The polymathematical James Tauber, author of the software that runs this site has announced that he’s working on the next version. Which has prompted me to dust-off one of my firmly held opinions about the best way to do HTML templates and offer it for consideration.

I think the best mechanism is to use plain-old HTML pages (best if it’s well-formed XML, XHTML even), with class names to mark the substitutable bits.

How does it look?

This is is a tiny sample. Elements with a class attribute that begins with ‘ins’ would have their content replaced with runtime data, in this case with a bit of data called ‘page-title’. The designer can also supply a CSS stylesheet with a rule for ‘page-title’. (You’re allowed to have multiple space-separated classes, apparently.)

<title class='ins page-title'>Placeholder title</title>
<h1 class='ins page-title>Placeholder title</h1>    

Why is this the ‘best’ way?

  • Templates are completely implementation-language neutral.
  • Designers can work with sample files with their web page and CSS design tools of choice, without worrying about yet another set of proprietary codes.
  • If the world was perfect, we could all agree on some standard names for page elements and then we could swap templates between applications. Yes I know, I am being silly.

Limitations? Well it is meant to be very simple.

First of all the fact that you can’t embed code in your template, is emphatically not a limitation. Code does not belong in a web page template. If you want that kind of thing go see the PHP, ASP or JSP people.

The minor limitation is that this approach makes it a bit hard to use a simple regular expression or string interpolation to populate your template, as the place-holder elements could contain fairly extensive sample content.

In the case of Leonardo you could pre-compile the templates, so

<title class='page-title'>...</title> 

ends up just the way it is in the current version of Leonardo:

<title class='page-title>%(page-title)s</title>

And the entire template can be populated by feeding it a dictionary with all your page data in it:

print template % pagestuff

Or even better, wrap it up in a plain-old HTML template class so you can say.

page = plainoldhtmltemplate('basic.htm')
page'page-title' = 'Home page'

The closest existing thing I can find is HTMLTemplate, which allows you to do what I’m talking about here, and couple of other things, including adding repeating elements, the only problem being that it will discard the class attributes.

But really, we are talking about only a few lines of Python here to build a minimal implementation, so this lunchtime I did:

  import libxml2

  class plainoldhtmltemplate(dict):
      templatestring = ""

      def __init__(self, templatefilename = None):
          if templatefilename:

      def loadtemplate(self, templatefilename):   
             rawtemplate = libxml2.parseFile(templatefilename, None)
              rawtemplate = libxml2.htmlParseFile(templatefilename, None)

          insertions = rawtemplate.xpathEval("//*starts-with(@class,'ins ')")
          #TODO: 'sub ' for places you want to replace the placeholder
          for insert in insertions:
              classname = insert.prop('class')4: #after insert
              selfclassname = ''
              insert.setContent('%%(%s)s' % classname)

          self.templatestring = rawtemplate.serialize()

      def render(self):
          return self.templatestring % self


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