Flask

Installation

Check this[1]

In the directory where you want to create your flask application:

apt install python3.11-venv  # make sure the venv module is there
python -m venv .venv   # Create a virtual environment
. .venv/bin/activate   # Activate the virtual environment
pip install Flask      # Install flask in the virtual environment

Basics

Hello world app:

#!/usr/bin/env python3

from flask import Flask
print(__name__)
app = Flask(__name__)

@app.route('/')

def welcome():
    return 'Welcome'

if __name__ == '__main__':         # When executed on the commandline (build in webserver)
    app.run(host='0.0.0.0',port=5000) # Listen to all on port 5000, you can also use flask run for options
    #app.run(host='0.0.0.0',port=8080,Debug=True)  # Run in debug mode, listen to all hosts on port 8080

Or run from the commandline so you can pass options to flask

export FLASK_APP=welcome_app.py 
flask run --host=0.0.0.0

Output:

welcome_app
 * Serving Flask app "welcome_app"
 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

Now you can browse to <serverIP>:5000

https://code.tutsplus.com/tutorials/creating-a-web-app-from-scratch-using-python-flask-and-mysql--cms-22972

Integrate with nginx

Mayor part is from this site [2].

• Install uwsgi and uwsgi-plugin-python3.

The inifile in

[uwsgi]
base = /var/www/flask/demoapp
plugins = python3

# Script to run without .py
app = hello
# It's unclear if the script to run is defined by module or callable
module = %(app)
callable = app

home = %(base)/.venv
pythonpath = %(base)

#socket file's location and permission
socket = /tmp/%n.sock      # %n is the name of the ini-file without .ini
chmod-socket    = 666

#location of log files
logto = /var/log/uwsgi/%n.log   # Funny, logging is done to /var/log/uwsgi/app/%n.log

• Create a virtual webserver (port 80 is save enough if you have a secure front-end server)

server {
    listen 80 ;
    listen [::]:80;

    root /var/www/flask;
    server_name <fqdn>;

    location @demoapp {
        include uwsgi_params;
        uwsgi_pass unix://tmp/demoapp_uwsgi.sock;
    }
}

That's it. Point your webbrowser to http://<your webserver fdqn> and it will show you what the app returns

Running applications

You can make any number of applications by adding more apps to uwsgi and a location in nginx. You can also run any number of programs by adding routes to an app or with just a few routes like this:

#!/usr/bin/env python3

from flask import Flask, render_template
import os
import subprocess 

### Application whitelist
whitelist = {
    'script as in URL' :'<scriptlocation>',
    'script as in URL' :'<another scriptlocation>',
    }
    
application = Flask(__name__)

@application.route('/app1/<script>/<path:arguments>')
def func1(script,arguments):
    if script in whitelist:
        arglist = arguments.split('/')
        result = subprocess.run([whitelist[script]]+arglist, stdout=subprocess.PIPE)
        output = result.stdout
    else:
        output = '{} does not exist in whitelist'.format(script)
    return output
    
@application.route('/app1/<script>')
def func2(script):
    if script in whitelist:
        result = subprocess.run([whitelist[script]], stdout=subprocess.PIPE)
        output = result.stdout
    else:
        output = '{} does not exist in whitelist'.format(script)
    return output

if __name__ == '__main__':
    application.run(host='0.0.0.0', port=8080,debug=True)

The route is the request URI as passed by the webserver. In this case 'app1' is the location. The things in <> are converted to python variables. <path:arguments> will put everything, including the '/'-es into 1 variable (arguments). So you can have a variable number of arguments that all will be passed to the pythonscript called by subprocess.

The whitelist is there for security to avoid one can run any script is knows the location for.

Generate web pages

Flask is by default using the jinja template engine. They have a nice tutorial for this too[3].

Very simple example is to put a index.html in <app-base>/templates/

#!/usr/bin/env python3

from flask import Flask, render_template

print(__name__)
app = Flask(__name__)

@app.route('/app1/html')
def static_page():
  return render_template('index.html')

if __name__ == '__main__':
    app.run(host='0.0.0.0',port=5000,debug=True)