PyMOTW: asyncore

By Doug Hellmann
March 1, 2009

asyncore – Asynchronous I/O handler

Purpose:Asynchronous I/O handler
Python Version:1.5.2 and later

The asyncore module includes tools for working with I/O objects such as sockets so they can be managed asynchronously (instead of, for example, using threads). The main class provided is dispatcher, a wrapper around a socket that provides hooks for handling events like connecting, reading, and writing when invoked from the main loop function, loop().

Clients

To create an asyncore-based client, subclass dispatcher and provide implementations for creating the socket, reading, and writing. Let’s examine this HTTP client, based on the one from the standard library documentation.

import asyncore
import logging
import socket
from cStringIO import StringIO
import urlparse

class HttpClient(asyncore.dispatcher):

def __init__(self, url):
self.url = url
self.logger = logging.getLogger(self.url)
self.parsed_url = urlparse.urlparse(url)
asyncore.dispatcher.__init__(self)
self.write_buffer = 'GET %s HTTP/1.0\r\n\r\n' % self.url
self.read_buffer = StringIO()
self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
address = (self.parsed_url.netloc, 80)
self.logger.debug('connecting to %s', address)
self.connect(address)

def handle_connect(self):
self.logger.debug('handle_connect()')

def handle_close(self):
self.logger.debug('handle_close()')
self.close()

def writable(self):
is_writable = (len(self.write_buffer) > 0)
if is_writable:
self.logger.debug('writable() -> %s', is_writable)
return is_writable

def readable(self):
self.logger.debug('readable() -> True')
return True

def handle_write(self):
sent = self.send(self.write_buffer)
self.logger.debug('handle_write() -> "%s"', self.write_buffer[:sent])
self.write_buffer = self.write_buffer[sent:]

def handle_read(self):
data = self.recv(8192)
self.logger.debug('handle_read() -> %d bytes', len(data))
self.read_buffer.write(data)

if __name__ == '__main__':
logging.basicConfig(level=logging.DEBUG,
format='%(name)s: %(message)s',
)

clients = [
HttpClient('http://www.python.org/'),
HttpClient('http://www.doughellmann.com/PyMOTW/contents.html'),
]

logging.debug('LOOP STARTING')

asyncore.loop()

logging.debug('LOOP DONE')

for c in clients:
response_body = c.read_buffer.getvalue()
print c.url, 'got', len(response_body), 'bytes'

First, the socket is created in __init__() using the base class method create_socket(). Alternative implementations of the method may be provided, but in this case we want a TCP/IP socket so the base class version is sufficient.

The handle_connect() hook is present simply to illustrate when it is called. Other types of clients that need to do some sort of hand-shaking or protocol negotiation should do the work in handle_connect().

handle_close() is similarly presented for the purposes of showing when the method is called. The base class version closes the socket correctly, so if you don’t need to do extra cleanup on close you can leave the method out.

The asyncore loop uses writable() and its sibling method readable() to decide what actions to take with each dispatcher. Actual use of poll() or select() on the sockets or file descriptors managed by each dispatcher is handled inside the asyncore code, so you don’t need to do that yourself. Simply indicate whether the dispatcher cares at all about reading or writing. In the case of this HTTP client, writable() returns True as long as there is data from the request to send to the server. readable() always returns True because we want to read all of the data.

Each time through the loop when writable() responds positively, handle_write() is invoked. In this version, the HTTP request string that was built in __init__() is sent to the server and the write buffer is reduced by the amount successfully sent.

Similarly, when readable() responds positively and there is data to read, handle_read() is invoked.

The example below the __main__ test configures logging for debugging then creates two clients to download two separate web pages. Creating the clients registers them in a “map” kept internally by asyncore. The downloading occurs as the loop iterates over the clients. When the client reads 0 bytes from a socket that seems readable, the condition is interpreted as a closed connection and handle_close() is called.

