18.5. asyncio – Asynchronous I/O, event loop, coroutines and tasks

Source code: Lib/asyncio/

This module provides infrastructure for writing single-threaded concurrent code using coroutines, multiplexing I/O access over sockets and other resources, running network clients and servers, and other related primitives. Here is a more detailed list of the package contents:

• a pluggable event loop with various system-specific implementations;
• transport and protocol abstractions (similar to those in Twisted);
• concrete support for TCP, UDP, SSL, subprocess pipes, delayed calls, and others (some may be system-dependent);
• a Future class that mimics the one in the concurrent.futures module, but adapted for use with the event loop;
• coroutines and tasks based on yield from (PEP 380), to help write concurrent code in a sequential fashion;
• cancellation support for Futures and coroutines;
• synchronization primitives for use between coroutines in a single thread, mimicking those in the threading module;
• an interface for passing work off to a threadpool, for times when you absolutely, positively have to use a library that makes blocking I/O calls.

Asynchronous programming is more complex than classical “sequential” programming: see the Develop with asyncio page which lists common traps and explains how to avoid them. Enable the debug mode during development to detect common issues.

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