Multithreading

Introduction

Threads allow Python programs to handle multiple functions at once as opposed to running a sequence of commands individually. This topic explains the principles behind threading and demonstrates its usage.

Advanced use of multithreads

This section will contain some of the most advanced examples realized using Multithreading.

Advanced printer (logger)

A thread that prints everything is received and modifies the output according to the terminal width. The nice part is that also the "already written" output is modified when the width of the terminal changes.

#!/usr/bin/env python2

import threading
import Queue
import time
import sys
import subprocess
from backports.shutil_get_terminal_size import get_terminal_size

printq = Queue.Queue()
interrupt = False
lines = []

def main():

    ptt = threading.Thread(target=printer) # Turn the printer on
    ptt.daemon = True
    ptt.start()

    # Stupid example of stuff to print
    for i in xrange(1,100):
        printq.put(' '.join([str(x) for x in range(1,i)]))           # The actual way to send stuff to the printer
        time.sleep(.5)

def split_line(line, cols):
    if len(line) > cols:
        new_line = ''
        ww = line.split()
        i = 0
        while len(new_line) <= (cols - len(ww[i]) - 1):
            new_line += ww[i] + ' '
            i += 1
            print len(new_line)
        if new_line == '':
            return (line, '')

        return (new_line, ' '.join(ww[i:]))
    else:
        return (line, '')


def printer():

    while True:
        cols, rows = get_terminal_size() # Get the terminal dimensions
        msg = '#' + '-' * (cols - 2) + '#\n' # Create the
        try:
            new_line = str(printq.get_nowait())
            if new_line != '!@#EXIT#@!': # A nice way to turn the printer
                                         # thread out gracefully
                lines.append(new_line)
                printq.task_done()
            else:
                printq.task_done()
                sys.exit()
        except Queue.Empty:
            pass

        # Build the new message to show and split too long lines
        for line in lines:
            res = line          # The following is to split lines which are
                                # longer than cols.
            while len(res) !=0:
                toprint, res = split_line(res, cols)
                msg += '\n' + toprint

        # Clear the shell and print the new output
        subprocess.check_call('clear') # Keep the shell clean
        sys.stdout.write(msg)
        sys.stdout.flush()
        time.sleep(.5)

Basics of multithreading

Using the threading module, a new thread of execution may be started by creating a new threading.Thread and assigning it a function to execute:

import threading

def foo():
  print "Hello threading!"

my_thread = threading.Thread(target=foo)

The target parameter references the function (or callable object) to be run. The thread will not begin execution until start is called on the Thread object.

Starting a Thread

my_thread.start() # prints 'Hello threading!'

Now that my_thread has run and terminated, calling start again will produce a RuntimeError. If you'd like to run your thread as a daemon, passing the daemon=True kwarg, or setting my_thread.daemon to True before calling start(), causes your Thread to run silently in the background as a daemon.

Joining a Thread

In cases where you split up one big job into several small ones and want to run them concurrently, but need to wait for all of them to finish before continuing, Thread.join() is the method you're looking for.

For example, let's say you want to download several pages of a website and compile them into a single page. You'd do this:

import requests
from threading import Thread
from queue import Queue

q = Queue(maxsize=20)
def put_page_to_q(page_num):
    q.put(requests.get('http://some-website.com/page_%s.html' % page_num)

def compile(q):
    # magic function that needs all pages before being able to be executed
    if not q.full():
        raise ValueError
    else:
        print("Done compiling!")

threads = []
for page_num in range(20):
     t = Thread(target=requests.get, args=(page_num,))
     t.start()
     threads.append(t)

# Next, join all threads to make sure all threads are done running before
# we continue. join() is a blocking call (unless specified otherwise using 
# the kwarg blocking=False when calling join)
for t in threads:
    t.join()

# Call compile() now, since all threads have completed
compile(q)

A closer look at how join() works can be found here.

Create a Custom Thread Class

Using threading.Thread class we can subclass new custom Thread class. we must override run method in a subclass.

from threading import Thread
import time

class Sleepy(Thread):

    def run(self):
        time.sleep(5)
        print("Hello form Thread")

if __name__ == "__main__":
    t = Sleepy()
    t.start()      # start method automatic call Thread class run method.
    # print 'The main program continues to run in foreground.'
    t.join()
    print("The main program continues to run in the foreground.")

Communicating between threads

There are multiple threads in your code and you need to safely communicate between them.

You can use a Queue from the queue library.

from queue import Queue
from threading import Thread

# create a data producer 
def producer(output_queue):
    while True:
        data = data_computation()
        
        output_queue.put(data)

# create a consumer
def consumer(input_queue):
    while True:
        # retrieve data (blocking)
        data = input_queue.get()

        # do something with the data

        # indicate data has been consumed
        input_queue.task_done()

Creating producer and consumer threads with a shared queue

q = Queue()
t1 = Thread(target=consumer, args=(q,))
t2 = Thread(target=producer, args=(q,))
t1.start()
t2.start()

Creating a worker pool

Using threading & queue:

from socket import socket, AF_INET, SOCK_STREAM
from threading import Thread
from queue import Queue
    
def echo_server(addr, nworkers):
    print('Echo server running at', addr)
    # Launch the client workers
    q = Queue()
    for n in range(nworkers):
        t = Thread(target=echo_client, args=(q,))
        t.daemon = True
        t.start()

    # Run the server
    sock = socket(AF_INET, SOCK_STREAM)
    sock.bind(addr)
    sock.listen(5)
    while True:
        client_sock, client_addr = sock.accept()
        q.put((client_sock, client_addr))

echo_server(('',15000), 128)

Using concurrent.futures.Threadpoolexecutor:

from socket import AF_INET, SOCK_STREAM, socket
from concurrent.futures import ThreadPoolExecutor

def echo_server(addr):
    print('Echo server running at', addr)
    pool = ThreadPoolExecutor(128)
    sock = socket(AF_INET, SOCK_STREAM)
    sock.bind(addr)
    sock.listen(5)
    while True:
        client_sock, client_addr = sock.accept()
        pool.submit(echo_client, client_sock, client_addr)

echo_server(('',15000))

Python Cookbook, 3rd edition, by David Beazley and Brian K. Jones (O’Reilly). Copyright 2013 David Beazley and Brian Jones, 978-1-449-34037-7.

Stoppable Thread with a while Loop

import threading
import time

class StoppableThread(threading.Thread):
    """Thread class with a stop() method. The thread itself has to check
    regularly for the stopped() condition."""

    def __init__(self):
        super(StoppableThread, self).__init__()
        self._stop_event = threading.Event()

    def stop(self):
        self._stop_event.set()

    def join(self, *args, **kwargs):
        self.stop()
        super(StoppableThread,self).join(*args, **kwargs)

    def run()
        while not self._stop_event.is_set():
            print("Still running!")
            time.sleep(2)
        print("stopped!"

Based on this Question.