One example of how this client app may run is:

$ python asyncore_http_client.py
http://www.python.org/: connecting to ('www.python.org', 80)
http://www.doughellmann.com/PyMOTW/contents.html: connecting to ('www.doughellmann.com', 80)
root: LOOP STARTING
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_connect()
http://www.doughellmann.com/PyMOTW/contents.html: handle_write() -> "GET http://www.doughellmann.com/PyMOTW/contents.html HTTP/1.0

"
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 3163 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 1448 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 2896 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 2896 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 2896 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 2896 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 1448 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 1448 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 1448 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 1448 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 895 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_close()
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 0 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.python.org/: handle_connect()
http://www.python.org/: handle_write() -> "GET http://www.python.org/ HTTP/1.0

"
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1257 bytes
http://www.python.org/: readable() -> True
http://www.python.org/: handle_close()
http://www.python.org/: handle_read() -> 0 bytes
root: LOOP DONE
http://www.python.org/ got 18009 bytes
http://www.doughellmann.com/PyMOTW/contents.html got 22882 bytes


Servers

The example below illustrates using asyncore on the server by re-implementing the EchoServer from the SocketServer examples. There are three classes: EchoServer receives incoming connections from clients and creates EchoHandler instances to deal with each. The EchoClient is an asyncore dispatcher similar to the HttpClient defined above.

import asyncore
import logging

class EchoServer(asyncore.dispatcher):
"""Receives connections and establishes handlers for each client.
"""

def __init__(self, address):
self.logger = logging.getLogger('EchoServer')
asyncore.dispatcher.__init__(self)
self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
self.bind(address)
self.address = self.socket.getsockname()
self.logger.debug('binding to %s', self.address)
self.listen(1)
return

def handle_accept(self):
# Called when a client connects to our socket
client_info = self.accept()
self.logger.debug('handle_accept() -> %s', client_info[1])
EchoHandler(sock=client_info[0])
# We only want to deal with one client at a time,
# so close as soon as we set up the handler.
# Normally you would not do this and the server
# would run forever or until it received instructions
# to stop.
self.handle_close()
return

def handle_close(self):
self.logger.debug('handle_close()')
self.close()
return

class EchoHandler(asyncore.dispatcher):
"""Handles echoing messages from a single client.
"""

def __init__(self, sock, chunk_size=256):
self.chunk_size = chunk_size
self.logger = logging.getLogger('EchoHandler%s' % str(sock.getsockname()))
asyncore.dispatcher.__init__(self, sock=sock)
self.data_to_write = []
return

def writable(self):
"""We want to write if we have received data."""
response = bool(self.data_to_write)
self.logger.debug('writable() -> %s', response)
return response

def handle_write(self):
"""Write as much as possible of the most recent message we have received."""
data = self.data_to_write.pop()
sent = self.send(data[:self.chunk_size])
if sent < len(data):
remaining = data[sent:]
self.data.to_write.append(remaining)
self.logger.debug('handle_write() -> (%d) "%s"', sent, data[:sent])
if not self.writable():
self.handle_close()

def handle_read(self):
"""Read an incoming message from the client and put it into our outgoing queue."""
data = self.recv(self.chunk_size)
self.logger.debug('handle_read() -> (%d) "%s"', len(data), data)
self.data_to_write.insert(0, data)

def handle_close(self):
self.logger.debug('handle_close()')
self.close()


class EchoClient(asyncore.dispatcher):
"""Sends messages to the server and receives responses.
"""

def __init__(self, host, port, message, chunk_size=512):
self.message = message
self.to_send = message
self.received_data = []
self.chunk_size = chunk_size
self.logger = logging.getLogger('EchoClient')
asyncore.dispatcher.__init__(self)
self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
self.logger.debug('connecting to %s', (host, port))
self.connect((host, port))
return

def handle_connect(self):
self.logger.debug('handle_connect()')

def handle_close(self):
self.logger.debug('handle_close()')
self.close()
received_message = ''.join(self.received_data)
if received_message == self.message:
self.logger.debug('RECEIVED COPY OF MESSAGE')
else:
self.logger.debug('ERROR IN TRANSMISSION')
self.logger.debug('EXPECTED "%s"', self.message)
self.logger.debug('RECEIVED "%s"', received_message)
return

