cpython/Doc/library/asyncore.rst

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

.. module:: asyncore
   :synopsis: A base class for developing asynchronous socket handling
              services.
.. moduleauthor:: Sam Rushing <rushing@nightmare.com>
.. sectionauthor:: Christopher Petrilli <petrilli@amber.org>
.. sectionauthor:: Steve Holden <sholden@holdenweb.com>
.. heavily adapted from original documentation by Sam Rushing


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 :cfunc:`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.


.. function:: loop([timeout[, use_poll[, map[,count]]]])

   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 :func:`select` or :func:`poll` call, measured
   in seconds; the default is 30 seconds.  The *use_poll* parameter, if true,
   indicates that :func:`poll` should be used in preference to :func:`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
   :class:`asyncore.dispatcher`, :class:`asynchat.async_chat` and subclasses
   thereof) can freely be mixed in the map.


.. class:: dispatcher()

   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_accept()``  | 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 :cfunc:`select`\ ed or
   :cfunc:`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:


   .. method:: handle_read()

      Called when the asynchronous loop detects that a :meth:`read` call on the
      channel's socket will succeed.


   .. method:: handle_write()

      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:]


   .. method:: handle_expt()

      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.


   .. method:: handle_connect()

      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.


   .. method:: handle_close()

      Called when the socket is closed.


   .. method:: handle_error()

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


   .. method:: handle_accept()

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


   .. method:: readable()

      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.


   .. method:: writable()

      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.


   .. method:: 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 :mod:`socket` documentation for
      information on creating sockets.


   .. method:: connect(address)

      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.


   .. method:: send(data)

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


   .. method:: recv(buffer_size)

      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.


   .. method:: listen(backlog)

      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).


   .. method:: bind(address)

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


   .. method:: accept()

      Accept a connection.  The socket must be bound to an address and listening
      for connections.  The return value is 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.


   .. method:: close()

      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:: file_dispatcher()

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

.. class:: file_wrapper()

   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:

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 http_client(asyncore.dispatcher):

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

       def handle_connect(self):
           pass

       def handle_close(self):
           self.close()

       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:]

   c = http_client('www.python.org', '/')

   asyncore.loop()