Flask API Development

Overview

Create efficient Flask APIs with blueprints for modular organization, SQLAlchemy for ORM, JWT authentication, comprehensive error handling, and proper request validation following REST principles.

When to Use

• Building RESTful APIs with Flask
• Creating microservices with minimal overhead
• Implementing lightweight authentication systems
• Designing API endpoints with proper validation
• Integrating with relational databases
• Building request/response handling systems

Instructions

1. Flask Application Setup

# app.py
from flask import Flask, request, jsonify
from flask_cors import CORS
from flask_sqlalchemy import SQLAlchemy
from flask_jwt_extended import JWTManager
import os

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = os.getenv('DATABASE_URL', 'sqlite:///app.db')
app.config['JWT_SECRET_KEY'] = os.getenv('JWT_SECRET_KEY', 'dev-secret')
app.config['JSON_SORT_KEYS'] = False

db = SQLAlchemy(app)
jwt = JWTManager(app)
CORS(app)

# Request ID middleware
@app.before_request
def assign_request_id():
    import uuid
    request.request_id = str(uuid.uuid4())

# Error handlers
@app.errorhandler(400)
def bad_request(error):
    return jsonify({
        'error': 'Bad Request',
        'message': str(error),
        'request_id': request.request_id
    }), 400

@app.errorhandler(404)
def not_found(error):
    return jsonify({
        'error': 'Not Found',
        'message': 'Resource does not exist',
        'request_id': request.request_id
    }), 404

@app.errorhandler(500)
def internal_error(error):
    db.session.rollback()
    return jsonify({
        'error': 'Internal Server Error',
        'request_id': request.request_id
    }), 500

if __name__ == '__main__':
    app.run(debug=os.getenv('ENV') != 'production')

2. Database Models with SQLAlchemy

# models.py
from datetime import datetime
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy.dialects.postgresql import UUID
import uuid

db = SQLAlchemy()

class User(db.Model):
    __tablename__ = 'users'

    id = db.Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
    email = db.Column(db.String(255), unique=True, nullable=False, index=True)
    password_hash = db.Column(db.String(255), nullable=False)
    first_name = db.Column(db.String(100))
    last_name = db.Column(db.String(100))
    role = db.Column(db.String(20), default='user', index=True)
    is_active = db.Column(db.Boolean, default=True)
    created_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
    updated_at = db.Column(db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)

    # Relationships
    posts = db.relationship('Post', backref='author', lazy='dynamic', cascade='all, delete-orphan')

    def __repr__(self):
        return f'<User {self.email}>'

    def set_password(self, password):
        from werkzeug.security import generate_password_hash
        self.password_hash = generate_password_hash(password)

    def verify_password(self, password):
        from werkzeug.security import check_password_hash
        return check_password_hash(self.password_hash, password)

    def to_dict(self):
        return {
            'id': str(self.id),
            'email': self.email,
            'first_name': self.first_name,
            'last_name': self.last_name,
            'role': self.role,
            'created_at': self.created_at.isoformat()
        }

class Post(db.Model):
    __tablename__ = 'posts'

    id = db.Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
    title = db.Column(db.String(255), nullable=False, index=True)
    content = db.Column(db.Text, nullable=False)
    published = db.Column(db.Boolean, default=False)
    user_id = db.Column(UUID(as_uuid=True), db.ForeignKey('users.id'), nullable=False)
    created_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
    updated_at = db.Column(db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)

    def to_dict(self):
        return {
            'id': str(self.id),
            'title': self.title,
            'content': self.content,
            'published': self.published,
            'author_id': str(self.user_id),
            'created_at': self.created_at.isoformat()
        }

3. Authentication and JWT

# auth.py
from flask import request, jsonify
from flask_jwt_extended import create_access_token, jwt_required, get_jwt_identity
from functools import wraps
from models import User, db

def authenticate_user(email, password):
    user = User.query.filter_by(email=email).first()
    if user and user.verify_password(password):
        return user
    return None

def login_required(f):
    @wraps(f)
    @jwt_required()
    def decorated_function(*args, **kwargs):
        identity = get_jwt_identity()
        user = User.query.get(identity)
        if not user or not user.is_active:
            return jsonify({'error': 'User not found or inactive'}), 401
        request.current_user = user
        return f(*args, **kwargs)
    return decorated_function

def admin_required(f):
    @wraps(f)
    @login_required
    def decorated_function(*args, **kwargs):
        if request.current_user.role != 'admin':
            return jsonify({'error': 'Admin access required'}), 403
        return f(*args, **kwargs)
    return decorated_function

# routes/auth.py
from flask import Blueprint, request, jsonify
from auth import authenticate_user, login_required
from models import User, db
from flask_jwt_extended import create_access_token

auth_bp = Blueprint('auth', __name__, url_prefix='/api/auth')

@auth_bp.route('/login', methods=['POST'])
def login():
    data = request.get_json()
    if not data or not data.get('email') or not data.get('password'):
        return jsonify({'error': 'Missing credentials'}), 400

    user = authenticate_user(data['email'], data['password'])
    if not user:
        return jsonify({'error': 'Invalid credentials'}), 401

    access_token = create_access_token(identity=str(user.id))
    return jsonify({
        'access_token': access_token,
        'user': user.to_dict()
    }), 200

@auth_bp.route('/register', methods=['POST'])
def register():
    data = request.get_json()
    if User.query.filter_by(email=data['email']).first():
        return jsonify({'error': 'Email already exists'}), 409

    user = User(email=data['email'], first_name=data.get('first_name'))
    user.set_password(data['password'])
    db.session.add(user)
    db.session.commit()

    return jsonify({'user': user.to_dict()}), 201

@auth_bp.route('/profile', methods=['GET'])
@login_required
def get_profile():
    return jsonify({'user': request.current_user.to_dict()}), 200