def writable(self):
self.logger.debug('writable() -> %s', bool(self.to_send))
return bool(self.to_send)

def handle_write(self):
sent = self.send(self.to_send[:self.chunk_size])
self.logger.debug('handle_write() -> (%d) "%s"', sent, self.to_send[:sent])
self.to_send = self.to_send[sent:]

def handle_read(self):
data = self.recv(self.chunk_size)
self.logger.debug('handle_read() -> (%d) "%s"', len(data), data)
self.received_data.append(data)

if __name__ == '__main__':
import socket
import threading

logging.basicConfig(level=logging.DEBUG,
format='%(name)s: %(message)s',
)

address = ('localhost', 0) # let the kernel give us a port
server = EchoServer(address)
ip, port = server.address # find out what port we were given

client = EchoClient(ip, port, message=open('lorem.txt', 'r').read())

asyncore.loop()

The EchoServer and EchoHandler are defined in separate classes because they do different things. When EchoServer accepts a connection, a new socket is established. Rather than try to dispatch to individual clients within EchoServer, an EchoHandler is created to take advantage of the socket map maintained by asyncore.

$ python asyncore_echo_server.py
EchoServer: binding to ('127.0.0.1', 52235)
EchoClient: connecting to ('127.0.0.1', 52235)
EchoClient: writable() -> True
EchoServer: handle_accept() -> ('127.0.0.1', 52236)
EchoServer: handle_close()
EchoClient: handle_connect()
EchoClient: handle_write() -> (512) "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec
egestas, enim et consectetuer ullamcorper, lectus ligula rutrum leo, a
elementum elit tortor eu quam. Duis tincidunt nisi ut ante. Nulla
facilisi. Sed tristique eros eu libero. Pellentesque vel arcu. Vivamus
purus orci, iaculis ac, suscipit sit amet, pulvinar eu,
lacus. Praesent placerat tortor sed nisl. Nunc blandit diam egestas
dui. Pellentesque habitant morbi tristique senectus et netus et
malesuada fames ac turpis egestas. Aliquam viverra f"
EchoClient: writable() -> True
EchoHandler('127.0.0.1', 52235): writable() -> False
EchoHandler('127.0.0.1', 52235): handle_read() -> (256) "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec
egestas, enim et consectetuer ullamcorper, lectus ligula rutrum leo, a
elementum elit tortor eu quam. Duis tincidunt nisi ut ante. Nulla
facilisi. Sed tristique eros eu libero. Pellentesque ve"
EchoClient: handle_write() -> (225) "ringilla
leo. Nulla feugiat augue eleifend nulla. Vivamus mauris. Vivamus sed
mauris in nibh placerat egestas. Suspendisse potenti. Mauris massa. Ut
eget velit auctor tortor blandit sollicitudin. Suspendisse imperdiet
justo.
"
EchoClient: writable() -> False
EchoHandler('127.0.0.1', 52235): writable() -> True
EchoHandler('127.0.0.1', 52235): handle_read() -> (256) "l arcu. Vivamus
purus orci, iaculis ac, suscipit sit amet, pulvinar eu,
lacus. Praesent placerat tortor sed nisl. Nunc blandit diam egestas
dui. Pellentesque habitant morbi tristique senectus et netus et
malesuada fames ac turpis egestas. Aliquam viverra f"
EchoHandler('127.0.0.1', 52235): handle_write() -> (256) "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec
egestas, enim et consectetuer ullamcorper, lectus ligula rutrum leo, a
elementum elit tortor eu quam. Duis tincidunt nisi ut ante. Nulla
facilisi. Sed tristique eros eu libero. Pellentesque ve"
EchoHandler('127.0.0.1', 52235): writable() -> True
EchoClient: writable() -> False
EchoHandler('127.0.0.1', 52235): writable() -> True
EchoClient: handle_read() -> (256) "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec
egestas, enim et consectetuer ullamcorper, lectus ligula rutrum leo, a
