Flask

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:

# 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

# 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:

# 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:

# 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:

export FLASK_APP=run.py
flask run

3. Request and Response Handling

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

# 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

# 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', required=True, help='Password is required')
        args = parser.parse_args()

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

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

        return {'id': user.id, 'email': user.email, 'name': user.name}, 201

class UserResource(Resource):
    """Single user endpoint."""

    @marshal_with(user_fields)
    def get(self, user_id):
        """Get user by ID."""
        user = User.query.get_or_404(user_id)
        return user

    @marshal_with(user_fields)
    def put(self, user_id):
        """Update user."""
        user = User.query.get_or_404(user_id)

        parser = reqparse.RequestParser()
        parser.add_argument('name')
        parser.add_argument('email')
        args = parser.parse_args()

        if args['name']:
            user.name = args['name']
        if args['email']:
            user.email = args['email']

        db.session.commit()
        return user

    def delete(self, user_id):
        """Delete user."""
        user = User.query.get_or_404(user_id)
        db.session.delete(user)
        db.session.commit()

        return '', 204

# app/__init__.py
from flask_restful import Api

def create_app():
    app = Flask(__name__)
    api = Api(app, prefix='/api/v1')

    # Register resources
    from app.api.resources import UserListResource, UserResource

    api.add_resource(UserListResource, '/users')
    api.add_resource(UserResource, '/users/<int:user_id>')

    return app

Request Validation

Marshmallow Schemas

# app/schemas/user_schema.py
from marshmallow import Schema, fields, validate, validates, ValidationError

class UserSchema(Schema):
    """User validation schema."""

    id = fields.Int(dump_only=True)
    email = fields.Email(required=True, validate=validate.Length(max=120))
    name = fields.Str(required=True, validate=validate.Length(min=2, max=80))
    password = fields.Str(
        required=True,
        load_only=True,
        validate=validate.Length(min=8)
    )
    created_at = fields.DateTime(dump_only=True)

    @validates('email')
    def validate_email(self, value):
        """Check email uniqueness."""
        from app.models.user import User
        if User.query.filter_by(email=value).first():
            raise ValidationError('Email already registered')

class UserUpdateSchema(Schema):
    """User update schema (partial updates)."""

    email = fields.Email(validate=validate.Length(max=120))
    name = fields.Str(validate=validate.Length(min=2, max=80))
    password = fields.Str(load_only=True, validate=validate.Length(min=8))

# Usage in routes
from marshmallow import ValidationError
from app.schemas.user_schema import UserSchema, UserUpdateSchema

user_schema = UserSchema()
users_schema = UserSchema(many=True)
user_update_schema = UserUpdateSchema()

@users_bp.route('/', methods=['POST'])
def create_user():
    """Create user with validation."""
    try:
        # Validate and deserialize
        data = user_schema.load(request.get_json())
    except ValidationError as err:
        return jsonify({"errors": err.messages}), 400

    user = User(**data)
    db.session.add(user)
    db.session.commit()

    # Serialize response
    return user_schema.dump(user), 201

@users_bp.route('/', methods=['GET'])
def list_users():
    """List users with serialization."""
    users = User.query.all()
    return jsonify(users_schema.dump(users))

Pydantic Validation

# app/schemas/user_schema_pydantic.py
from pydantic import BaseModel, EmailStr, Field, validator
from typing import Optional
from datetime import datetime

class UserCreate(BaseModel):
    """User creation schema."""

    email: EmailStr
    name: str = Field(..., min_length=2, max_length=80)
    password: str = Field(..., min_length=8)

    @validator('password')
    def password_complexity(cls, v):
        """Validate password complexity."""
        if not any(char.isdigit() for char in v):
            raise ValueError('Password must contain at least one digit')
        if not any(char.isupper() for char in v):
            raise ValueError('Password must contain at least one uppercase letter')
        return v

class UserUpdate(BaseModel):
    """User update schema."""

    email: Optional[EmailStr] = None
    name: Optional[str] = Field(None, min_length=2, max_length=80)

class UserResponse(BaseModel):
    """User response schema."""

    id: int
    email: str
    name: str
    created_at: datetime

    class Config:
        orm_mode = True

# Usage
from pydantic import ValidationError

@users_bp.route('/', methods=['POST'])
def create_user():
    """Create user with Pydantic validation."""
    try:
        user_data = UserCreate(**request.get_json())
    except ValidationError as e:
        return jsonify({"errors": e.errors()}), 400

    user = User(
        email=user_data.email,
        name=user_data.name
    )
    user.set_password(user_data.password)

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

    return UserResponse.from_orm(user).dict(), 201

SQLAlchemy Integration

Models with Flask-SQLAlchemy

# app/models/user.py
from datetime import datetime
from werkzeug.security import generate_password_hash, check_password_hash
from app.extensions import db

class User(db.Model):
    """User model."""

