17.7. asynchat — Asynchronous socket command/response handler

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 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 string. 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.


Sample implementation of a data collection rutine to be used in conjunction with _get_data() in a user-specified found_terminator().


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.


Will return and clear the data received with the sample _collect_incoming_data() implementation.


Returns the current terminator for the channel.


Called when the channel is closed. The default method silently closes the channel’s socket.


Called when a read event fires on the channel’s socket in the asynchronous loop. The default method checks for the termination condition established by set_terminator(), which can be either the appearance of a particular string in the input stream or the receipt of a particular number of characters. When the terminator is found, handle_read() calls the found_terminator() method after calling collect_incoming_data() with any data preceding the terminating condition.


Called when the application may write data to the channel. The default method calls the initiate_send() method, which in turn will call refill_buffer() to collect data from the producer fifo associated with the channel.


Creates a simple_producer object (see below) containing the data and pushes it on to the channel’s producer_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.


Should return True for the channel to be included in the set of channels tested by the select() loop for readability.


Refills the output buffer by calling the more() method of the producer at the head of the fifo. If it is exhausted then the producer is popped off the fifo and the next producer is activated. If the current producer is, or becomes, None then the channel is closed.


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.


Should return True as long as items remain on the producer fifo, or the channel is connected and the channel’s output buffer is non-empty.

17.7.1. asynchat - Auxiliary Classes and Functions

class class asynchat.simple_producer(data[, buffer_size=512])

A simple_producer takes a chunk of data and an optional buffer size. Repeated calls to its more() method yield successive chunks of the data no larger than buffer_size.


Produces the next chunk of information from the producer, or returns the empty string.

class class asynchat.fifo([list=None])

Each channel maintains 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.

The asynchat module also defines one utility function, which may be of use in network and textual analysis operations.

asynchat.find_prefix_at_end(haystack, needle)

Returns True if string haystack ends with any non-empty prefix of string needle.

17.7.2. 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.

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 = “” self.set_terminator(“rnrn”) self.reading_headers = True 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):
if self.reading_headers:

self.reading_headers = False self.parse_headers(“”.join(self.ibuffer)) self.ibuffer = [] if self.op.upper() == “POST”:

clen = self.headers.getheader(“content-length”) self.set_terminator(int(clen))
self.handling = True self.set_terminator(None) self.handle_request()
elif not self.handling:
self.set_terminator(None) # browsers sometimes over-send self.cgi_data = parse(self.headers, “”.join(self.ibuffer)) self.handling = True self.ibuffer = [] self.handle_request()