elementum elit tortor eu quam. Duis tincidunt nisi ut ante. Nulla
facilisi. Sed tristique eros eu libero. Pellentesque ve"
EchoHandler('127.0.0.1', 52235): handle_read() -> (225) "ringilla
leo. Nulla feugiat augue eleifend nulla. Vivamus mauris. Vivamus sed
mauris in nibh placerat egestas. Suspendisse potenti. Mauris massa. Ut
eget velit auctor tortor blandit sollicitudin. Suspendisse imperdiet
justo.
"
EchoHandler('127.0.0.1', 52235): handle_write() -> (256) "l arcu. Vivamus
purus orci, iaculis ac, suscipit sit amet, pulvinar eu,
lacus. Praesent placerat tortor sed nisl. Nunc blandit diam egestas
dui. Pellentesque habitant morbi tristique senectus et netus et
malesuada fames ac turpis egestas. Aliquam viverra f"
EchoHandler('127.0.0.1', 52235): writable() -> True
EchoClient: writable() -> False
EchoHandler('127.0.0.1', 52235): writable() -> True
EchoClient: handle_read() -> (256) "l arcu. Vivamus
purus orci, iaculis ac, suscipit sit amet, pulvinar eu,
lacus. Praesent placerat tortor sed nisl. Nunc blandit diam egestas
dui. Pellentesque habitant morbi tristique senectus et netus et
malesuada fames ac turpis egestas. Aliquam viverra f"
EchoHandler('127.0.0.1', 52235): handle_write() -> (225) "ringilla
leo. Nulla feugiat augue eleifend nulla. Vivamus mauris. Vivamus sed
mauris in nibh placerat egestas. Suspendisse potenti. Mauris massa. Ut
eget velit auctor tortor blandit sollicitudin. Suspendisse imperdiet
justo.
"
EchoHandler('127.0.0.1', 52235): writable() -> False
EchoHandler('127.0.0.1', 52235): handle_close()
EchoClient: writable() -> False
EchoClient: handle_read() -> (225) "ringilla
leo. Nulla feugiat augue eleifend nulla. Vivamus mauris. Vivamus sed
mauris in nibh placerat egestas. Suspendisse potenti. Mauris massa. Ut
eget velit auctor tortor blandit sollicitudin. Suspendisse imperdiet
justo.
"
EchoClient: writable() -> False
EchoClient: handle_close()
EchoClient: RECEIVED COPY OF MESSAGE
EchoClient: handle_read() -> (0) ""

In this example the server, handler, and client objects are all being maintained in the same socket map by asyncore in a single process. To separate the server from the client, simply instantiate them from separate scripts and run asyncore.loop() in both. When a dispatcher is closed, it is removed from the map maintained by asyncore and the loop exits when the map is empty.


Working with Other Event Loops

It is sometimes necessary to integrate the asyncore event loop with an event loop from the parent application. For example, a GUI application would not want the UI to block until all asynchronous transfers are handled – that would defeat the purpose of making them asynchronous. To make this sort of integration easy, asyncore.loop() accepts arguments to set a timeout and to limit the number of times the loop is run. We can see their effect on the client by re-using HttpClient from the first example.

import asyncore
import logging

from asyncore_http_client import HttpClient

logging.basicConfig(level=logging.DEBUG,
format='%(name)s: %(message)s',
)

clients = [
HttpClient('http://www.doughellmann.com/PyMOTW/contents.html'),
HttpClient('http://www.python.org/'),
]

loop_counter = 0
while asyncore.socket_map:
loop_counter += 1
logging.debug('loop_counter=%s', loop_counter)
asyncore.loop(timeout=1, count=1)

Here we see that the client is only asked to read or data once per call into asyncore.loop(). Instead of our own while loop, we could call asyncore.loop() like this from a GUI toolkit idle handler or other mechanism for doing a small amount of work when the UI is not busy with other event handlers.