    __tablename__ = 'users'

    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(120), unique=True, nullable=False, index=True)
    name = db.Column(db.String(80), nullable=False)
    password_hash = db.Column(db.String(200), nullable=False)
    is_active = db.Column(db.Boolean, default=True)
    created_at = db.Column(db.DateTime, default=datetime.utcnow)
    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 set_password(self, password):
        """Hash and set password."""
        self.password_hash = generate_password_hash(password)

    def check_password(self, password):
        """Verify password."""
        return check_password_hash(self.password_hash, password)

    def to_dict(self):
        """Serialize to dictionary."""
        return {
            'id': self.id,
            'email': self.email,
            'name': self.name,
            'is_active': self.is_active,
            'created_at': self.created_at.isoformat(),
            'updated_at': self.updated_at.isoformat()
        }

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

# app/models/post.py
class Post(db.Model):
    """Post model."""

    __tablename__ = 'posts'

    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(200), nullable=False)
    content = db.Column(db.Text, nullable=False)
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'), nullable=False)
    created_at = db.Column(db.DateTime, default=datetime.utcnow)

    def to_dict(self):
        return {
            'id': self.id,
            'title': self.title,
            'content': self.content,
            'user_id': self.user_id,
            'author': self.author.name,
            'created_at': self.created_at.isoformat()
        }

# Database operations
@users_bp.route('/<int:user_id>/posts', methods=['GET'])
def get_user_posts(user_id):
    """Get user posts with pagination."""
    user = User.query.get_or_404(user_id)
    page = request.args.get('page', 1, type=int)

    posts = user.posts.paginate(page=page, per_page=20)

    return jsonify({
        'posts': [p.to_dict() for p in posts.items],
        'total': posts.total,
        'page': posts.page
    })

Database Migrations

# Initialize migrations
flask db init

# Create migration
flask db migrate -m "Create users table"

# Apply migration
flask db upgrade

# Rollback
flask db downgrade

# migrations/versions/xxx_create_users.py (auto-generated)
def upgrade():
    op.create_table(
        'users',
        sa.Column('id', sa.Integer(), nullable=False),
        sa.Column('email', sa.String(length=120), nullable=False),
        sa.Column('name', sa.String(length=80), nullable=False),
        sa.Column('password_hash', sa.String(length=200), nullable=False),
        sa.Column('created_at', sa.DateTime(), nullable=True),
        sa.PrimaryKeyConstraint('id'),
        sa.UniqueConstraint('email')
    )
    op.create_index('ix_users_email', 'users', ['email'])

def downgrade():
    op.drop_index('ix_users_email', table_name='users')
    op.drop_table('users')

Authentication

Flask-Login Session-Based Auth

# app/extensions.py
from flask_login import LoginManager

login_manager = LoginManager()

# app/__init__.py
def create_app():
    app = Flask(__name__)
    login_manager.init_app(app)
    login_manager.login_view = 'auth.login'

    @login_manager.user_loader
    def load_user(user_id):
        from app.models.user import User
        return User.query.get(int(user_id))

    return app

# app/models/user.py
from flask_login import UserMixin

class User(UserMixin, db.Model):
    # ... existing fields ...

    def get_id(self):
        return str(self.id)

# app/routes/auth.py
from flask import Blueprint, request, jsonify
from flask_login import login_user, logout_user, login_required, current_user
from app.models.user import User

auth_bp = Blueprint('auth', __name__)

@auth_bp.route('/login', methods=['POST'])
def login():
    """User login."""
    data = request.get_json()

    user = User.query.filter_by(email=data['email']).first()
    if not user or not user.check_password(data['password']):
        return jsonify({"error": "Invalid credentials"}), 401

    login_user(user, remember=data.get('remember', False))
    return jsonify(user.to_dict())

@auth_bp.route('/logout', methods=['POST'])
@login_required
def logout():
    """User logout."""
    logout_user()
    return jsonify({"message": "Logged out successfully"})

@auth_bp.route('/me', methods=['GET'])
@login_required
def get_current_user():
    """Get current authenticated user."""
    return jsonify(current_user.to_dict())

JWT Authentication

# app/extensions.py
from flask_jwt_extended import JWTManager

jwt = JWTManager()

# app/__init__.py
def create_app():
    app = Flask(__name__)
    app.config['JWT_SECRET_KEY'] = 'super-secret-key'
    app.config['JWT_ACCESS_TOKEN_EXPIRES'] = timedelta(hours=1)
    app.config['JWT_REFRESH_TOKEN_EXPIRES'] = timedelta(days=30)

    jwt.init_app(app)

    return app

# app/routes/auth.py
from flask_jwt_extended import (
    create_access_token,
    create_refresh_token,
    jwt_required,
    get_jwt_identity
)

