Simon Willison’s Weblog

Ludicrously simple templates with Python

A long, long time ago I wrote my first ever PHP templating system. It was pretty simple; it consisted of a function that took two arguments: the name of a template file, and an associative array of replacements to make on that file.

I’ve finally got around to playing with Python CGIs for web development recently, and decided I needed a similar system. Thanks to Python’s powerful string formatting operator, it ended up as a one-liner:

def template(file, vars):
    return open(templatedir.template, 'r').read() % vars

Presuming you’ve set templatedir at the top of the script, the above function lets you load a template and make some simple replacements on it with a single function call. For example:

<h3>%(title)s</h3>

%(body)s

<p class="footer">Posted: %(date)s</p>

With the above saved in the template directory as “entry.tpl”, the template function above can be used thus:

print template('entry.tpl', {
    'title':'A blog entry', 
    'body':'Entry goes here...',
    'date':'3rd July 2003'})

The work is all done by the % vars bit at the end of the line. Since vars is a dictionary, Python substitutes the named items in the dictionary for their corresponding %(varname)s tokens in the string loaded from the template file. More information on string formatting operations can be found in the manual.

As templating systems go, it’s far from the most useful or complete solution. It does however show that a little Python can go quite a long way.