Chapter 2 of 10
A minimal Flask app is 5 lines of Python. That simplicity is intentional — Flask gets out of your way so you can focus on what makes your application unique.
# Create virtual environment
python -m venv venv
source venv/bin/activate # Mac/Linux
venvScriptsactivate # Windows
# Install Flask
pip install flask
# app.py
from flask import Flask
app = Flask(__name__)
@app.route("/")
def index():
return "Hello from Flask!"
if __name__ == "__main__":
app.run(debug=True)# Option 1: run directly
python app.py
# Option 2: using Flask CLI (recommended)
export FLASK_APP=app.py # Mac/Linux
set FLASK_APP=app.py # Windows
export FLASK_DEBUG=1 # enables auto-reload
flask run
# Visit http://127.0.0.1:5000/TIP
Use an app factory pattern for larger projects. For anything beyond a single file, use an application factory function (create_app()). This makes your app testable, configurable, and ready for Blueprints. We'll cover this in the Blueprints chapter.