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+
+: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>
+
+
+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
+ :cfunc:`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 first-in-first-out queue (fifo) 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 string. At this point the
+ :class:`async_chat` object removes the producer from the fifo and starts
+ using the next producer, if any. When the producer fifo 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 fifo. When this producer is popped off
+ the fifo 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._collect_incoming_data(data)
+
+ Sample implementation of a data collection rutine to be used in conjunction
+ with :meth:`_get_data` in a user-specified :meth:`found_terminator`.
+
+
+.. method:: async_chat.discard_buffers()
+
+ In emergencies this method will discard any data held in the input and/or
+ output buffers and the producer fifo.
+
+
+.. 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_data()
+
+ Will return and clear the data received with the sample
+ :meth:`_collect_incoming_data` implementation.
+
+
+.. method:: async_chat.get_terminator()
+
+ Returns the current terminator for the channel.
+
+
+.. method:: async_chat.handle_close()
+
+ Called when the channel is closed. The default method silently closes the
+ channel's socket.
+
+
+.. method:: async_chat.handle_read()
+
+ 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 :meth:`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, :meth:`handle_read` calls the
+ :meth:`found_terminator` method after calling :meth:`collect_incoming_data`
+ with any data preceding the terminating condition.
+
+
+.. method:: async_chat.handle_write()
+
+ Called when the application may write data to the channel. The default
+ method calls the :meth:`initiate_send` method, which in turn will call
+ :meth:`refill_buffer` to collect data from the producer fifo associated
+ with the channel.
+
+
+.. method:: async_chat.push(data)
+
+ Creates a :class:`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.
+
+
+.. method:: async_chat.push_with_producer(producer)
+
+ 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 :meth:`more`
+ method and send the data to the remote endpoint.
+
+
+.. method:: async_chat.readable()
+
+ Should return ``True`` for the channel to be included in the set of
+ channels tested by the :cfunc:`select` loop for readability.
+
+
+.. method:: async_chat.refill_buffer()
+
+ Refills the output buffer by calling the :meth:`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.
+
+
+.. 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.
+
+
+.. method:: async_chat.writable()
+
+ 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.
+
+
+asynchat - Auxiliary Classes and Functions
+------------------------------------------
+
+
+.. class:: simple_producer(data[, buffer_size=512])
+
+ A :class:`simple_producer` takes a chunk of data and an optional buffer
+ size. Repeated calls to its :meth:`more` method yield successive chunks of
+ the data no larger than *buffer_size*.
+
+
+ .. method:: more()
+
+ Produces the next chunk of information from the producer, or returns the
+ empty string.
+
+
+.. class:: fifo([list=None])
+
+ Each channel maintains a :class:`fifo` holding data which has been pushed
+ by the application but not yet popped for writing to the channel. A
+ :class:`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.
+
+
+ .. method:: is_empty()
+
+ Returns ``True`` if and only if the fifo is empty.
+
+
+ .. method:: first()
+
+ Returns the least-recently :meth:`push`\ ed item from the fifo.
+
+
+ .. method:: push(data)
+
+ Adds the given data (which may be a string or a producer object) to the
+ producer fifo.
+
+
+ .. method:: pop()
+
+ If the fifo is not empty, returns ``True, first()``, deleting the popped
+ item. Returns ``False, None`` for an empty fifo.
+
+The :mod:`asynchat` module also defines one utility function, which may be of
+use in network and textual analysis operations.
+
+
+.. function:: find_prefix_at_end(haystack, needle)
+
+ Returns ``True`` if string *haystack* ends with any non-empty prefix of
+ string *needle*.
+
+
+.. _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. ::
+
+ 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("\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("".join(self.ibuffer))
+ self.ibuffer = []
+ if self.op.upper() == "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, "".join(self.ibuffer))
+ self.handling = True
+ self.ibuffer = []
+ self.handle_request()