$ python asyncore_loop.py
http://www.doughellmann.com/PyMOTW/contents.html: connecting to ('www.doughellmann.com', 80)
http://www.python.org/: connecting to ('www.python.org', 80)
root: loop_counter=1
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: writable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_connect()
http://www.doughellmann.com/PyMOTW/contents.html: handle_write() -> "GET http://www.doughellmann.com/PyMOTW/contents.html HTTP/1.0

"
root: loop_counter=2
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 267 bytes
root: loop_counter=3
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 8192 bytes
root: loop_counter=4
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 6288 bytes
root: loop_counter=5
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 1448 bytes
root: loop_counter=6
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 1448 bytes
root: loop_counter=7
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 2896 bytes
root: loop_counter=8
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 2343 bytes
root: loop_counter=9
http://www.doughellmann.com/PyMOTW/contents.html: readable() -> True
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.doughellmann.com/PyMOTW/contents.html: handle_close()
http://www.doughellmann.com/PyMOTW/contents.html: handle_read() -> 0 bytes
root: loop_counter=10
http://www.python.org/: readable() -> True
http://www.python.org/: writable() -> True
http://www.python.org/: handle_connect()
http://www.python.org/: handle_write() -> "GET http://www.python.org/ HTTP/1.0

"
root: loop_counter=11
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=12
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=13
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=14
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=15
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=16
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=17
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=18
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=19
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=20
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=21
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=22
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1396 bytes
root: loop_counter=23
http://www.python.org/: readable() -> True
http://www.python.org/: handle_read() -> 1257 bytes
root: loop_counter=24
http://www.python.org/: readable() -> True
http://www.python.org/: handle_close()
http://www.python.org/: handle_read() -> 0 bytes


Working with Files

Normally you would want to use asyncore with sockets, but there are times when it is useful to read files asynchronously, too (to use files when testing network servers without requiring the network setup, or to read or write large data files in parts). For these situations, asyncore provides the file_dispatcher and file_wrapper classes.

import asyncore
import os

class FileReader(asyncore.file_dispatcher):

def writable(self):
return False

def handle_read(self):
data = self.recv(256)
print 'READ: (%d) "%s"' % (len(data), data)

def handle_expt(self):
# Ignore events that look like out of band data
pass

def handle_close(self):
self.close()

lorem_fd = os.open('lorem.txt', os.O_RDONLY)
reader = FileReader(lorem_fd)
asyncore.loop()

This example was tested under Python 2.5.2, so I am using os.open() to get a file descriptor for the file. For Python 2.6 and later, file_dispatcher automatically converts anything with a fileno() method to a file descriptor.

$ python asyncore_file_dispatcher.py
READ: (256) "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec
egestas, enim et consectetuer ullamcorper, lectus ligula rutrum leo, a
elementum elit tortor eu quam. Duis tincidunt nisi ut ante. Nulla
facilisi. Sed tristique eros eu libero. Pellentesque ve"
READ: (256) "l arcu. Vivamus
purus orci, iaculis ac, suscipit sit amet, pulvinar eu,
lacus. Praesent placerat tortor sed nisl. Nunc blandit diam egestas
dui. Pellentesque habitant morbi tristique senectus et netus et
malesuada fames ac turpis egestas. Aliquam viverra f"
READ: (225) "ringilla
leo. Nulla feugiat augue eleifend nulla. Vivamus mauris. Vivamus sed
mauris in nibh placerat egestas. Suspendisse potenti. Mauris massa. Ut
eget velit auctor tortor blandit sollicitudin. Suspendisse imperdiet
justo.
"
READ: (0) ""

See also

asyncore
The standard library documentation for this module.
asynchat
The asynchat module builds on asyncore to make it easier to create clients
and servers communicate by passing messages back and forth using a set protocol.
SocketServer
The SocketServer module article includes another example of the EchoServer with
threading and forking variants.

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