: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.

--------------

.. note::

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

recommend using :mod:`asyncio`.

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.

.. class:: async_chat()

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:

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()