# :mod:asynchat --- Asynchronous socket command/response handler

Source code: :source:Lib/asynchat.py

This module builds on the :mod:asyncore infrastructure, simplifying asynchronous clients and servers and making it easier to handle protocols whose elements are terminated by arbitrary strings, or are of variable length. :mod:asynchat defines the abstract class :class:async_chat that you subclass, providing implementations of the :meth:collect_incoming_data and :meth:found_terminator methods. It uses the same asynchronous loop as :mod:asyncore, and the two types of channel, :class:asyncore.dispatcher and :class:asynchat.async_chat, can freely be mixed in the channel map. Typically an :class:asyncore.dispatcher server channel generates new :class:asynchat.async_chat channel objects as it receives incoming connection requests.

This class is an abstract subclass of :class:asyncore.dispatcher. To make practical use of the code you must subclass :class:async_chat, providing meaningful :meth:collect_incoming_data and :meth:found_terminator methods. The :class:asyncore.dispatcher methods can be used, although not all make sense in a message/response context.

Like :class:asyncore.dispatcher, :class:async_chat defines a set of events that are generated by an analysis of socket conditions after a :c:func:select call. Once the polling loop has been started the :class:async_chat object's methods are called by the event-processing framework with no action on the part of the programmer.

Two class attributes can be modified, to improve performance, or possibly even to conserve memory.

Unlike :class:asyncore.dispatcher, :class:async_chat allows you to define a first-in-first-out queue (fifo) of producers. A producer need have only one method, :meth:more, which should return data to be transmitted on the channel. The producer indicates exhaustion (i.e. that it contains no more data) by having its :meth:more method return the empty string. At this point the :class:async_chat object removes the producer from the fifo and starts using the next producer, if any. When the producer fifo is empty the :meth:handle_write method does nothing. You use the channel object's :meth:set_terminator method to describe how to recognize the end of, or an important breakpoint in, an incoming transmission from the remote endpoint.

To build a functioning :class:async_chat subclass your input methods :meth:collect_incoming_data and :meth:found_terminator must handle the data that the channel receives asynchronously. The methods are described below.

## asynchat - Auxiliary Classes

A :class:fifo holding data which has been pushed by the application but not yet popped for writing to the channel. A :class:fifo is a list used to hold data and/or producers until they are required. If the list argument is provided then it should contain producers or data items to be written to the channel.

## asynchat Example

The following partial example shows how HTTP requests can be read with :class:async_chat. A web server might create an :class:http_request_handler object for each incoming client connection. Notice that initially the channel terminator is set to match the blank line at the end of the HTTP headers, and a flag indicates that the headers are being read.

Once the headers have been read, if the request is of type POST (indicating that further data are present in the input stream) then the Content-Length: header is used to set a numeric terminator to read the right amount of data from the channel.

The :meth:handle_request method is called once all relevant input has been marshalled, after setting the channel terminator to None to ensure that any extraneous data sent by the web client are ignored.

class http_request_handler(asynchat.async_chat):

def __init__(self, sock, addr, sessions, log):
asynchat.async_chat.__init__(self, sock=sock)
self.sessions = sessions
self.ibuffer = []
self.obuffer = ""
self.set_terminator("\r\n\r\n")
self.handling = False
self.cgi_data = None
self.log = log

def collect_incoming_data(self, data):
"""Buffer the data"""
self.ibuffer.append(data)

def found_terminator(self):
self.ibuffer = []
if self.op.upper() == "POST":