17.6. asyncore — Asynchronous socket handler

Source code: Lib/asyncore.py

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 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 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 asynchat module is invaluable.

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

Once the initial channel(s) is(are) created, calling the 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.


Enter a polling loop that terminates after count passes or all open channels have been closed. All arguments are optional. The count parameter defaults to None, resulting in the loop terminating only when all channels have been closed. The timeout argument sets the timeout parameter for the appropriate select() or poll() call, measured in seconds; the default is 30 seconds. The use_poll parameter, if true, indicates that poll() should be used in preference to select() (the default is False).

The map parameter is a dictionary whose items are the channels to watch. As channels are closed they are deleted from their map. If map is omitted, a global map is used. Channels (instances of asyncore.dispatcher, asynchat.async_chat and subclasses thereof) can freely be mixed in the map.

class asyncore.dispatcher

The 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:




Implied by the first read or write event


Implied by a read event with no data available


Implied by a read event on a listening socket

During asynchronous processing, each mapped channel’s readable() and writable() methods are used to determine whether the channel’s socket should be added to the list of channels select()ed or 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:


Called when the asynchronous loop detects that a read() call on the channel’s socket will succeed.


Called when the asynchronous loop detects that a writable socket can be written. Often this method will implement the necessary buffering for performance. For example:

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

Called when there is out of band (OOB) data for a socket connection. This will almost never happen, as OOB is tenuously supported and rarely used.


Called when the active opener’s socket actually makes a connection. Might send a “welcome” banner, or initiate a protocol negotiation with the remote endpoint, for example.


Called when the socket is closed.


Called when an exception is raised and not otherwise handled. The default version prints a condensed traceback.


Called on listening channels (passive openers) when a connection can be established with a new remote endpoint that has issued a connect() call for the local endpoint.


Called each time around the asynchronous loop to determine whether a channel’s socket should be added to the list on which read events can occur. The default method simply returns True, indicating that by default, all channels will be interested in read events.


Called each time around the asynchronous loop to determine whether a channel’s socket should be added to the list on which write events can occur. The default method simply returns True, indicating that by default, all channels will be interested in write events.

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

create_socket(family, type)

This is identical to the creation of a normal socket, and will use the same options for creation. Refer to the socket documentation for information on creating sockets.


As with the normal socket object, address is a tuple with the first element the host to connect to, and the second the port number.


Send data to the remote end-point of the socket.


Read at most buffer_size bytes from the socket’s remote end-point. An empty string implies that the channel has been closed from the other end.

Note that recv() may raise socket.error with EAGAIN or EWOULDBLOCK, even though select.select() or select.poll() has reported the socket ready for reading.


Listen for connections made to the socket. The backlog argument specifies the maximum number of queued connections and should be at least 1; the maximum value is system-dependent (usually 5).


Bind the socket to address. The socket must not already be bound. (The format of address depends on the address family — refer to the socket documentation for more information.) To mark the socket as re-usable (setting the SO_REUSEADDR option), call the dispatcher object’s set_reuse_addr() method.


Accept a connection. The socket must be bound to an address and listening for connections. The return value can be either None or a pair (conn, address) where conn is a new socket object usable to send and receive data on the connection, and address is the address bound to the socket on the other end of the connection. When None is returned it means the connection didn’t take place, in which case the server should just ignore this event and keep listening for further incoming connections.


Close the socket. All future operations on the socket object will fail. The remote end-point will receive no more data (after queued data is flushed). Sockets are automatically closed when they are garbage-collected.

class asyncore.dispatcher_with_send
A dispatcher subclass which adds simple buffered output capability, useful for simple clients. For more sophisticated usage use asynchat.async_chat.
class asyncore.file_dispatcher
A file_dispatcher takes a file descriptor or file object along with an optional map argument and wraps it for use with the poll() or loop() functions. If provided a file object or anything with a fileno() method, that method will be called and passed to the file_wrapper constructor. Availability: UNIX.
class asyncore.file_wrapper
A file_wrapper takes an integer file descriptor and calls 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 file_dispatcher class. Availability: UNIX.

17.6.1. asyncore Example basic HTTP client

Here is a very basic HTTP client that uses the 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) )

    def handle_connect(self):

    def handle_close(self):

    def handle_read(self):
        print self.recv(8192)

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

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

client = HTTPClient('www.python.org', '/')

17.6.2. asyncore Example basic echo server

Here is a basic echo server that uses the 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_accept(self):
        pair = self.accept()
        if pair is not None:
            sock, addr = pair
            print 'Incoming connection from %s' % repr(addr)
            handler = EchoHandler(sock)

server = EchoServer('localhost', 8080)