Introduction to multithreading in Python

A Python application runs on a single thread, unless you explicitly enable multithreading.

Why is multithreading useful? Code in Python is ran in sequence, one instruction after another.

If you define a function that sleeps 3 seconds and then prints something, like this:

import time

def greet():
    time.sleep(3)
    print('hello')

greet()
print('world')

The world string is printed after 3 seconds, because we call time.sleep(3) inside the greet() function.

This is just a silly example, but imagine processing an image, getting a resource from the network, or writing a big file to disk. Anything that can take a lot of time.

With multithreading we can run the function that takes a lot of time into a separate thread, and go on with our program in the meantime.

The threading standard library module helps with implementing multithreading. You import Thread from it:

from threading import Thread

Then we pass the function we must execute as the target argument to the Thread() function, getting a thread object:

t = Thread(target=greet)

then we call its start() method to start the thread:

t.start()

Try running this code:

from threading import Thread
import time

def greet():
    time.sleep(3)
    print('hello')

t = Thread(target=greet)
t.start()

print('world')

You will now see world printed 3 seconds before hello shows up in the console.

The program does not end until the thread (or all the threads it started) end, unless you start a thread as a deamon.

Those are the basics of multithreading. This is complex, and can lead to the introduction of bugs if not done well.