Getting Started with Flask #

Install #

Probably best to set this up in a virtual environment.

Let’s assume we’re already there.

pip3 install flask

import flask

Setup #

In a working project directory, start with a single file. Let’s call it initial.py:

from flask import Flask # Flask class
app = Flask(__name__) # instantiates a Flask application at the app variable

Routes #

from flask import Flask # Flask class
app = Flask(__name__) # instantiates a Flask application at the app variable

@app.route("/") 

def hello():
    return "Hello World!"

Route decorator (@app) - handles backend complexity for routing.

"/" bit here defines the route.

Chunk beneath that represents what actually happens.

Adding extra routes is as simple as adding:

@app.route("/about")

def about():
    return "Ahoy!" # this would cause an about page to appear with the text "Ahoy!"

Multiple routes can invoke the same function, just stack the routes.

All URL paths must start with a /.

Dynamic #

Routes can be specified with variables:

@app.route('/orders/<user_id>/<int:order_id>')
def orders(user_id, order_id):
    return f'Order {order_id} for {user_id}.'

Content #

HTML can be embedded:

@app.route('/')
def home():
    return '<h1>Hello, World!</h1>'

Note that the html is within the ticks.

This can also be mult-lined:

@app.route('/')
def home():
    return '''
        <h1>Hello, World!</h1>
    '''

Running Locally #

With environment variables #

tl;dr: Decorators add additional functionality to existing functions.

Prior to running (without the name/main chunk below), export variable while within working directory to point at main file:

export FLASK_APP=initial.py

To execute, run flask run from the command line. This should bring up a simple Hello world! page.

To show changes without having to reload application, run in debug mode.

export FLASK_DEBUG=1

Without environment variables #

Alternatively (my preferred way), add the following to the bottom of the script to enable the application to run with python initial.py without having to set environment variables:

if __name__ == '__main__':
    app.run(debug=True) # run the app in debug mode