Rolling Your Own Servers in Python

Most of the Internet modules we looked at in the last few chapters deal with client-side interfaces such as FTP and POP, or special server-side protocols such as CGI that hide the underlying server itself. If you want to build servers in Python by hand, you can do so either manually or by using higher-level tools.

15.10.1 Coding Solutions

We saw the sort of code needed to build servers manually in Chapter 10. Python programs typically implement servers either by using raw socket calls with threads, forks, or selects to handle clients in parallel, or by using the SocketServer module.

In either case, to serve requests made in terms of higher-level protocols such as FTP, NNTP, and HTTP, you must listen on the protocol's port and add appropriate code to handle the protocol's message conventions. If you go this route, the client-side protocol modules in Python's standard library can help you understand the message conventions used. You may also be able to uncover protocol server examples in the Demos and Tools directories of the Python source distribution and on the Net at large (search http://www.python.org). See prior chapters for more details on writing socket-based servers.

As a higher-level interface, Python also comes with precoded HTTP web protocol server implementations, in the form of three standard modules. BaseHTTPServer implements the server itself; this class is derived from the standard SocketServer.TCPServer class. SimpleHTTPServer and CGIHTTPServer implement standard handlers for incoming HTTP requests; the former handles simple web page file requests, while the latter also runs referenced CGI scripts on the server machine by forking processes.

For example, to start a CGI-capable HTTP server, simply run Python code like that shown in Example 15-19 on the server machine.

Example 15-19. PP2EInternetOtherwebserver.py

#!/usr/bin/python ############################################ # implement a HTTP server in Python which # knows how to run server-side CGI scripts; # change root dir for your server machine ############################################ import os from BaseHTTPServer import HTTPServer from CGIHTTPServer import CGIHTTPRequestHandler os.chdir("/home/httpd/html") # run in html root dir srvraddr = ("", 80) # my hostname, portnumber srvrobj = HTTPServer(srvraddr, CGIHTTPRequestHandler) srvrobj.serve_forever( ) # run as perpetual demon

This assumes that you have appropriate permissions to run such a script, of course; see the Python library manual for more details on precoded HTTP server and request handler modules. Once you have your server running, you can access it in any web browser or by using either the Python httplib module, which implements the client side of the HTTP protocol, or the Python urllib module, which provides a file-like interface to data fetched from a named URL address (see the urllib examples in Chapter 11Chapter 11, and Chapter 13, and use a URL of the form "http://..." to access HTTP documents).

15.10.2 Packaged Solutions

Finally, you can deploy full-blown, open source, and Python-friendly web servers and tools that are freely available on the Net. These may change over time too, but here are a few current options:

Medusa, asyncore

The Medusa system (http://www.nightmare.com/medusa) is an architecture for building long-running, high-performance network servers in Python, and is used in several mission-critical systems. Beginning in Python 1.5.2, the core of Medusa is now standard in Python, in the form of the asyncore and asynchat library modules. These standard modules may be used by themselves to build high-performance network servers, based on an asynchronous, multiplexing, single-process model. They use an event loop built using the select system call presented in Chapter 10 of this book to provide concurrency without spawning threads or processes, and are well-suited to handling short-lived transactions. See the Python library for details. The complete Medusa system (not shipped with Python) also provides precoded HTTP and FTP servers; it is free for noncommercial use, but requires a license otherwise.

Zope

If you are doing any server-side work at all, be sure to consider the Zope open source web application server, described earlier in this chapter and at http://www.zope.org. Zope provides a full-featured web framework that implements an object model that is well beyond standard server-side CGI scripting. The Zope world has also developed full-blown servers (e.g., Zserver).

Mailman

If you are looking for email list support, be sure to explore the GNU mailing list manager, otherwise known as Mailman. Written in Python, Mailman provides a robust, quick, and feature-rich email discussion list tool. Mailman allows users to subscribe over the Web, supports web-based administration, and provides mail-to-news gateways and integrated spam prevention (spam of the junk mail variety, that is). At this time, http://www.list.org is the place to find more Mailman details.

Apache

If you are adventurous, you may be interested in the highly configurable Apache open source web server. Apache is one of the dominant servers used on the Web today, despite its free nature. Among many other things, it supports running Python server-side scripts in a variety of modes; see the site http://www.apache.org for details on Apache itself.

PyApache

If you use Apache, also search the Python web site for information on the PyApache Apache server module (sometimes called mod_pyapache), which embeds a Python interpreter inside Apache to speed up the process of launching Python server-side scripts. CGI scripts are passed to the embedded interpreter directly, avoiding interpreter startup costs. PyApache also opens up the possibility of scripting Apache's internal components.

mod_python

As I wrote this chapter, another package for embedding Python within the Apache web server appeared on the open source landscape: mod_python, available at http://www.modpython.org. According to its release notes, mod_python also allows Python to be embedded in Apache, with a substantial boost in performance and added flexibility. The beta release announcement for this system appeared on comp.lang.python the very week that this section was written, so check the Web for its current status.

Be sure to watch http://www.python.org for new developments on the server front, as well as late-breaking advances in Python web scripting techniques in general.