Using custom django template helpers with Google App Engine

This is dedicated to all you django lovers out there. If you wondered how to use custom filter functions with your Google App Engine applications, here you go:

Create a new file (I created mine in common/templatefilters.py) to hold your template helper functions.

In this file, we need to register our custom filters, so:

# import the webapp module
from google.appengine.ext import webapp
# get registry, we need it to register our filter later.
register = webapp.template.create_template_register()

After we got the registry, we now can define our custom function:

def truncate(value,maxsize,stopper = '...'):
    """ truncates a string to a given maximum
        size and appends the stopper if needed """
    stoplen = len(stopper)
    if len(value) > maxsize and maxsize > stoplen:
       return value[:(maxsize-stoplen)] + stopper
    else:
       return value[:maxsize]

Then, we need to register our filter as following:

register.filter(truncate)

Now go to your bootstrap file (the one with your main function - in my case base.py - see image above) and add the library (if you used a different module than common.templatefilters, you need to specifiy it here):

webapp.template.register_template_library(
 'common.templatefilters')

After adding this line of code, your template functions will get added to django and are available to use within your html templates:

This will truncate the variable somevar to 20 characters and add "..." to the end, if needed.