flask

## When to use this skill

Use this skill whenever the user wants to:
- Build Python web applications with Flask routing and templates
- Configure WSGI, blueprints, extensions, and deployment
- Integrate Flask-SQLAlchemy, Flask-Login, or other extensions
- Create REST APIs or serve server-rendered pages with Jinja2

## How to use this skill

### Workflow

1. **Create app** — instantiate Flask and define routes
2. **Add templates** — use Jinja2 for HTML rendering
3. **Organize with blueprints** — split features into blueprint modules
4. **Configure and deploy** — set environment config, deploy with Gunicorn

### Quick Start Example

```python
# app.py
from flask import Flask, render_template, request, jsonify

app = Flask(__name__)
app.config['SECRET_KEY'] = 'change-me-in-production'

@app.route('/')
def index():
    return render_template('index.html', title='Home')

@app.route('/api/items', methods=['GET', 'POST'])
def items():
    if request.method == 'POST':
        data = request.get_json()
        # Process and save item
        return jsonify({'status': 'created', 'item': data}), 201
    return jsonify({'items': []})

if __name__ == '__main__':
    app.run(debug=True)
```

### Application Factory Pattern

```python
# __init__.py
from flask import Flask

def create_app(config_name='default'):
    app = Flask(__name__)
    app.config.from_object(config[config_name])

    # Initialize extensions
    db.init_app(app)
    login_manager.init_app(app)

    # Register blueprints
    from .auth import auth_bp
    app.register_blueprint(auth_bp, url_prefix='/auth')

    from .main import main_bp
    app.register_blueprint(main_bp)

    return app
```

```bash
# Development
flask --app app run --debug

# Production
gunicorn -w 4 -b 0.0.0.0:8000 "app:create_app()"
```

### Blueprint Example

```python
# auth/routes.py
from flask import Blueprint, render_template, redirect, url_for

auth_bp = Blueprint('auth', __name__, template_folder='templates')

@auth_bp.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        # Authenticate user
        return redirect(url_for('main.index'))
    return render_template('auth/login.html')
```

## Best Practices

- Use the application factory pattern with blueprints for modularity
- Keep sensitive configuration out of source code — use environment variables
- Deploy with Gunicorn or uWSGI behind a reverse proxy (nginx)
- Enable CSRF protection with Flask-WTF; set security headers
- Use `flask shell` for interactive debugging with app context

## Reference

- Official documentation: https://flask.palletsprojects.com/

## Keywords

flask, Python web, routing, templates, Jinja2, blueprints, WSGI, REST API, Gunicorn