Python 3.6.5 Documentation >  "asynchat" — Asynchronous socket command/response handler

"asynchat" — Asynchronous socket command/response handler
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**Source code:** Lib/asynchat.py

Deprecated since version 3.6: Please use "asyncio" instead.

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Note: This module exists for backwards compatibility only. For new
code we recommend using "asyncio".

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
"collect_incoming_data()" and "found_terminator()" methods. It uses
the same asynchronous loop as "asyncore", and the two types of
channel, "asyncore.dispatcher" and "asynchat.async_chat", can freely
be mixed in the channel map. Typically an "asyncore.dispatcher" server
channel generates new "asynchat.async_chat" channel objects as it
receives incoming connection requests.

class asynchat.async_chat

This class is an abstract subclass of "asyncore.dispatcher". To
make practical use of the code you must subclass "async_chat",
providing meaningful "collect_incoming_data()" and
"found_terminator()" methods. The "asyncore.dispatcher" methods can
be used, although not all make sense in a message/response context.

Like "asyncore.dispatcher", "async_chat" defines 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_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.

ac_in_buffer_size

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

ac_out_buffer_size

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

Unlike "asyncore.dispatcher", "async_chat" allows you to define a
FIFO (first-in, first-out) queue 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_chat" object removes the producer from the queue and starts
using the next producer, if any. When the producer queue 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_chat" subclass your input methods
"collect_incoming_data()" and "found_terminator()" must handle the
data that the channel receives asynchronously. The methods are
described below.

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.

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
"NotImplementedError" exception.

async_chat.discard_buffers()

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

async_chat.found_terminator()

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

async_chat.get_terminator()

Returns the current terminator for the channel.

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.

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 "more()" method and send the data to the remote
endpoint.

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 "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 |
+-------------+-----------------------------------------------+
| "None" | The channel continues to collect data forever |
+-------------+-----------------------------------------------+

Note that any data following the terminator will be available for
reading by the channel after "found_terminator()" is called.

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 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 "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()