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decorated template tags in django

published: 06-01-2012 / updated: 06-01-2012
posted in: development, programming, python, tips
by Daniel Molina Wegener

Django provides an API to create custom template tags for those applications mounted over this nice Web Application Framework. Sometimes we need to decorate our functions, but you cannot decorate a function registered as tag in Django. The most elegant solution to solve this problem is to use a closure which will wrap the template tag function to be used decorated template tag.

from django import template
from django.conf import settings

from owner.common.decorators import html_escape_output

register = template.Library()

def fmt_owner_name(_owner):
    Formats the Owner Name
    def _fmt_owner_name(owner):
        nms = u'%s %s' % (first_word_on_string(owner.first_name),
        if len(nms) > settings.MAX_NAME_ON_CARD:
            return u'%s...' % (nms[0:settings.MAX_NAME_ON_CARD])
            return nms
    return _format_owner_name(_owner)
fmt_owner_name = register.simple_tag(fmt_owner_name)

So, in this example, the template tag will escape the owner name, where all template tags in Django are considering HTML output as safe output, so they do not escape the HTML output automatically, even if you use is_safe and needs_autoescape properties, they require special treatment when you need to escape the HTML tags. In the example above the closure _fmt_owner_name is decorated with @html_escape_output which forces escaped HTML output to the Django template.

Also you can stack any number of decorators that you want. I think that this avoids using cgi.escape and similar stuff on each return statement. This helps you on Django template tag processing with common operations, like the example above. And it reduces the amount of code, making it more legible and simple, where we are using the fourth aphorism from the Zen of Python: «complex is better than complicated». Then we apply the template tag in the Django template as follows.

<div class="owner-name">
  {% fmt_owner_name owner_obj %}

I hope that this will help you on the task of reducing the amount of repeated calls.

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