The asynchat module

This module is an extension to asyncore. It provides additional support for line oriented protocols. It also provides improved buffering support, via the push methods and the “producer” mechanism.

To configure the class, call the set_terminator method to specify what terminator to use. The asyncore layer will then call collect_incoming_data repeatedly, as data arrives, until it sees the given terminator, and calls found_terminator.

To read a given number of bytes without checking for terminators (e.g. a HTTP body), you can set the terminator to a byte count (an integer). To disable the terminator check, set the terminator to None.

The following example implements a very minimal HTTP responder. It simply returns a HTML document containing information from HTTP request (the output appears in the browser window):

 
Example: Using the asynchat module to implement a minimal HTTP server
# File: asynchat-example-1.py

import asyncore, asynchat
import os, socket, string

PORT = 8000

class HTTPChannel(asynchat.async_chat):

    def __init__(self, server, sock, addr):
        asynchat.async_chat.__init__(self, sock)
        self.set_terminator("\r\n")
        self.request = None
        self.data = ""
        self.shutdown = 0

    def collect_incoming_data(self, data):
        self.data = self.data + data

    def found_terminator(self):
        if not self.request:
            # got the request line
            self.request = string.split(self.data, None, 2)
            if len(self.request) != 3:
                self.shutdown = 1
            else:
                self.push("HTTP/1.0 200 OK\r\n")
                self.push("Content-type: text/html\r\n")
                self.push("\r\n")
            self.data = self.data + "\r\n"
            self.set_terminator("\r\n\r\n") # look for end of headers
        else:
            # return payload.
            self.push("<html><body><pre>\r\n")
            self.push(self.data)
            self.push("</pre></body></html>\r\n")
            self.close_when_done()

class HTTPServer(asyncore.dispatcher):

    def __init__(self, port):
        asyncore.dispatcher.__init__(self)
        self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
        self.bind(("", port))
        self.listen(5)

    def handle_accept(self):
        conn, addr = self.accept()
        HTTPChannel(self, conn, addr)

#
# try it out

s = HTTPServer(PORT)
print "serving at port", PORT, "..."
asyncore.loop()

GET / HTTP/1.1
Accept: */*
Accept-Language: en, sv
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; Bruce/1.0)
Host: localhost:8000
Connection: Keep-Alive

The producer interface allows you to “push” objects that are too large to store in memory. asyncore calls the producer’s more method whenever it needs more data. To signal end of file, just return an empty string.

The following example implements a very simple file-based HTTP server, using a simple FileProducer class that reads data from a file, a few kilobytes at the time.

 
Example: Using the asynchat module to implement a simple HTTP server
# File: asynchat-example-2.py

import asyncore, asynchat
import os, socket, string, sys
import StringIO, mimetools

ROOT = "."

PORT = 8000

class HTTPChannel(asynchat.async_chat):

    def __init__(self, server, sock, addr):
        asynchat.async_chat.__init__(self, sock)
        self.server = server
        self.set_terminator("\r\n\r\n")
        self.header = None
        self.data = ""
        self.shutdown = 0

    def collect_incoming_data(self, data):
        self.data = self.data + data
        if len(self.data) > 16384:
            # limit the header size to prevent attacks
            self.shutdown = 1

    def found_terminator(self):
        if not self.header:
            # parse http header
            fp = StringIO.StringIO(self.data)
            request = string.split(fp.readline(), None, 2)
            if len(request) != 3:
                # badly formed request; just shut down
                self.shutdown = 1
            else:
                # parse message header
                self.header = mimetools.Message(fp)
                self.set_terminator("\r\n")
                self.server.handle_request(
                    self, request[0], request[1], self.header
                    )
                self.close_when_done()
            self.data = ""
        else:
            pass # ignore body data, for now

    def pushstatus(self, status, explanation="OK"):
        self.push("HTTP/1.0 %d %s\r\n" % (status, explanation))


class FileProducer:
    # a producer which reads data from a file object

    def __init__(self, file):
        self.file = file

    def more(self):
        if self.file:
            data = self.file.read(2048)
            if data:
                return data
            self.file = None
        return ""


class HTTPServer(asyncore.dispatcher):

    def __init__(self, port=None, request=None):
        asyncore.dispatcher.__init__(self)
        if not port:
            port = 80
        self.port = port
        if request:
            self.handle_request = request # external request handler
        self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
        self.bind(("", port))
        self.listen(5)

    def handle_accept(self):
        conn, addr = self.accept()
        HTTPChannel(self, conn, addr)

    def handle_request(self, channel, method, path, header):
        try:
            # this is not safe!
            while path[:1] == "/":
                path = path[1:]
            filename = os.path.join(ROOT, path)
            print path, "=>", filename
            file = open(filename, "r")
        except IOError:
            channel.pushstatus(404, "Not found")
            channel.push("Content-type: text/html\r\n")
            channel.push("\r\n")
            channel.push("<html><body>File not found.</body></html>\r\n")
        else:
            channel.pushstatus(200, "OK")
            channel.push("Content-type: text/html\r\n")
            channel.push("\r\n")
            channel.push_with_producer(FileProducer(file))

#
# try it out

s = HTTPServer(PORT)
print "serving at port", PORT
asyncore.loop()

serving at port 8000
log: adding channel <HTTPServer  at 8e54d0>
log: adding channel <HTTPChannel  at 8e64a0>
samples/sample.htm => .\samples/sample.htm
log: closing channel 96:<HTTPChannel connected at 8e64a0>

Note: In Python 2.4 and later, you must call the parent’s __init__ method when you subclass from asyncore.dispatcher. In earlier versions, that call is optional.

 

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