flask

What this skill does

# Flask - Lightweight Python Web Framework

## Overview

Flask is a micro-framework for Python web development, designed for building microservices, REST APIs, and flexible web applications. Its minimalist core and extensive extension ecosystem make it ideal for projects requiring lightweight architecture, rapid development, and full control over components.

**Key Features**:
- Micro-framework philosophy (minimal core, extensible)
- Flask-RESTful for API development
- Blueprints for modular application structure
- SQLAlchemy integration via Flask-SQLAlchemy
- Jinja2 templating engine
- Built-in development server with auto-reload
- Werkzeug WSGI toolkit foundation
- Large extension ecosystem (Flask-Login, Flask-JWT, Flask-CORS)
- Production deployment with Gunicorn/uWSGI

**Installation**:
```bash
# Basic Flask
pip install flask

# Flask with common extensions
pip install flask flask-restful flask-sqlalchemy flask-login flask-cors

# With database support
pip install flask flask-sqlalchemy psycopg2-binary  # PostgreSQL

# Full microservices stack
pip install flask flask-restful marshmallow flask-jwt-extended redis
```

## Basic Application Patterns

### 1. Minimal Flask App

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

app = Flask(__name__)

@app.route('/')
def hello():
    return jsonify({"message": "Hello, World!"})

@app.route('/api/users/<int:user_id>')
def get_user(user_id):
    return jsonify({"id": user_id, "name": f"User {user_id}"})

@app.route('/api/users', methods=['POST'])
def create_user():
    data = request.get_json()
    return jsonify({"id": 123, **data}), 201

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

**Run:**
```bash
# Development server
python app.py

# Or using flask CLI
export FLASK_APP=app.py
export FLASK_ENV=development
flask run

# Custom port
flask run --port 8000 --host 0.0.0.0
```

### 2. Application Factory Pattern

**Recommended for production and testing:**

```python
# app/__init__.py
from flask import Flask
from app.config import Config
from app.extensions import db, migrate, jwt

def create_app(config_class=Config):
    """Application factory pattern."""
    app = Flask(__name__)
    app.config.from_object(config_class)

    # Initialize extensions
    db.init_app(app)
    migrate.init_app(app, db)
    jwt.init_app(app)

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

    return app

# app/extensions.py
from flask_sqlalchemy import SQLAlchemy
from flask_migrate import Migrate
from flask_jwt_extended import JWTManager

db = SQLAlchemy()
migrate = Migrate()
jwt = JWTManager()

# app/config.py
import os

class Config:
    SECRET_KEY = os.environ.get('SECRET_KEY') or 'dev-secret-key'
    SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URL') or 'sqlite:///app.db'
    SQLALCHEMY_TRACK_MODIFICATIONS = False
    JWT_SECRET_KEY = os.environ.get('JWT_SECRET_KEY') or 'jwt-secret'

class DevelopmentConfig(Config):
    DEBUG = True
    TESTING = False

class ProductionConfig(Config):
    DEBUG = False
    TESTING = False

# run.py
from app import create_app

app = create_app()

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

**Run:**
```bash
export FLASK_APP=run.py
flask run
```

### 3. Request and Response Handling

```python
from flask import Flask, request, jsonify, make_response, abort

app = Flask(__name__)

@app.route('/api/data', methods=['GET', 'POST'])
def handle_data():
    # GET request
    if request.method == 'GET':
        # Query parameters
        page = request.args.get('page', 1, type=int)
        limit = request.args.get('limit', 10, type=int)

        return jsonify({
            "page": page,
            "limit": limit,
            "data": [...]
        })

    # POST request
    if request.method == 'POST':
        # JSON body
        data = request.get_json()

        # Validation
        if not data or 'name' not in data:
            abort(400, description="Missing required field: name")

        # Custom response with headers
        response = make_response(jsonify({"id": 1, **data}), 201)
        response.headers['X-Custom-Header'] = 'value'
        return response

# Error handling
@app.errorhandler(404)
def not_found(error):
    return jsonify({"error": "Resource not found"}), 404

@app.errorhandler(400)
def bad_request(error):
    return jsonify({"error": str(error.description)}), 400
```

## Blueprints - Modular Applications

### Blueprint Structure

```
app/
├── __init__.py          # Application factory
├── extensions.py        # Extension instances
├── config.py            # Configuration
├── models/
│   ├── __init__.py
│   ├── user.py
│   └── product.py
├── routes/
│   ├── __init__.py
│   ├── auth.py          # Authentication routes
│   ├── users.py         # User management routes
│   └── products.py      # Product routes
└── services/
    ├── __init__.py
    ├── user_service.py
    └── auth_service.py
```

### Blueprint Implementation

```python
# app/routes/users.py
from flask import Blueprint, jsonify, request
from app.models.user import User
from app.extensions import db

users_bp = Blueprint('users', __name__)

@users_bp.route('/', methods=['GET'])
def list_users():
    """List all users."""
    page = request.args.get('page', 1, type=int)
    per_page = request.args.get('per_page', 20, type=int)

    users = User.query.paginate(page=page, per_page=per_page)

    return jsonify({
        "users": [u.to_dict() for u in users.items],
        "total": users.total,
        "page": users.page,
        "pages": users.pages
    })

@users_bp.route('/<int:user_id>', methods=['GET'])
def get_user(user_id):
    """Get user by ID."""
    user = User.query.get_or_404(user_id)
    return jsonify(user.to_dict())

@users_bp.route('/', methods=['POST'])
def create_user():
    """Create new user."""
    data = request.get_json()

    user = User(
        email=data['email'],
        name=data['name']
    )
    user.set_password(data['password'])

    db.session.add(user)
    db.session.commit()

    return jsonify(user.to_dict()), 201

@users_bp.route('/<int:user_id>', methods=['PUT'])
def update_user(user_id):
    """Update user."""
    user = User.query.get_or_404(user_id)
    data = request.get_json()

    if 'name' in data:
        user.name = data['name']
    if 'email' in data:
        user.email = data['email']

    db.session.commit()
    return jsonify(user.to_dict())

@users_bp.route('/<int:user_id>', methods=['DELETE'])
def delete_user(user_id):
    """Delete user."""
    user = User.query.get_or_404(user_id)
    db.session.delete(user)
    db.session.commit()

    return '', 204

# app/__init__.py
def create_app():
    app = Flask(__name__)

    # Register blueprints
    from app.routes.users import users_bp
    from app.routes.products import products_bp

    app.register_blueprint(users_bp, url_prefix='/api/users')
    app.register_blueprint(products_bp, url_prefix='/api/products')

    return app
```

## Flask-RESTful for APIs

### Basic REST API

```python
# app/api/resources.py
from flask import request
from flask_restful import Resource, Api, reqparse, fields, marshal_with
from app.models.user import User
from app.extensions import db

# Response serialization
user_fields = {
    'id': fields.Integer,
    'email': fields.String,
    'name': fields.String,
    'created_at': fields.DateTime(dt_format='iso8601')
}

class UserListResource(Resource):
    """User collection endpoint."""

    @marshal_with(user_fields)
    def get(self):
        """List all users."""
        users = User.query.all()
        return users

    def post(self):
        """Create new user."""
        parser = reqparse.RequestParser()
        parser.add_argument('email', required=True, help='Email is required')
        parser.add_argument('name', required=True, help='Name is required')
        parser.add_argument('password', r