A minimal Flask application

Let's see what a minimal Flask application looks like:

from flask import Flask
app = Flask(__name__)
@app.route('/')
def index():
  return 'Hello World!'

That's it, we are done with the minimal Flask application. It's very simple to configure and create a microservice with Flask.

Let's discuss what exactly the preceding code is doing and how we would run the program:

1. First, we imported a Flask class.
2. Next, we created an instance of the Flask class. This instance will be our WSGI application. This first argument will be the name of the module or package. Here, we created a single module, hence we used __name__. This is needed so that Flask knows where to look for templates, static, and other directories.
3. Then, we used app.route as a decorator with a URL name as a parameter. This will define and map the route with the specified function.
4. The function will be invoked to the HTTP request with the URL specified in the route decorator.

To run this program, you can either use the flask command or python -m flask, but before that, you need to set an environment variable as FLASK_APP with a module file name of where the Flask instance was defined:

$ export FLASK_APP=hello_world.py
$ flask run
* Serving Flask app "flask_todo.hello_world"
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

This launches a built-in server that is good enough for testing and debugging locally. The following is a screenshot of the localhost running in the browser:

Of course, it wouldn't work with production, but Flask provides numerous options for deployment. You can have a look at http://flask.pocoo.org/docs/0.12/deploying/#deployment for more information, but in our case, we are going to deploy to a serverless environment on AWS Lambda and API Gateway using Zappa.