Threading and threads


In Python you can create threads using the thread module in Python 2.x or _thread module in Python 3.  We will use the threading module to interact with it.

A thread is an operating system process with different features than a normal process:

  • threads exist as a subset of a process
  • threads share memory and resources
  • processes have a different address space (in memory)

When would you use threading? Usually when you want a function to occur at the same time as your program.  If you create server software, you want the server not only listens to one connection but to many connections. In short, threads enable programs to execute multiple tasks at once.

Python threading

Let’s create a thread program. In this program we will start 10 threads which will each output their id.

import threading
 
# Our thread class
class MyThread (threading.Thread):
 
    def __init__(self,x):
        self.__x = x
        threading.Thread.__init__(self)
 
    def run (self):
          print str(self.__x)
 
# Start 10 threads.
for x in xrange(10):
    MyThread(x).start()

Output:

0
1
...
9

Threads do not have to stop if run once. Threads could be timed, where a threads functionality is repeated every x seconds.

Timed threads

In Python, the Timer class is a subclass of the Thread class. This means it behaves similar. We can use the timer class to create timed threads. Timers are started with the .start() method call, just like regular threads. The program below creates a thread that starts after 5 seconds.

#!/usr/bin/env python
from threading import *
 
def hello():
    print "hello, world"
 
# create thread
t = Timer(10.0, hello)
 
# start thread after 10 seconds
t.start()

Repeating functionality using threads
We can execute threads endlessly like this:

#!/usr/bin/env python
from threading import *
import time
 
def handleClient1():
    while(True):
        print "Waiting for client 1..."
        time.sleep(5) # wait 5 seconds      
 
def handleClient2():
    while(True):
        print "Waiting for client 2..."
        time.sleep(5) # wait 5 seconds
 
# create threads
t = Timer(5.0, handleClient1)
t2 = Timer(3.0, handleClient2)
 
# start threads
t.start()
t2.start()

 


13 thoughts on “Threading and threads

  1. Ma - July 7, 2016

    Hi,Frank
    I run the first example and get the output “Disallowed system call: SYS_pipe”
    Should the last line “MyThread(x).start()” be changed as “MyThread(x).run()” ?

  2. Kate - May 11, 2016

    When I run the very first example, one or two of the numbers will sometimes be indented when printed out. Sometimes they will just be listed like:
    Do you know what causes this?

    1. Frank - May 14, 2016

      Threads do not always execute in chronological order, that causes the listing to be “out of order” sometimes. The indenting is may be something in the thread.