Flask

Making Python-based web apps with Flask

Introduction

Flask is a framework for creating webapps in Python. There are other frameworks for Python, Django being one of the most popular. The advantage of Flask is that it is easy for beginners; the “Hello, World” webapp is a single file (see below.

Hello World in Flask

These five lines of code (with one blank line) set up a webserver on your local machine at the web address http://localhost:5000. Each time you request the default page for the site, i.e. /, you get back the message Hello, World:

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'

To try this code:

• Try running it either in IDLE, or by saving it in hello.py and running python hello.py at command line.
  • If you get an error message on the from flask import flask line, you may need to do pip install -U flask first before you can run this code. Other than that, it should just work.
• Once its running, it sets up a web server running on localhost (i.e. the machine where you run the code) at port 5000.
• To access that server, point your browser to http://localhost:5000 in your browser.
• You should see `Hello, World!’ in the browser.
• Try changing the message to Hello, Friends or Hello, Gauchos or Hello, Tritons so you can see that you have control over the webapp.

Deployment

The following is true of webapps written in Python using Flask:

• They can be deployed in the cloud on Heroku
• Continous integration is available through Travis-CI

Tutorial information about Flask

Some tutorial information is available at these links. Eventually I’d like to migrate some of this into this website directly:

• Flask tutorial created for UCSD CSE SPIS 2015: https://sites.google.com/a/eng.ucsd.edu/spis/home/AcademicProgram/foundations/final-projects/web-apps
• Flask tutorials for UCSD CSE SPIS 2016 (scroll down to webapps section) http://ucsd-cse-spis-2016.github.io/