@auth_bp.route('/login', methods=['POST'])
def login():
    """Login and return JWT tokens."""
    data = request.get_json()

    user = User.query.filter_by(email=data['email']).first()
    if not user or not user.check_password(data['password']):
        return jsonify({"error": "Invalid credentials"}), 401

    access_token = create_access_token(identity=user.id)
    refresh_token = create_refresh_token(identity=user.id)

    return jsonify({
        "access_token": access_token,
        "refresh_token": refresh_token,
        "user": user.to_dict()
    })

@auth_bp.route('/refresh', methods=['POST'])
@jwt_required(refresh=True)
def refresh():
    """Refresh access token."""
    current_user_id = get_jwt_identity()
    access_token = create_access_token(identity=current_user_id)

    return jsonify({"access_token": access_token})

@auth_bp.route('/me', methods=['GET'])
@jwt_required()
def get_current_user():
    """Get current user from JWT."""
    current_user_id = get_jwt_identity()
    user = User.query.get_or_404(current_user_id)

    return jsonify(user.to_dict())

# Protected route example
@users_bp.route('/<int:user_id>', methods=['DELETE'])
@jwt_required()
def delete_user(user_id):
    """Delete user (authenticated)."""
    current_user_id = get_jwt_identity()

    # Authorization check
    if current_user_id != user_id:
        return jsonify({"error": "Unauthorized"}), 403

    user = User.query.get_or_404(user_id)
    db.session.delete(user)
    db.session.commit()

    return '', 204

Configuration Management

Environment-Based Configuration

# app/config.py
import os
from datetime import timedelta

class Config:
    """Base configuration."""

    # Flask
    SECRET_KEY = os.getenv('SECRET_KEY', 'dev-secret-key-change-in-production')

    # SQLAlchemy
    SQLALCHEMY_DATABASE_URI = os.getenv('DATABASE_URL', 'sqlite:///app.db')
    SQLALCHEMY_TRACK_MODIFICATIONS = False
    SQLALCHEMY_ECHO = False

    # JWT
    JWT_SECRET_KEY = os.getenv('JWT_SECRET_KEY', 'jwt-secret-change-in-production')
    JWT_ACCESS_TOKEN_EXPIRES = timedelta(hours=1)
    JWT_REFRESH_TOKEN_EXPIRES = timedelta(days=30)

    # CORS
    CORS_ORIGINS = os.getenv('CORS_ORIGINS', '*').split(',')

    # Pagination
    ITEMS_PER_PAGE = 20
    MAX_ITEMS_PER_PAGE = 100

    # Logging
    LOG_LEVEL = os.getenv('LOG_LEVEL', 'INFO')
    LOG_FILE = os.getenv('LOG_FILE', 'app.log')

class DevelopmentConfig(Config):
    """Development configuration."""
    DEBUG = True
    TESTING = False
    SQLALCHEMY_ECHO = True

class TestingConfig(Config):
    """Testing configuration."""
    TESTING = True
    SQLALCHEMY_DATABASE_URI = 'sqlite:///:memory:'
    WTF_CSRF_ENABLED = False

class ProductionConfig(Config):
    """Production configuration."""
    DEBUG = False
    TESTING = False

    # Strict security requirements
    SECRET_KEY = os.getenv('SECRET_KEY')
    JWT_SECRET_KEY = os.getenv('JWT_SECRET_KEY')

    # Production database (PostgreSQL recommended)
    SQLALCHEMY_DATABASE_URI = os.getenv('DATABASE_URL')

    @classmethod
    def init_app(cls, app):
        """Production-specific initialization."""
        # Log to syslog or external service
        import logging
        from logging.handlers import SysLogHandler

        syslog_handler = SysLogHandler()
        syslog_handler.setLevel(logging.WARNING)
        app.logger.addHandler(syslog_handler)

# Configuration factory
config_by_name = {
    'development': DevelopmentConfig,
    'testing': TestingConfig,
    'production': ProductionConfig,
    'default': DevelopmentConfig
}

def get_config(env_name='default'):
    """Get configuration by environment name."""
    return config_by_name.get(env_name, DevelopmentConfig)

# app/__init__.py
def create_app(config_name='default'):
    app = Flask(__name__)
    app.config.from_object(get_config(config_name))

    return app

.env File Support

# .env
FLASK_APP=run.py
FLASK_ENV=development
SECRET_KEY=your-secret-key-here
JWT_SECRET_KEY=your-jwt-secret-here
DATABASE_URL=postgresql://user:password@localhost/dbname
CORS_ORIGINS=http://localhost:3000,http://localhost:8000

# Load environment variables
from dotenv import load_dotenv
load_dotenv()

# app/__init__.py
import os


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