How to use threading in Python

Question

I am trying to understand threading in Python. I've looked at the documentation and examples, but quite frankly, many examples are overly sophisticated and I'm having trouble understanding them.

How do you clearly show tasks being divided for multi-threading

Answer 1

Here's a simple example: you need to try a few alternative URLs and return the contents of the first one to respond.

import Queue

import threading

import urllib2

# called by each thread

def get_url(q, url):

q.put(urllib2.urlopen(url).read())

theurls = ["http://google.com", "http://yahoo.com"]

q = Queue.Queue()

for u in theurls:

t = threading.Thread(target=get_url, args = (q,u))

t.daemon = True t.start()

s = q.get()

print s

This is a case where threading is used as a simple optimization: each subthread is waiting for a URL to resolve and respond, in order to put its contents on the queue; each thread is a daemon (won't keep the process up if main thread ends -- that's more common than not); the main thread starts all subthreads, does a get on the queue to wait until one of them has done a put, then emits the results and terminates (which takes down any subthreads that might still be running, since they're daemon threads).

Proper use of threads in Python is invariably connected to I/O operations (since CPython doesn't use multiple cores to run CPU-bound tasks anyway, the only reason for threading is not blocking the process while there's a wait for some I/O). Queues are almost invariably the best way to farm out work to threads and/or collect the work's results, by the way, and they're intrinsically threadsafe so they save you from worrying about locks, conditions, events, semaphores, and other inter-thread coordination/communication concepts.

Answer 2

 Thread is the smallest unit of processing that can be performed in an operating system. you can assume that a thread is simply a subset of a process.  thread is a sequence of such instructions within a program that can be executed independently of other code.

 In Python concept multi threading is defined as the ability of a processor to execute multiple threads concurrently. example of multi threading .

import thread
import time
# define functrion
def print_time( threadName, delay):
   count = 0
   while count < 5:
      time.sleep(delay)
      count += 1
      print "%s: %s" % ( threadName, time.ctime(time.time()) )
# create thread
try:
   thread.start_new_thread( print_time, ("Thread-1", 2, ) )
except:
   print "Error: unable to start thread"
while 1:
   pass

output:

Thread-1: Sat Apr 06 11:42:19 20019
Thread-1: Sat Apr 06 11:42:21 20019
Thread-1: Sat Apr 06 11:42:23 20019