Taking notes on useful Google App Engine tutorials and articles…

Trying to figure out where JSON or simplejson exists in Google App Engine. The answer is in Using AJAX to enable RPC requests.

from django.utils import simplejson

in the Articles section.

Community help for Google App Engine has less than stellar because nobody takes a moment to explain how to do the supposedly “basic” python stuff. It took me a long time but I finally figured out the process to install Cheetah Templates for use with Google App Engine.

Here is what I had to do on Mac OS X 10.5:

• download Cheetah and extract (double clicked on Mac)
• cd Cheetah-2.0.1

• run the command:

python setup.py install --install-lib /path/to/myapp/

Running the command above compiled and installed Cheetah templates into a new Cheetah subdirectory of my single Google App Engine application. For Mac it was something like /Users/bart/myapp/Cheetah.

Now, in your code file you can import Cheetah! In the Google example helloworld.py I would put this line near the top with the other includes

from Cheetah.Template import Template

Then, to actually have a Cheetah Template return something, you would add something like this to the MainPage class to output the rendered template.

from Cheetah.Template import Template #woohoo! Cheetah!
import os
import wsgiref.handlers
from google.appengine.ext import webapp
from google.appengine.api import users
class MainHandler(webapp.RequestHandler):
def get(self):
        #find the desired file
        path = os.path.join(os.path.dirname(__file__), 'templates/cheetahtest.tmpl')
        user = users.get_current_user()
        template_values = { 'user':user,}
        #render template with values inserted
        tmpl = Template( file = path, searchList = (template_values,) )
        #output the template to screen
        self.response.out.write(tmpl)

My template might be this:

$user is using Cheetah

Let us assume that the logged in user is ‘Jeff’ for sample output. The rendered output would be:

Jeff is using Cheetah

Notes

If you have more applications you probably need to copy or re-install the Cheetah directory into each different application. You need to do this because the applications get deployed to Google.

Google says that App Engine works with “Pure Python” extensions and the process is simply to upload your extensions with the app. Since I put the install in /path/to/myapp/Cheetah I presume you can put other extensions and/or frameworks. Just download the Python source and configure/compile them to go into the app’s folder.

Why Cheetah?

Why did I go through this? First I should explain that I do not like the Django templating system. I have gotten too used to Cheetah. Django’s hardline stance is to separate logic from presentation. Yes, the Django approach works for many but I just want to make my own fricking little app and I like running loops and instantiating usable variables within my templates, a luxury afforded by Cheetah.

Maybe in the future I will learn how to use a different app Framework instead of Google’s default webapp Framework.

I want to try using the recently released Google App Engine on my Mac OS X 10.5.2 Leopard machine.

The SDK requires Python 2.5 but I need Python 2.4 because of work. The webapp framework included with the App Engine SDK requires the wsgi module which is part of Python 2.5 but not in Python 2.4. Consequently the Hello World application in the Google App Engine tutorials will not work.

I am trying to get around the version problem and I think I am getting close.

Useful knowledge: the which command will tell you the path for executables that you run. Useful in the case you have the same executable in different locations. which python give /opt/local/bin/python

Update 5/10/08 method 3

Another user just sent me this:

find your path first

echo $PATH

/Users/ryan/bin:/usr/local/bin:/opt/local/bin: /opt/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin: /usr/local/bin:/usr/X11/bin

now route your path to find 2.5, which i believe is on our macs already from the leopard update

export PATH=/Users/ryan/bin: /usr/local/bin: /usr/bin:/bin: /usr/sbin: /sbin: /usr/local/bin: /usr/X11/bin

Update 5/8/08: method 2

Rather than compiling your own Python using the method above, my friend told me he followed the README included with the App Engine SDK. Following that he was able to get Python 2.5 by just downloading it from the Python website and running the installer. Then he ran the Google installer.

INSTALLING ON Mac OSX
=================
1) Download and install Python 2.5 from http://www.python.org/download/
2) Download the SDK installer from http://code.google.com/appengine/downloads
3) Install the SDK by double-clicking on the GoogleAppEngine.dmg file and running the installer.

To run it you would use this

python2.5 /usr/local/google_appengine/dev_appserver.py \
<app location>

Update April ’08: method 1

Rather than trying to trick Google App Engine SDK into cooperating with Python 2.4, a guy in the UK has handy instructions for getting things to work without trying to have 2 globale versions of Python. Instead, I can have a global Python (2.4) and a local version in my home directory (2.5) for tinkering with the SDK.

Down the wrong path

Do not do it this way because it doesn’t work. I found an ONLamp article that introduces the use of WSGI. Apparently you can checkout WSGI and run it on Python 2.4. Simply checkout the library and copy the entire WSGI directory to your python site-packages directory.

svn co http://svn.python.org/projects/python/trunk/Lib/wsgiref

The site-packages directory on my Mac OS X 10.5 system is

/opt/local/lib/python2.4/site-packages