sandbox/morph / Doc / library / asynchat.rst

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

Source code: :source:`Lib/`

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.

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.

asynchat - Auxiliary Classes

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.

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.reading_headers = True
        self.handling = False
        self.cgi_data = None
        self.log = log

    def collect_incoming_data(self, data):
        """Buffer the data"""

    def found_terminator(self):
        if self.reading_headers:
            self.reading_headers = False
            self.ibuffer = []
            if self.op.upper() == "POST":
                clen = self.headers.getheader("content-length")
                self.handling = True
        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 = []