Python 3.6.5 Documentation >  "asyncore" — Asynchronous socket handler

"asyncore" — Asynchronous socket handler
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**Source code:** Lib/asyncore.py

Deprecated since version 3.6: Please use "asyncio" instead.

======================================================================

Note: This module exists for backwards compatibility only. For new
code we recommend using "asyncio".

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_accepted()" | 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. Deprecated in version
3.2; use "handle_accepted()" instead.

Deprecated since version 3.2.

handle_accepted(sock, addr)

Called on listening channels (passive openers) when a connection
has been established with a new remote endpoint that has issued
a "connect()" call for the local endpoint. *sock* is a *new*
socket object usable to send and receive data on the connection,
and *addr* is the address bound to the socket on the other end
of the connection.

New in version 3.2.

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=socket.AF_INET, type=socket.SOCK_STREAM)

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.

Changed in version 3.3: *family* and *type* arguments can be
omitted.

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 bytes object implies that the channel has been
closed from the other end.

Note that "recv()" may raise "BlockingIOError" , 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.

asyncore Example basic HTTP client
==================================

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

import asyncore

class HTTPClient(asyncore.dispatcher):

def __init__(self, host, path):
asyncore.dispatcher.__init__(self)
self.create_socket()
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):
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()

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

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()
self.set_reuse_addr()
self.bind((host, port))
self.listen(5)

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

server = EchoServer('localhost', 8080)
asyncore.loop()