Adding gzip encoding to Python-based XML-RPC clients is pretty simple.  My
approach is based upon the observation that (in my code anyway - database
queries) the request is usually much smaller than the response.  The
client's request is never gzip'd.  It only tells the server if it will
accept a gzip'd response.

To add gzip-awareness to xmlrpclib.py take the following steps:

1. Near the top of the file add the following lines:

    try:
	import zlib
    except ImportError:
	zlib = None

2. In Transport.request, add an Accept-Encoding header if zlib was imported
   okay: 

    if zlib is not None:
	h.putheader("Accept-Encoding", "gzip")

3. Replace the "return self.parse_response..." with

    try:
	content_encoding = headers["content-encoding"]
	if content_encoding and content_encoding == "x-gzip":
	    return self.parse_response_gzip(h.getfile())
	elif content_encoding:
	    raise ProtocolError(host + handler,
				500,
				"Unknown encoding type: %s" %
				content_encoding,
				headers)
	else:
	    return self.parse_response(h.getfile())
    except KeyError:
	return self.parse_response(h.getfile())

4. Add a parse_response_gzip method to the Transport class:

    def parse_response_gzip(self, f):
	# read response from input file, and parse it

        #t = time.time()
        dc = zlib.decompressobj()
	p, u = getparser()
        resp = []

        append = resp.append
        read = f.read

	while 1:
	    response = read(8192)
	    if not response: break
            append(response)

        response = dc.decompress(string.join(resp, ""))
        #print "decompress time: %.2f" % (time.time()-t)
        #t = time.time()
        p.feed(response)
        rest = dc.flush()
        if rest: p.feed(rest)
        
	f.close()
	p.close()

        #print "parse time: %.2f" % (time.time()-t)
	return u.close()

To modify an XML-RPC server to handle gzip encoding the steps are similar.

1. Try and import zlib:

    try:
	import zlib
    except ImportError:
	zlib = None

2. In do_POST, check to see if gzip was sent as an encoding:

    try:
	accept_encoding = self.headers["accept-encoding"]
	if accept_encoding:
	    accept_encoding = map(string.strip,
				  string.split(accept_encoding, ","))
	else:
	    accept_encoding = []
    except KeyError:
	accept_encoding = []        

3. Add appropriate tests later in do_POST to make sure you can and should
   send a gzip'd response.  The requirements are that zlib was imported and
   gzip was one of the encodings sent by the client.  I also avoid gzip'ing
   short responses.

    if (zlib is not None and
	"gzip" in accept_encoding and
	len(response) > 1000):
	self.send_header("Content-encoding", "x-gzip")
	response = zlib.compress(response)
    self.send_header("Content-length", "%d" % len(response))