Python Standard Library

Chapter 3. Threads and Processes

“Well, since you last asked us to stop, this thread has moved from discussing languages suitable for professional programmers via accidental users to computer-phobic users. A few more iterations can make this thread really interesting...”

—eff-bot, June 1996

Overview

This chapter describes the thread-support modules provided with the standard Python interpreter. Note that thread support is optional and may not be available in your Python interpreter.

This chapter also covers some modules that allow you to run external processes on Unix and Windows systems.

Threads

When you run a Python program, execution starts at the top of the main module and proceeds downwards. Loops can be used to repeat portions of the program, and function and method calls transfer control to a different part of the program (but only temporarily).

With threads, your program can do several things at one time. Each thread has its own flow of control. While one thread might be reading data from a file, another thread can keep the screen updated.

To keep two threads from accessing the same internal data structure at the same time, Python uses a global interpreter lock. Only one thread at a time can execute Python code; Python automatically switches to the next thread after a short period of time, or when a thread does something that may take a while (like waiting for the next byte to arrive over a network socket, or reading data from a file).

The global lock isn’t enough to avoid ...