Flask Framework Guide

Applies to: Flask 3.0+, REST APIs, Microservices, Web Applications Language Guide: @.claude/skills/python-guide/SKILL.md

Overview

Flask is a lightweight WSGI web framework providing the basics for building web applications while allowing flexibility in choosing components.

Use Flask when:

Building microservices or small-to-medium APIs
You want flexibility to choose your own ORM, auth, etc.
Rapid prototyping is needed
You prefer explicit over implicit behavior

Consider alternatives when:

You need async support (use FastAPI or Quart)
You want batteries-included (use Django)
High performance async is critical (use FastAPI)

Guardrails

Flask-Specific Guidelines

Use the application factory pattern (never use global app instances)
Use blueprints for modular routing
Use Flask extensions appropriately (init in extensions.py)
Configure via environment variables with class-based config
Use Marshmallow for validation/serialization
Implement proper error handlers for all error types
Use Flask-Migrate for database migrations
Use Flask-JWT-Extended for authentication

Security Guidelines

Never store plain passwords (use werkzeug.security)
Use environment variables for all secrets
Implement proper authentication and authorization
Validate all user input with Marshmallow schemas
Use HTTPS in production
Set secure cookie options (HTTPONLY, SECURE, SAMESITE)
Sanitize all database queries via SQLAlchemy ORM
Configure CORS restrictively in production

Testing Guidelines

Use pytest with Flask test client
Use fixtures for test data and app context
Test both success and error cases for every endpoint
Mock external services (never call real APIs in tests)
Use a separate test database
Coverage target: >80% for business logic

Project Structure

myproject/
├── app/
│   ├── __init__.py           # Application factory
│   ├── config.py             # Configuration classes
│   ├── extensions.py         # Flask extensions (single init point)
│   ├── models/
│   │   ├── __init__.py
│   │   ├── base.py           # Base model class with mixins
│   │   └── user.py
│   ├── api/
│   │   ├── __init__.py       # API blueprint registration
│   │   ├── users.py          # User endpoints
│   │   └── auth.py           # Auth endpoints
│   ├── services/
│   │   ├── __init__.py
│   │   └── user_service.py   # Business logic (no Flask imports)
│   ├── schemas/
│   │   ├── __init__.py
│   │   └── user.py           # Marshmallow schemas
│   └── utils/
│       ├── __init__.py
│       ├── errors.py         # Custom exceptions and handlers
│       └── decorators.py     # Auth and role decorators
├── migrations/               # Alembic migrations (via Flask-Migrate)
├── tests/
│   ├── conftest.py           # Fixtures: app, client, db_session
│   ├── test_api/
│   │   └── test_users.py
│   └── test_services/
│       └── test_user_service.py
├── .env.example
├── requirements.txt
├── requirements-dev.txt
├── pyproject.toml
└── run.py                    # Entry point

Key conventions:

app/__init__.py contains create_app() factory only
app/extensions.py centralizes all extension instances
app/services/ holds business logic, no Flask imports
app/schemas/ holds Marshmallow validation/serialization
app/utils/errors.py defines custom exceptions and handlers

Application Factory

Always use the factory pattern. Never use a global app = Flask(__name__).

"""Flask application factory."""
from flask import Flask

from app.config import config
from app.extensions import db, migrate, ma, jwt, cors


def create_app(config_name: str = "development") -> Flask:
    """Create and configure the Flask application."""
    app = Flask(__name__)
    app.config.from_object(config[config_name])

    register_extensions(app)
    register_blueprints(app)
    register_error_handlers(app)
    register_commands(app)

    return app


def register_extensions(app: Flask) -> None:
    """Initialize Flask extensions."""
    db.init_app(app)
    migrate.init_app(app, db)
    ma.init_app(app)
    jwt.init_app(app)
    cors.init_app(app)


def register_blueprints(app: Flask) -> None:
    """Register Flask blueprints."""
    from app.api import api_bp
    app.register_blueprint(api_bp, url_prefix="/api/v1")


def register_error_handlers(app: Flask) -> None:
    """Register error handlers."""
    from app.utils.errors import (
        handle_app_error,
        handle_validation_error,
        handle_not_found,
        handle_internal_error,
    )
    from marshmallow import ValidationError

    app.register_error_handler(ValidationError, handle_validation_error)
    app.register_error_handler(404, handle_not_found)
    app.register_error_handler(500, handle_internal_error)


def register_commands(app: Flask) -> None:
    """Register CLI commands."""
    from app.commands import seed_db
    app.cli.add_command(seed_db)

Blueprints

Organize routes into blueprints. Register them in the factory.

"""API blueprint registration."""
from flask import Blueprint

api_bp = Blueprint("api", __name__)

# Import routes to register them
from app.api import users, auth  # noqa: F401, E402

Blueprint endpoints follow REST conventions:

Method	Route	Handler	Description
GET	`/resources`	`get_resources()`	List with pagination
GET	`/resources/<id>`	`get_resource(id)`	Get single resource
POST	`/resources`	`create_resource()`	Create resource
PATCH	`/resources/<id>`	`update_resource(id)`	Partial update
DELETE	`/resources/<id>`	`delete_resource(id)`	Delete resource

Configuration

Use class-based configuration with environment variable overrides.

