Python: Flask

WSGI

• The Web Server Gateway Interface (WSGI, pronounced whiskey) is a Python specification which specifies how a web server can communicate with web applications in order to satisfy requests.
• Flask is a WSGI application
• A WSGI server is used to run the application. It converts incoming HTTP requests to WSGI requests and WSGI responses to HTTP responses.
• WSGI servers have HTTP servers built-in.
• However, for a multitude of reasons (security, etc.) WSGI servers are usually fronted by a dedicated HTTP server such as Apache or Nginx

Gunicorn - a WSGI Server

• Gunicorn is a WSGI server with a simple configuration which supports multiple worker implementations for performance tuning
• Here is how you start Gunicorn

# -w = number of workers (rule of thumb - #cpu*2)
gunicorn -w 4 'hello:app'       # equivalent to 'from hello import app' - 4 workers

• Gunicorn should not be run as root, because that would cause your applicaiton code to also run as root - which is not secure
• However, this means you will not be able to bind to port 80 or 443
• If you need to be listening on these ports, a reverse proxy such as nginx or Apache should be used in front of gunicorn

• A Flask application is an instance of the Flask class.
• The recommended method to create a Flask object is inside a factory method

Minimal Application

# hello_world.py

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"

• To execute hello_world.py, use the flask command or python3 -m flask, ensuring you set and export the FLASK_APP environment variable

$ export FLASK_APP="$PWD/hello_world.py"
$ flask run &
  * Running on http://127.0.0.1:5000
$ curl 127.0.0.1:5000

...

#
# flask_hello_world.py
#

import Flask

# Flask needs to 
app = Flask(__name__)

# "/" should trigger hello_world()
@app.route("/")
def index()
    return <"p>index</p>"

@app.route("/hello_world")
def hello_world()
    return "<p>Hello, World!</p>"

• export FLASK_APP=flask_hello_world.py; flask run
• curl http://127.0.0.1:5000
• To have an externally visible URL, use flask run --host=0.0.0.0 - which instructs Flask to listen on all IP addresses

[Suggested] Directory Structure

application
|-flaskr/               # application code boes here
|   __init__.py

Acknowledgements