Using pyjade with webapp2 on Google App Engine

And now for something completely different. I got a little distracted again by an idea for a simple social networking mashup I’ve had and decided that the best place to deploy it would probably on Google App Engine.  App Engine supports Scala and I’ve seen one or two positive reports about using Scalatra on it, so I thought it wouldn’t be too hard to spin something up quickly using my existing code.

It turned out not to be as easy as that, unfortunately; I may try to elaborate on the reasons in a later post, but it seems to come down to Scalate always checking the filesystem for its template routines.  In any event, I found that I was spending a lot of time trying to glue weird bits of infrastructure to one another rather than working on my idea.

With that in mind, I decided to start over in Python, and of course as my first task I spent a long time glueing random bits of infrastructure together.  In this case I’ve been successful, though, so here’s a brief writeup on getting Jade templates to work with Google App Engine and Google’s webapp2 in python.

1. Add dependencies to include webapp2’s jinja2 module in your project.

This is as simple as adding this bit of code to app.yaml:

- name: jinja2
  version: latest

2. Install pyjade in your environment.

I’ve been using a layout similar to the one mentioned in this Stack Overflow answer.  It’s puzzling that Google doesn’t really mention anything about a standard way to get libraries and stuff from the wider Python ecosystem installed in a development environment, but essentially I set up a virtualenv, ran pip install pyjade, and then symlinked the pyjade directory from the virtualenv lib directory into the project’s src directory.

3. Add a custom Jinja2 factory.

This is where the magic happens.  We write a factory function, jinja2_factory, whose purpose is to add pyjade’s included Jinja2 extension into the Jinja2 instance’s Environment.

  def jade_factory(app):
    j = jinja2.Jinja2(app)
    return j

Then, in our handler’s jinja2 method, we pass along the factory method to it:

  def jinja2(self):
    return jinja2.get_jinja2(, factory=jade_factory)

(This is based on moraes’s Stack Overflow answer here.)

4. Use the factory and get a template.

Here’s the full source code for a simple “hello world” app. I’m following the examples on Google’s jinja2 page and creating a JadeHandler subclass of RequestHandler.

import os
import webapp2
from webapp2_extras import jinja2

class JadeHandler(webapp2.RequestHandler):
  # Per
  def jade_factory(app):
    j = jinja2.Jinja2(app)
    return j
  def jinja2(self):
    return jinja2.get_jinja2(, factory=JadeHandler.jade_factory)

  def render_response(self, _template, **context):
    # Renders a template and writes the result to the response.
    rv = self.jinja2.render_template(_template, **context)

class MainPage(JadeHandler):
  def get(self):
    context = {'message': 'Hello, world!'}
    self.render_response('index.jade', **context)

app = webapp2.WSGIApplication([('/', MainPage)], debug=True)


I’m by no means an expert in Google App Engine, and I’m a little worried that there’s something badly inefficient or similarly wrong about this approach. In particular, I’m hoping the returned templates are cached, but I’m not certain, and I’ve seen reports that Jinja2 Environment creation is an expensive operation on GAE. More to the point, pyjade seems to work as a preprocessor for Jinja2, so it would obviously be better not to need to run it for every request. In any event, this does at least find jade templates and render them properly as HTML.

Edit: as usual, the current code is available on GitHub.