Flask

Short introduction guide on Flask. http://flask.pocoo.org/ (Used for XPPL, so to see a more advanced use in connection with database, see XPPL)

Basic Flask

Flask is a microframework for python to create web applications. It basically connects the webserver with your python code.

Install

$ pip install Flask

Simple text serving

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

with @app.route you can define the url flask respons to.
the function definition after is mandetory as well as the return.
everything that comes after return gets sent back to the browser (http GET request)

Save the code above as hello.py.

Run it with:

$ flask --app hello run

And if you want to enable the debugger (recommended!):

$ flask --app hello run

Paths

You can use any route you like

@app.route("/any/route/you/like")

You can also use variable routes (example for int)

@app.route("/book/<int:id>")
def book(id):

as you can see you can grab the variable in the url through the function’s parameter

(example for string)

@app.route("/book/<bookname>")
def book(bookname):

Http Methods

Methods like GET or POST (DELETE, PUT…) can be handled by flask

Therefore you need to add the wanted methods to the route definition like:

from flask import Flask
app = Flask(__name__)

@app.route('/address_to_post', methods= ['POST','GET'])
def respond_to_post():
    answer = ""
    if request.method == 'GET':
        answer = "get"
    if request.method == 'POST':
        answer = "get"
    return answer

with request.method you can determine the incoming kind of request.

Templates

To be able return full html pages, flask uses templates using Jinja to insert variable content.

@app.route('/')
def home():
    message = "Welcome Home!"
    return render_template('home.html', message=message)

you can pass as many variables to the template as you want. in this example we pass message to the template:

The html looks something like:

<html>
<p>{{message}}</p>
<html>

!important: The html file needs to be saved inside a templates folder called "templates" inside your project folder-

Helpful function

404 Page not found

@app.errorhandler(404)
def page_not_found(error):
    """Custom 404 page."""
    return render_template('404.html'), 404

Deploying

Running with uwsgi

Flask warns you that using "the development server" isn't good for "production use".

Using software like uwsgi to run your flask project on a server is a good idea. It:

Is able to deal with many users at the same time
When configured right lets large static files (like images / videos) be served in a way that works well

https://hackersandslackers.dev/deploy-flask-uwsgi-nginx/

Making a systemd service file

See: https://blog.miguelgrinberg.com/post/running-a-flask-application-as-a-service-with-systemd

Useful: https://containersolutions.github.io/runbooks/posts/linux/debug-systemd-service-units/

$ sudo nano /etc/systemd/system/myflaskapp.service

[Unit]
Description=<a description of your application>
After=network.target

[Service]
User=<username>
WorkingDirectory=<path to your app>
ExecStart=<app start command>
Restart=always

[Install]
WantedBy=multi-user.target

When the service file is new or changed, you need (one time) to:

$ sudo systemctl daemon-reload

Then you can:

$ sudo systemctl start myflaskapp
$ sudo systemctl status myflaskapp
$ sudo systemctl restart myflaskapp
$ sudo systemctl stop myflaskapp

Then finally when you see that start works (checking status, checking that it actually is running , etc)

$ sudo systemctl enable myflaskapp

Will make the "service" auto start when the pi restarts.

To view the log file (errors):

$ sudo journalctl -u myflaskapp -f

You can find documentation here:

https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html