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

.. module:: asynchat
 :synopsis: Support for asynchronous command/response protocols.

.. moduleauthor:: Sam Rushing <rushing@nightmare.com>

.. sectionauthor:: Steve Holden <sholden@holdenweb.com>

Source code: :source:`Lib/asynchat.py`

.. deprecated:: 3.6
 Please use :mod:`asyncio` instead.

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.

.. data:: ac_in_buffer_size

 The asynchronous input buffer size (default ``4096``).

.. data:: ac_out_buffer_size

 The asynchronous output buffer size (default ``4096``).

Unlike :class:`asyncore.dispatcher`, :class:`async_chat` allows you to define a :abbr:`FIFO (first-in, first-out)` queue 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 bytes object. At this point the :class:`async_chat` object removes the producer from the queue and starts using the next producer, if any. When the producer queue 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.

.. method:: async_chat.close_when_done()

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

.. method:: async_chat.collect_incoming_data(data)

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

.. method:: async_chat.discard_buffers()

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

.. method:: async_chat.found_terminator()

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

.. method:: async_chat.get_terminator()

 Returns the current terminator for the channel.

.. method:: async_chat.push(data)

 Pushes data on to the channel's queue 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.

.. method:: async_chat.push_with_producer(producer)

 Takes a producer object and adds it to the producer queue associated with
 the channel. When all currently-pushed producers have been exhausted the
 channel will consume this producer's data by calling its :meth:`more`
 method and send the data to the remote endpoint.

.. method:: async_chat.set_terminator(term)

 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 :meth:`found_terminator` when the |
 | | string is found in the input stream |
 +-----------+---------------------------------------------+
 | *integer* | Will call :meth:`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 :meth:`found_terminator` is called.

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.

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()

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