Python Cookbook, 3rd Edition

Chapter 12. Concurrency

Python has long supported different approaches to concurrent programming, including programming with threads, launching subprocesses, and various tricks involving generator functions. In this chapter, recipes related to various aspects of concurrent programming are presented, including common thread programming techniques and approaches for parallel processing.

As experienced programmers know, concurrent programming is fraught with potential peril. Thus, a major focus of this chapter is on recipes that tend to lead to more reliable and debuggable code.

12.1. Starting and Stopping Threads

Problem

You want to create and destroy threads for concurrent execution of code.

Solution

The threading library can be used to execute any Python callable in its own thread. To do this, you create a Thread instance and supply the callable that you wish to execute as a target. Here is a simple example:

# Code to execute in an independent thread
import time
def countdown(n):
    while n > 0:
        print('T-minus', n)
        n -= 1
        time.sleep(5)


# Create and launch a thread
from threading import Thread
t = Thread(target=countdown, args=(10,))
t.start()

When you create a thread instance, it doesn’t start executing until you invoke its start() method (which invokes the target function with the arguments you supplied).

Threads are executed in their own system-level thread (e.g., a POSIX thread or Windows threads) that is fully managed by the host operating system. Once started, threads run independently ...