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18.7. asynchat — Asynchronous socket command/response handler

Source code: Lib/


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

This module builds on the 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. asynchat defines the abstract class async_chat that you subclass, providing implementations of the collect_incoming_data() and found_terminator() methods. It uses the same asynchronous loop as asyncore, and the two types of channel, asyncore.dispatcher and asynchat.async_chat, can freely be mixed in the channel map. Typically an asyncore.dispatcher server channel generates new asynchat.async_chat channel objects as it receives incoming connection requests.

class asynchat.async_chat

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

Like asyncore.dispatcher, async_chat defines a set of events that are generated by an analysis of socket conditions after a select() call. Once the polling loop has been started the 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.


The asynchronous input buffer size (default 4096).


The asynchronous output buffer size (default 4096).

Unlike asyncore.dispatcher, async_chat allows you to define a first-in-first-out queue (fifo) of producers. A producer need have only one method, 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 more() method return the empty bytes object. At this point the async_chat object removes the producer from the fifo and starts using the next producer, if any. When the producer fifo is empty the handle_write() method does nothing. You use the channel object’s 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 async_chat subclass your input methods collect_incoming_data() and found_terminator() must handle the data that the channel receives asynchronously. The methods are described below.


Pushes a None on to the producer fifo. When this producer is popped off the fifo it causes the channel to be closed.


Called with data holding an arbitrary amount of received data. The default method, which must be overridden, raises a NotImplementedError exception.


In emergencies this method will discard any data held in the input and/or output buffers and the producer fifo.


Called when the incoming data stream matches the termination condition set by set_terminator(). The default method, which must be overridden, raises a NotImplementedError exception. The buffered input data should be available via an instance attribute.


Returns the current terminator for the channel.


Pushes data on to the channel’s fifo to ensure its transmission. This is all you need to do to have the channel write the data out to the network, although it is possible to use your own producers in more complex schemes to implement encryption and chunking, for example.


Takes a producer object and adds it to the producer fifo associated with the channel. When all currently-pushed producers have been exhausted the channel will consume this producer’s data by calling its more() method and send the data to the remote endpoint.


Sets the terminating condition to be recognized on the channel. term may be any of three types of value, corresponding to three different ways to handle incoming protocol data.

term Description
string Will call found_terminator() when the string is found in the input stream
integer Will call found_terminator() when the indicated number of characters have been received
None The channel continues to collect data forever

Note that any data following the terminator will be available for reading by the channel after found_terminator() is called.

18.7.1. asynchat - Auxiliary Classes

class asynchat.fifo(list=None)

A fifo holding data which has been pushed by the application but not yet popped for writing to the channel. A 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.


Returns True if and only if the fifo is empty.


Returns the least-recently push()ed item from the fifo.


Adds the given data (which may be a string or a producer object) to the producer fifo.


If the fifo is not empty, returns True, first(), deleting the popped item. Returns False, None for an empty fifo.

18.7.2. asynchat Example

The following partial example shows how HTTP requests can be read with async_chat. A web server might create an 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 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.

import asynchat

class http_request_handler(asynchat.async_chat):

    def __init__(self, sock, addr, sessions, log):
        asynchat.async_chat.__init__(self, sock=sock)
        self.addr = addr
        self.sessions = sessions
        self.ibuffer = []
        self.obuffer = b""
        self.reading_headers = True
        self.handling = False
        self.cgi_data = None
        self.log = log

    def collect_incoming_data(self, data):
        """Buffer the data"""

    def found_terminator(self):
        if self.reading_headers:
            self.reading_headers = False
            self.ibuffer = []
            if self.op.upper() == b"POST":
                clen = self.headers.getheader("content-length")
                self.handling = True
        elif not self.handling:
            self.set_terminator(None) # browsers sometimes over-send
            self.cgi_data = parse(self.headers, b"".join(self.ibuffer))
            self.handling = True
            self.ibuffer = []