asynchat — Asynchronous socket command/response handler
Source code: Lib/asynchat.py
This module exists for backwards compatibility only. For new code we
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
found_terminator() methods. It uses the same asynchronous loop as
asyncore, and the two types of channel,
asynchat.async_chat, can freely be mixed in the channel map.
asyncore.dispatcher server channel generates new
asynchat.async_chat channel objects as it receives incoming
This class is an abstract subclass of
asyncore.dispatcher. To make practical use of the code you must subclass
async_chat, providing meaningful
asyncore.dispatchermethods can be used, although not all make sense in a message/response context.
async_chatdefines 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_chatobject’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
The asynchronous output buffer size (default
async_chatallows 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_chatobject 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_chatsubclass your input methods
found_terminator()must handle the data that the channel receives asynchronously. The methods are described below.
Noneon 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
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
NotImplementedErrorexception. 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.
termmay 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
The channel continues to collect data forever
Note that any data following the terminator will be available for reading by the channel after
18.7.1. 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
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.
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.set_terminator(b"\r\n\r\n") 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(b"".join(self.ibuffer)) self.ibuffer =  if self.op.upper() == b"POST": clen = self.headers.getheader("content-length") self.set_terminator(int(clen)) else: 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, b"".join(self.ibuffer)) self.handling = True self.ibuffer =  self.handle_request()