Recipe14.13.Turning Data into Web Pages Through Templates

Recipe 14.13. Turning Data into Web Pages Through Templates

Credit: Valentino Volonghi

Problem

You need to turn some Python data into web pages based on templates, meaning files or strings of HTML code in which the data gets suitably inserted.

Solution

Templating with Python can be accomplished in an incredible number of ways. but my favorite is Nevow.

The Nevow web toolkit works with the Twisted networking framework to provide excellent templating capabilities to web sites that are coded on the basis of Twisted's powerful asynchronous model. For example, here's one way to render a list of dictionaries into a web page according to a template, with Nevow and Twisted:

from twisted.application import service, internet from nevow import rend, loaders, appserver dct = [{'name':'Mark', 'surname':'White', 'age':'45'},        {'name':'Valentino', 'surname':'Volonghi', 'age':'21'},        {'name':'Peter', 'surname':'Parker', 'age':'Unknown'},       ] class Pg(rend.Page):     docFactory = loaders.htmlstr("""     <html><head><title>Names, Surnames and Ages</title></head>         <body>             <ul nevow:data="dct" nevow:render="sequence">                 <li nevow:pattern="item" nevow:render="mapping">                     <span><nevow:slot name="name"/>&nbsp;</span>                     <span><nevow:slot name="surname"/>&nbsp;</span>                     <span><nevow:slot name="age"/></span>                 </li>             </ul>         </body>     </html>     """)     def _ _init_ _(self, dct):         self.data_dct = dct         rend.Page._ _init_ _(self) site = appserver.NevowSite( Pg(dct) ) application = service.Application("example") internet.TCPServer(8080, site).setServiceParent(application)

Save this code to nsa.tac. Now, entering at a shell command prompt twistd -noy nsa.tac serves up the data, formatted into HTML as the template specifies, as a tiny web site. You can visit the site, at http://localhost:8080, by running a browser on the same computer where the twistd command is running. On the command window where twistd is running, you'll see a lot of information, roughly equivalent to a typical web server's log file.

Discussion

This recipe uses Twisted (http://www.twistedmatrix.com) for serving a little web site built with Nevow (http://nevow.com/). Twisted is a large and powerful framework for writing all kinds of Python programs that interact with the network (including, of course, web servers). Nevow is a web application construction kit, normally used in cooperation with a Twisted server but usable in other ways. For example, you could write Nevow CGI scripts that can run with any web server. (Unfortunately, CGI scripts' performance might prove unsatisfactory for many applications, while Twisted's performance and scalability are outstanding.)

A vast range of choices is available for packages you can use to perform templating with Python. You can look up some of them at http://www.webwareforpython.org/Papers/Templates/ (which lists a dozen packages suitable for use with the Webware web development toolkit), and specific ones at http://htmltmpl.sourceforge.net/, http://freespace.virgin.net/hamish.sanderson/htmltemplate.html, http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52305, http://www.alcyone.com/pyos/empy/, http://www.entrian.com/PyMeld/... and many, many more besides. I definitely don't claim to have thoroughly tried each and every one of these dozens of templating systems in production situations, and I wonder whether anyone can truthfully make such a claim! However, out of all I have tried, my favorite is Nevow.

Nevow builds web pages by working on the HTML DOM tree. Recipe 14.14 shows how you can build such a DOM tree from within your program by using the stan subsystem of Nevow. This recipe shows that you can also building a DOM tree from HTML source, known as a template. In this case, for simplicity, we keep that template source in a string in our code, and load the DOM for it by calling loaders.htmlstr; more commonly, we keep the template source in a separate .html file, and load the DOM for it by calling loaders.htmlfile.

Examining the HTML string, you will notice it contains, besides standard HTML tags and attributes, a few attributes and one tag from the 'nevow:' namespace, such as 'nevow:slot', 'nevow:data' and 'nevow:render'. These additions are in accord with the HTML standards, and also, in practice, the additions work with all browsers. They amount to Nevow defining its own small supplementary namespace, so that HTML templates can express directives to Nevow for building a dynamic page from the template together with data coming from Python code. Note that the attributes and tags in the 'nevow:' namespace do not remain in the HTML output from Nevow: you can verify that, as you visit the web page served by this recipe's script, by asking your browser to "view source". Nevertheless, it's important that template files are perfectly correct HTML: this means those files can be edited with all kinds of specialized HTML editor programs! So, like many other templating systems, Nevow chooses to have correct HTML as its input, as well as (of course) as its output.

The 'nevow:data' directive defines the source of the data for the page: in this case, we use the data_dct attribute of the Pg class instance which is building the page. The 'nevow:render' directive defines the method to use for rendering the data into HTML strings. In this case, we use two standard rendering methods supplied by Nevow itself: sequence, for rendering a sequence of items, such as a list, one after the other; and mapping, for rendering items of a mapping, such as a dictionary, based on the items' keys appearing as name attributes of nevow:slot tags. More generally, we could code our own rendering methods in any class that subclasses rend.Page.

After defining the Pg class, the recipe continues by building a site object, then an application object, then a TCP server on port 8080 using that site and applicationall of this building makes up a common Twisted idiom. The source file nsa.tac into which you save the code from this recipe is not meant to be run with the usual python interpreter. Rather, you should run nsa.tac with the twistd command that you installed as part of Twisted's own installation procedure: twistd handles all the startup, daemonization, and logging issues, depending on the flags we pass to it. That is exactly why, by convention, one should normally use file extension .tac, rather than .py, for source files that are meant to be run with twistd, rather than directly with pythonto avoid any confusion.

Given the experimental, toy-like nature of this recipe, you should pass the flags -noy, to ask twistd to run in the foreground and to "log" information to standard output rather than to some file. An even better idea is to read up on twistd in the Twisted documentation, to learn about all the options for the flags.

Recipe14.13.Turning Data into Web Pages Through Templates