Flask

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 --debug

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:

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

!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