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Revision as of 10:57, 31 January 2023
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)
Run it with:
$ FLASK_APP=hello.py flask 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
When you deploy your application on a server, you need to make sure the application runs uninterrupted. If the application crashes, you'd want it to automatically restart, and if the server experiences a power outage, you'd want the application to start immediately once power is restored.
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/YOUR_NAME_OF_THE_APP.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
Here is an example:
[Unit]
Description=Bla Bla Bla made in Flask
After=network.target
[Service]
User=YOUR_USERNAME
WorkingDirectory=/YOUR/PATH/
ExecStart=/YOUR/PATH/.venv/bin/flask --app NAME_OF_YOUR_PY_FILE run --port 51?? #DO NOT PUT .py AT THE END + #PUT YOUR_PORT
Restart=always
[Install]
WantedBy=multi-user.target
1st thing to do when your .service file is new or changed
$ sudo systemctl daemon-reload
Then in order:
$ sudo systemctl start YOUR_NAME_OF_THE_APP $ sudo systemctl status YOUR_NAME_OF_THE_APP
When you see that it's working (checking its status, that it actually is running, etc...)
$ sudo systemctl enable YOUR_NAME_OF_THE_APP
Will make the "service" auto start when the pi restarts.
Additional commands:
$ sudo systemctl restart YOUR_NAME_OF_THE_APP $ sudo systemctl stop YOUR_NAME_OF_THE_APP
To view the log file (errors):
$ sudo journalctl -u YOUR_NAME_OF_THE_APP -f
You can find documentation here:
https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html