Python/Flask

Installation

## Linux / macOS
mkdir myproject
cd myproject
# python3 -m venv .flask
python3 -m venv .venv
. .venv/bin/activate

## Windows
mkdir myproject
cd myproject
py -3 -m venv .venv
.venv\Scripts\activate

## Next Common Step
# pip install Flask
pip3 install Flask

# End
deactivate

References:

• https://flask.palletsprojects.com/en/stable/installation/

Quick Start

Flask Hello World

helloworld.py

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>\n"

Run the server:

flask --app helloworld run

NOTE: As a shortcut, if the file is named app.py or wsgi.py, you don’t have to use --app. See Command Line Interface for more details.

• Meaning, just simply rename helloworld.py to app.py (or wsgi.py) and you can simply run run:

flask run

WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.

Note:

* Running on http://127.0.0.1:5000

Test the server:

curl http://127.0.0.1:5000
   <p>Hello, World!</p>

References:

• https://flask.palletsprojects.com/en/stable/quickstart/

Hello World Headers

$ curl -i http://127.0.0.1:5000
HTTP/1.1 200 OK
Server: Werkzeug/3.1.4 Python/3.12.3
Date: Sat, 13 Dec 2025 22:05:42 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 20
Connection: close

<p>Hello, World!</p>

WSGI

Apache mod_wsgi

$ cd hello-app
$ python -m venv .venv
$ . .venv/bin/activate
$ pip install .  # install your application
$ pip install mod_wsgi

wsgi.py :

## IIMPORT PATTERN
from hello import app
application = app

## OR

## FACTORY PATTERN
from hello import create_app
application = create_app()

Configure mod_wsgi-express and specify the number of worker processes:

mod_wsgi-express start-server wsgi.py --processes 4

Example running mod_wsgi-express as root, to bind to port 80/443, but then drop permissions and binding app as different user:

sudo /home/hello/.venv/bin/mod_wsgi-express start-server /home/hello/wsgi.py \
   --user hello --group hello --port 80 --processes 4

Reference: https://flask.palletsprojects.com/en/stable/deploying/mod_wsgi/

Reverse Proxy

Tell Flask it is being reverse Proxied:

from werkzeug.middleware.proxy_fix import ProxyFix

app.wsgi_app = ProxyFix(
    app.wsgi_app, x_for=1, x_proto=1, x_host=1, x_prefix=1
)

reference: https://flask.palletsprojects.com/en/stable/deploying/proxy_fix/

Apapache Configuration:

LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_http_module modules/mod_proxy_http.so
ProxyPass / http://127.0.0.1:8000/
RequestHeader set X-Forwarded-Proto http
RequestHeader set X-Forwarded-Prefix /

reference: https://flask.palletsprojects.com/en/stable/deploying/apache-httpd/

Systemd Flask Service

/etc/systemd/system/myproject.service

[Unit]
Description=Gunicorn instance to serve myproject
After=network.target

[Service]
User=youruser
WorkingDirectory=/path/to/your/project
Environment="FLASK_ENV=production"
ExecStart=/path/to/your/project/venv/bin/gunicorn -w 4 'app:app' -b 0.0.0.0:8000
Restart=always

[Install]
WantedBy=multi-user.target

sudo systemctl daemon-reload
sudo systemctl start myproject
sudo systemctl enable myproject

Check logs with:

sudo systemctl status myproject
journalctl -u myproject