"""Application configuration."""
import os
from datetime import timedelta
from typing import Type


class Config:
    """Base configuration."""
    SECRET_KEY = os.getenv("SECRET_KEY", "change-in-production")
    SQLALCHEMY_TRACK_MODIFICATIONS = False
    SQLALCHEMY_ENGINE_OPTIONS = {
        "pool_pre_ping": True,
        "pool_recycle": 300,
    }
    JWT_SECRET_KEY = os.getenv("JWT_SECRET_KEY", SECRET_KEY)
    JWT_ACCESS_TOKEN_EXPIRES = timedelta(hours=1)
    JWT_REFRESH_TOKEN_EXPIRES = timedelta(days=30)
    CORS_ORIGINS = os.getenv("CORS_ORIGINS", "*").split(",")


class DevelopmentConfig(Config):
    DEBUG = True
    SQLALCHEMY_DATABASE_URI = os.getenv(
        "DATABASE_URL",
        "postgresql://postgres:postgres@localhost:5432/myapp_dev"
    )
    SQLALCHEMY_ECHO = True


class TestingConfig(Config):
    TESTING = True
    SQLALCHEMY_DATABASE_URI = os.getenv(
        "TEST_DATABASE_URL",
        "postgresql://postgres:postgres@localhost:5432/myapp_test"
    )


class ProductionConfig(Config):
    DEBUG = False
    SQLALCHEMY_DATABASE_URI = os.environ["DATABASE_URL"]
    SESSION_COOKIE_SECURE = True
    SESSION_COOKIE_HTTPONLY = True
    SESSION_COOKIE_SAMESITE = "Lax"


config: dict[str, Type[Config]] = {
    "development": DevelopmentConfig,
    "testing": TestingConfig,
    "production": ProductionConfig,
}

Extensions

Centralize all Flask extension instances in a single file.

"""Flask extensions initialization."""
from flask_sqlalchemy import SQLAlchemy
from flask_migrate import Migrate
from flask_marshmallow import Marshmallow
from flask_jwt_extended import JWTManager
from flask_cors import CORS

db = SQLAlchemy()
migrate = Migrate()
ma = Marshmallow()
jwt = JWTManager()
cors = CORS()

Common extensions and their purposes:

Extension	Purpose
Flask-SQLAlchemy	ORM and database integration
Flask-Migrate	Alembic database migrations
Flask-Marshmallow	Serialization and validation
Flask-JWT-Extended	JWT authentication
Flask-CORS	Cross-origin resource sharing
Flask-Limiter	Rate limiting
Flask-Caching	Response and data caching
Flask-Mail	Email sending

Error Handling

Define a custom exception hierarchy and register handlers.

"""Custom exceptions and error handlers."""
from flask import jsonify


class AppError(Exception):
    """Base application error."""
    def __init__(self, message: str, status_code: int = 400):
        super().__init__(message)
        self.message = message
        self.status_code = status_code


class NotFoundError(AppError):
    def __init__(self, message: str = "Resource not found"):
        super().__init__(message, status_code=404)


class UnauthorizedError(AppError):
    def __init__(self, message: str = "Unauthorized"):
        super().__init__(message, status_code=401)


class ForbiddenError(AppError):
    def __init__(self, message: str = "Forbidden"):
        super().__init__(message, status_code=403)


class ConflictError(AppError):
    def __init__(self, message: str = "Conflict"):
        super().__init__(message, status_code=409)


def handle_validation_error(error):
    return jsonify({"error": "Validation error", "details": error.messages}), 400


def handle_app_error(error: AppError):
    return jsonify({"error": error.message}), error.status_code


def handle_not_found(error):
    return jsonify({"error": "Not found"}), 404


def handle_internal_error(error):
    return jsonify({"error": "Internal server error"}), 500

Jinja2 Templates

When building server-rendered pages (not just APIs):

Templates live in app/templates/ with a base.html layout
Use template inheritance: {% extends "base.html" %}
Use {% block content %} for page-specific content
Escape user content automatically (Jinja2 autoescape is on by default)
Use url_for() for all URLs in templates
Keep logic out of templates; use filters and context processors

{# app/templates/base.html #}
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>{% block title %}My App{% endblock %}</title>
</head>
<body>
    {% with messages = get_flashed_messages(with_categories=true) %}
        {% for category, message in messages %}
            <div class="alert alert-{{ category }}">{{ message }}</div>
        {% endfor %}
    {% endwith %}
    {% block content %}{% endblock %}
</body>
</html>

CLI Commands

"""Custom CLI commands."""
import click
from flask.cli import with_appcontext
from app.extensions import db


@click.command("seed-db")
@with_appcontext
def seed_db():
    """Seed database with initial data."""
    # Create initial records
    db.session.commit()
    click.echo("Database seeded.")

Commands Reference

# Install dependencies
pip install -r requirements.txt
pip install -r requirements-dev.txt

# Set environment variables
export FLASK_APP=run.py
export FLASK_ENV=development

# Initialize database
flask db init
flask db migrate -m "Initial migration"
flask db upgrade

# Seed database
flask seed-db

# Run development server
flask run

# Run with Gunicorn (production)
gunicorn -w 4 -b 0.0.0.0:5000 "app:create_app('production')"

# Run tests
pytest
pytest -v --cov=app --cov-report=html

# Lint and format
black .
isort .
ruff check .
mypy app/

Dependencies

requirements.txt

Flask>=3.0.0
Flask-SQLAlchemy>=3.1.0
Flask-Migrate>=4.0.0
Flask-Marshmallow>=0.15.0
Flask-JWT-Extended>=4.6.0
Flask-CORS>=4.0.0
marshmallow-sqlalchemy>=0.29.0
psycopg2-binary>=2.9.9
python-dotenv>=1.0.0
gunicorn>=21.0.0

requirements-dev.txt

-r requirements.txt
pytest>=7.4.0
pytest-flask>=1.2.0
pytest-cov>=4.1.0
black>=23.0.0
isort>=5.12.0
mypy>=1.5.0
ruff>=0.1.0

Advanced Topics

For detailed code examples and advanced patterns, see:

references/patterns.md -- Models, schemas, services, API endpoints, authentication, decorators, testing, and deployment patterns

flask