4. Blueprints for Modular API Design

# routes/users.py
from flask import Blueprint, request, jsonify
from auth import login_required, admin_required
from models import User, db
from sqlalchemy import or_

users_bp = Blueprint('users', __name__, url_prefix='/api/users')

@users_bp.route('', methods=['GET'])
@login_required
def list_users():
    page = request.args.get('page', 1, type=int)
    limit = request.args.get('limit', 20, type=int)
    search = request.args.get('q', '', type=str)

    query = User.query
    if search:
        query = query.filter(or_(
            User.email.ilike(f'%{search}%'),
            User.first_name.ilike(f'%{search}%')
        ))

    paginated = query.paginate(page=page, per_page=limit)
    return jsonify({
        'data': [user.to_dict() for user in paginated.items],
        'pagination': {
            'page': page,
            'limit': limit,
            'total': paginated.total,
            'pages': paginated.pages
        }
    }), 200

@users_bp.route('/<user_id>', methods=['GET'])
@login_required
def get_user(user_id):
    user = User.query.get(user_id)
    if not user:
        return jsonify({'error': 'User not found'}), 404
    return jsonify({'user': user.to_dict()}), 200

@users_bp.route('/<user_id>', methods=['PATCH'])
@login_required
def update_user(user_id):
    if str(request.current_user.id) != user_id:
        return jsonify({'error': 'Unauthorized'}), 403

    user = User.query.get(user_id)
    if not user:
        return jsonify({'error': 'User not found'}), 404

    data = request.get_json()
    if 'first_name' in data:
        user.first_name = data['first_name']
    if 'last_name' in data:
        user.last_name = data['last_name']

    db.session.commit()
    return jsonify({'user': user.to_dict()}), 200

@users_bp.route('/<user_id>', methods=['DELETE'])
@admin_required
def delete_user(user_id):
    user = User.query.get(user_id)
    if not user:
        return jsonify({'error': 'User not found'}), 404

    db.session.delete(user)
    db.session.commit()
    return '', 204

5. Request Validation

# validators.py
from flask import request, jsonify
from functools import wraps

def validate_json(*required_fields):
    def decorator(f):
        @wraps(f)
        def decorated_function(*args, **kwargs):
            if not request.is_json:
                return jsonify({'error': 'Request body must be JSON'}), 400

            data = request.get_json()
            missing = [field for field in required_fields if field not in data]

            if missing:
                return jsonify({
                    'error': 'Missing required fields',
                    'missing_fields': missing
                }), 400

            return f(*args, **kwargs)
        return decorated_function
    return decorator

def validate_email(email):
    import re
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    return re.match(pattern, email) is not None

# Usage
@users_bp.route('', methods=['POST'])
@validate_json('email', 'password', 'first_name')
def create_user():
    data = request.get_json()
    if not validate_email(data['email']):
        return jsonify({'error': 'Invalid email format'}), 400
    # ... rest of logic

6. Application Factory and Configuration

# config.py
import os

class Config:
    SQLALCHEMY_TRACK_MODIFICATIONS = False
    JSON_SORT_KEYS = False

class DevelopmentConfig(Config):
    DEBUG = True
    TESTING = False
    SQLALCHEMY_DATABASE_URI = 'sqlite:///app.db'

class ProductionConfig(Config):
    DEBUG = False
    TESTING = False
    SQLALCHEMY_DATABASE_URI = os.getenv('DATABASE_URL')
    JWT_SECRET_KEY = os.getenv('JWT_SECRET_KEY')

class TestingConfig(Config):
    TESTING = True
    SQLALCHEMY_DATABASE_URI = 'sqlite:///:memory:'

# factory.py
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_jwt_extended import JWTManager

def create_app(config_name='development'):
    app = Flask(__name__)

    if config_name == 'production':
        from config import ProductionConfig
        app.config.from_object(ProductionConfig)
    else:
        from config import DevelopmentConfig
        app.config.from_object(DevelopmentConfig)

    db = SQLAlchemy(app)
    jwt = JWTManager(app)

    # Register blueprints
    from routes.auth import auth_bp
    from routes.users import users_bp
    app.register_blueprint(auth_bp)
    app.register_blueprint(users_bp)

    return app

Best Practices

✅ DO

• Use blueprints for modular organization
• Implement proper authentication with JWT
• Validate all user input
• Use SQLAlchemy ORM for database operations
• Implement comprehensive error handling
• Use pagination for collection endpoints
• Log errors and important events
• Return appropriate HTTP status codes
• Implement CORS properly
• Use environment variables for configuration

❌ DON'T

• Store secrets in code
• Use global variables for shared state
• Ignore database transactions
• Trust user input without validation
• Return stack traces in production
• Use mutable default arguments
• Forget to handle database connection errors
• Implement authentication in route handlers

Complete Example

from flask import Flask, request, jsonify
from flask_sqlalchemy import SQLAlchemy
from flask_jwt_extended import JWTManager, create_access_token

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://user:pass@localhost/db'
db = SQLAlchemy(app)
jwt = JWTManager(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String, unique=True)
    password = db.Column(db.String)

@app.route('/api/login', methods=['POST'])
def login():
    data = request.json
    user = User.query.filter_by(email=data['email']).first()
    if user:
        token = create_access_token(identity=user.id)
        return jsonify({'token': token}), 200
    return jsonify({'error': 'Invalid'}), 401

@app.route('/api/users', methods=['GET'])
def get_users():
    users = User.query.all()
    return jsonify([{'id': u.id, 'email': u.email} for u in users]), 200

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