cpython-pep302 / Doc / library / asyncore.rst

:mod:`asyncore` --- Asynchronous socket handler

Source code: :source:`Lib/`

This module provides the basic infrastructure for writing asynchronous socket service clients and servers.

There are only two ways to have a program on a single processor do "more than one thing at a time." Multi-threaded programming is the simplest and most popular way to do it, but there is another very different technique, that lets you have nearly all the advantages of multi-threading, without actually using multiple threads. It's really only practical if your program is largely I/O bound. If your program is processor bound, then pre-emptive scheduled threads are probably what you really need. Network servers are rarely processor bound, however.

If your operating system supports the :c:func:`select` system call in its I/O library (and nearly all do), then you can use it to juggle multiple communication channels at once; doing other work while your I/O is taking place in the "background." Although this strategy can seem strange and complex, especially at first, it is in many ways easier to understand and control than multi-threaded programming. The :mod:`asyncore` module solves many of the difficult problems for you, making the task of building sophisticated high-performance network servers and clients a snap. For "conversational" applications and protocols the companion :mod:`asynchat` module is invaluable.

The basic idea behind both modules is to create one or more network channels, instances of class :class:`asyncore.dispatcher` and :class:`asynchat.async_chat`. Creating the channels adds them to a global map, used by the :func:`loop` function if you do not provide it with your own map.

Once the initial channel(s) is(are) created, calling the :func:`loop` function activates channel service, which continues until the last channel (including any that have been added to the map during asynchronous service) is closed.

The :class:`dispatcher` class is a thin wrapper around a low-level socket object. To make it more useful, it has a few methods for event-handling which are called from the asynchronous loop. Otherwise, it can be treated as a normal non-blocking socket object.

The firing of low-level events at certain times or in certain connection states tells the asynchronous loop that certain higher-level events have taken place. For example, if we have asked for a socket to connect to another host, we know that the connection has been made when the socket becomes writable for the first time (at this point you know that you may write to it with the expectation of success). The implied higher-level events are:

Event Description
handle_connect() Implied by the first read or write event
handle_close() Implied by a read event with no data available
handle_accepted() Implied by a read event on a listening socket

During asynchronous processing, each mapped channel's :meth:`readable` and :meth:`writable` methods are used to determine whether the channel's socket should be added to the list of channels :c:func:`select`ed or :c:func:`poll`ed for read and write events.

Thus, the set of channel events is larger than the basic socket events. The full set of methods that can be overridden in your subclass follows:

In addition, each channel delegates or extends many of the socket methods. Most of these are nearly identical to their socket partners.

A :class:`dispatcher` subclass which adds simple buffered output capability, useful for simple clients. For more sophisticated usage use :class:`asynchat.async_chat`.

A file_dispatcher takes a file descriptor or :term:`file object` along with an optional map argument and wraps it for use with the :c:func:`poll` or :c:func:`loop` functions. If provided a file object or anything with a :c:func:`fileno` method, that method will be called and passed to the :class:`file_wrapper` constructor. Availability: UNIX.

A file_wrapper takes an integer file descriptor and calls :func:`os.dup` to duplicate the handle so that the original handle may be closed independently of the file_wrapper. This class implements sufficient methods to emulate a socket for use by the :class:`file_dispatcher` class. Availability: UNIX.

asyncore Example basic HTTP client

Here is a very basic HTTP client that uses the :class:`dispatcher` class to implement its socket handling:

import asyncore, socket

class HTTPClient(asyncore.dispatcher):

    def __init__(self, host, path):
        self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
        self.connect( (host, 80) )
        self.buffer = bytes('GET %s HTTP/1.0\r\nHost: %s\r\n\r\n' %
                            (path, host), 'ascii')

    def handle_connect(self):

    def handle_close(self):

    def handle_read(self):

    def writable(self):
        return (len(self.buffer) > 0)

    def handle_write(self):
        sent = self.send(self.buffer)
        self.buffer = self.buffer[sent:]

 client = HTTPClient('', '/')

asyncore Example basic echo server

Here is a basic echo server that uses the :class:`dispatcher` class to accept connections and dispatches the incoming connections to a handler:

import asyncore
import socket

class EchoHandler(asyncore.dispatcher_with_send):

    def handle_read(self):
        data = self.recv(8192)
        if data:

class EchoServer(asyncore.dispatcher):

    def __init__(self, host, port):
        self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
        self.bind((host, port))

    def handle_accepted(self, sock, addr):
        print('Incoming connection from %s' % repr(addr))
        handler = EchoHandler(sock)

server = EchoServer('localhost', 8080)