Chapter 18 ■ rpC

338

Listing 18-4. Using XML-RPC Multicall

#!/usr/bin/env python3

Foundations of Python Network Programming, Third Edition

github.com/brandon-rhodes/fopnp/blob/m/py3/chapter18/xmlrpc_multicall.py

XML-RPC client performing a multicall

import xmlrpc.client

def main():
proxy = xmlrpc.client.ServerProxy('http://127.0.0.1:7001')
multicall = xmlrpc.client.MultiCall(proxy)
multicall.addtogether('a', 'b', 'c')
multicall.quadratic(2, -4, 0)
multicall.remote_repr([1, 2.0, 'three'])
for answer in multicall():
print(answer)

if name == 'main':
main()

When you run this script, you can watch the server’s command window to confirm that only a single HTTP
request is made in order to answer all three function calls that get made:

localhost - - [04/Oct/2010 00:16:19] "POST /RPC2 HTTP/1.0" 200 -

The ability to log messages like the preceding one can be turned off, by the way; such logging is controlled by one
of the options in SimpleXMLRPCServer. Note that the default URL used by both the server and the client is the path
/RPC2, unless you consult the documentation and configure the client and server differently.
Three final points are worth mentioning before I move on to examine another RPC mechanism:

•    There are two additional data types that sometimes prove hard to live without, so many

XML-RPC mechanisms support them: dates and the value that Python calls None (other

languages call this null or nil). Python’s client and server both support options that will

enable the transmission and reception of these nonstandard values.

•    Keyword arguments are, alas, not supported by XML-RPC because few languages are

sophisticated enough to include them. Some services get around this by allowing a dictionary

to be passed as a function’s final argument—or by disposing of positional arguments

altogether and using a single dictionary argument for every function that specifies all of its

parameters by name.

•    Finally, keep in mind that dictionaries can only be passed if all of their keys are strings,

whether normal or Unicode. See the “Self-Documenting Data” section later in this chapter for

more information on how to think about this restriction.

Although the entire point of an RPC protocol like XML-RPC is to let you forget about the details of network
transmission and focus on normal programming, you should see what your calls will look like on the wire at least
once! Here is the first call to quadratic() that the sample client program makes:

<?xml version='1.0'?>