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.

asyncore.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 "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:

   +------------------------+------------------------------------------+
   | 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 "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:

   handle_read()

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

   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:]

   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.

   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.

   handle_close()

      Called when the socket is closed.

   handle_error()

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

   handle_accept()

      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.

   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.

   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.

   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.

   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.

   send(data)

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

   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.

      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(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).

   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 "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()

      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()

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

       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:]


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


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:
               self.send(data)

   class EchoServer(asyncore.dispatcher):

       def __init__(self, host, port):
           asyncore.dispatcher.__init__(self)
           self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
           self.set_reuse_addr()
           self.bind((host, port))
           self.listen(5)

       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)
   asyncore.loop()
