Starter Python Application

This document describes how to start a Passenger application in Python language.

Basic application

passenger_wsgi.py is the entrypoint for any python application.

cd ~/ondemand/dev
mkdir python-hello-world
cd python-hello-world
touch passenger_wsgi.py

Now with the passenger_wsgi.py file created, we can add this content to serve a response to a request.

# passenger_wsgi.py
import sys

def application(environ, start_response):
    start_response('200 OK', [('Content-type', 'text/plain')])
    return ["Hello World from Open OnDemand (Python WSGI)!\n\n" + sys.version]

Boot the application

Now that the app’s all setup and implemented, you should be able to boot it up. To do so, simply navigate to My Sandbox Apps (Development) in the Develop menu of your OnDemand installation.

There you should see this application at the top of the list. Clicking Launch Python Hello World will launch this application in a new tab.

When the new tab opens you should see a blank page with the text Hello World with some extra text about the system. This is your new Python application!

Application using Flask and a virtual environment

The basic application above is fine, but you’ll likely need to add more dependencies and load those dependencies at runtime.

So this section goes over adding the Flask web framework and having the application load the virtual environment that has your dependencies in it.

Create the virtual environmet

First, we need to create the virtual environment. Issue this command below to create one. This will create a subdirectory python-hello-world with a bin/activate file you can use to activate the environment.

python3 -m venv python-hello-world

Now, let’s create the requriements.txt file where we’ll add the application’s required libraries. Here, we’re only adding flask of any version.

# requirements.txt
flask

source python-hello-world/bin/activate
python3 -m pip install -r requirements.txt

Create the python files

In the basic example above, the entire implementation is held within a passenger_wsgi.py. This project is more advanced, so it will include two files. passenger_wsgi.py and app.py. app.py will hold the logic for the application.

passenger_wsgi.py simply imports the app from the app.py file. This is all that’s required for this file.

# passenger_wsgi.py
from app import MyApp as application

app.py on the other hand, has logic associcated with the web application in it. It imports the Flask libraries, configures the routes and starts the flask server.

# app.py
from flask import Flask
import sys

MyApp = Flask('python_hello_world')

@MyApp.route("/")
def index():
  return 'Hello World!<br>' + sys.version

if __name__ == "__main__":
  MyApp.run()

Using the virtual environment

At this point, the app is basically done, but won’t boot up because it can’t find Flask libraries. We created a virtual environment in a previous step, now we have to get OnDemand to recognize this environment.

To do this, we need to create a bin/python wrapper file to load the appropriate virtual environment.

#!/usr/bin/env bash

SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
source $SCRIPT_DIR/../python-hello-world/bin/activate

exec /bin/env python3 "$@"

Warning

Ensure that this bin/python file has executable permissons on it. Issue the command chmod +x bin.python to give it executable permissions.

Now, with the python wrapper script to load the environment for your application, it should boot up correctly.

Publish App

Publishing an app requires two steps:

1. Updating the manifest.yml to specify the category and optionally subcategory, which indicates where in the dashboard menu the app appears.

2. Having an administrator checkout a copy of the production version to a directory under /var/www/ood/apps/sys.

Steps:

1. Add category to manifest so the app appears in the Files menu:

   name: Quota
   description: Display quotas
   icon: fa://hdd-o
   +category: Files
   +subcategory: Utilities
   

2. Version these changes. Click Shell button on app details view, and then commit the changes:

   git add .
   git commit -m "update manifest for production"
   
   # if there is an external remote associated with this, push to that
   git push origin master
   

3. As the admin, sudo copy or git clone this repo to production

   # as sudo on OnDemand host:
   cd /var/www/ood/apps/sys
   git clone /users/PZS0562/efranz/ondemand/dev/quota
   

4. Reload the dashboard.

Fig. 9 Every user can now launch the Quota from the Files menu.

Warning

Accessing this new app for the first time will cause your NGINX server to restart, killing all websocket connections, which means resetting your active web-based OnDemand Shell sessions.