, is currently running.

  • It can be interrupted
  • It can temporarily be put on hold while other threads are running – this is called yielding.

Starting a New Thread

We need to call the following method available in the thread module,

thread.start_new_thread ( function, args[, kwargs] )

This method call enables a fast and efficient way to create new threads in both Linux and Windows. The method call returns immediately and the child thread starts and calls a function with the passed list of args. When the function returns, the thread terminates.

Here, args is a tuple of arguments; use an empty tuple to call the function without passing any arguments. kwargs is an optional dictionary of keyword arguments.

Example code to start a new Thread

#!/usr/bin/python
import thread
import time
# Define a function for the thread
def print_time( threadName, delay):
  count = 0
  while count < 5:
    time.sleep(delay)
    count += 1
    print "%s: %s" % ( threadName, time.ctime(time.time()) )
# Create two threads as follows
try:
  thread.start_new_thread( print_time, ("Thread-1", 2, ) )
  thread.start_new_thread( print_time, ("Thread-2", 4, ) )
except:
  print "Error: unable to start thread"
while 1:
  pass

Output

Thread-1: Thu Jan 22 15:42:17 2009
Thread-1: Thu Jan 22 15:42:19 2009
Thread-2: Thu Jan 22 15:42:19 2009
Thread-1: Thu Jan 22 15:42:21 2009
Thread-2: Thu Jan 22 15:42:23 2009
Thread-1: Thu Jan 22 15:42:23 2009
Thread-1: Thu Jan 22 15:42:25 2009
Thread-2: Thu Jan 22 15:42:27 2009
Thread-2: Thu Jan 22 15:42:31 2009
Thread-2: Thu Jan 22 15:42:35 2009

Multithreaded Priority Queue

The Queue module allows you to create a new queue object that can hold a specific number of items. The following methods to control the Queue such as

  • get() − The get() removes and returns an item from the queue.
  • put() − The put adds an item to a queue.
  • qsize() − The qsize() returns the number of items that are currently in the queue.
  • empty() − The empty( ) returns True if queue is empty; otherwise, False.
  • full() − the full() returns True if queue is full; otherwise, False.

Example for multithreaded priority queue

#!/usr/bin/python
import Queue
import threading
import time
exitFlag = 0
class myThread (threading.Thread):
  def __init__(self, threadID, name, q):
    threading.Thread.__init__(self)
    self.threadID = threadID
    self.name = name
    self.q = q
 def run(self):
    print "Starting " + self.name
    process_data(self.name, self.q)
    print "Exiting " + self.name
def process_data(threadName, q):
  while not exitFlag:
    queueLock.acquire()
     if not workQueue.empty():
       data = q.get()
       queueLock.release()
       print "%s processing %s" % (threadName, data)
     else:
       queueLock.release()
     time.sleep(1)
threadList = ["Thread-1", "Thread-2", "Thread-3"]
nameList = ["One", "Two", "Three", "Four", "Five"]
queueLock = threading.Lock()
workQueue = Queue.Queue(10)
threads = []
threadID = 1
# Create new threads
for tName in threadList:
  thread = myThread(threadID, tName, workQueue)
  thread.start()
  threads.append(thread)
  threadID += 1
# Fill the queue
queueLock.acquire()
for word in nameList:
  workQueue.put(word)
queueLock.release()
# Wait for a queue to empty
while not workQueue.empty():
  pass
# Notify threads it's time to exit
exitFlag = 1
# Wait for all threads to complete
for t in threads:
  t.join()
print "Exiting Main Thread"

Output

Starting Thread-1
Starting Thread-2
Starting Thread-3
Thread-1 processing One
Thread-2 processing Two
Thread-3 processing Three
Thread-1 processing Four
Thread-2 processing Five
Exiting Thread-3
Exiting Thread-1
Exiting Thread-2
Exiting Main Thread

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