Flask REST API with flask-smorest Pattern

This skill helps you set up a Flask REST API following a standardized pattern with flask-smorest for OpenAPI documentation, blueprint architecture, and best practices for production-ready APIs.

When to Use This Skill

Use this skill when:

• Starting a new Flask REST API project
• You want automatic OpenAPI/Swagger documentation
• You need a clean, modular blueprint architecture
• You want type-safe request/response handling with Marshmallow schemas
• You're building a production-ready API server

What This Skill Creates

1. Main application file - Flask app initialization with flask-smorest
2. Blueprint structure - Modular endpoint organization
3. Schema files - Marshmallow schemas for validation and docs
4. Singleton manager pattern - Centralized service/database initialization
5. CORS support - Cross-origin request handling
6. Requirements file - All necessary dependencies

Step 1: Gather Project Information

IMPORTANT: Before creating files, ask the user these questions:

1. "What is your project name?" (e.g., "materia-server", "trading-api", "myapp")

   • Use this to derive:
     • Main module: {project_name}.py (e.g., materia_server.py)
     • Port number (suggest based on project, default: 5000)

2. "What features/endpoints do you need?" (e.g., "users", "tokens", "orders")

   • Each feature will become a blueprint

3. "Do you need database integration?" (yes/no)

   • If yes, reference postgres-setup skill for database layer

4. "What port should the server run on?" (default: 5000)

Step 2: Create Directory Structure

Create these directories if they don't exist:

{project_root}/
├── blueprints/          # Blueprint modules (one per feature)
│   ├── __init__.py
│   ├── {feature}.py
│   └── {feature}_schemas.py
├── src/                 # Optional: for package code (database drivers, models, etc.)
│   └── {project_name}/
└── {project_name}.py    # Main application file

Step 3: Create Main Application File

Create {project_name}.py using this template:

import os
import logging
from flask import Flask
from flask_cors import CORS
from flask_smorest import Api

logging.basicConfig(level=logging.INFO)
logger = logging.getLogger(__name__)

def create_app():
    app = Flask(__name__)
    app.config['API_TITLE'] = '{Project Name} API'
    app.config['API_VERSION'] = 'v1'
    app.config['OPENAPI_VERSION'] = '3.0.2'
    app.config['OPENAPI_URL_PREFIX'] = '/'
    app.config['OPENAPI_SWAGGER_UI_PATH'] = '/swagger'
    app.config['OPENAPI_SWAGGER_UI_URL'] = 'https://cdn.jsdelivr.net/npm/swagger-ui-dist/'

    CORS(app)
    api = Api(app)

    from blueprints.{feature} import blp as {feature}_blp
    api.register_blueprint({feature}_blp)

    logger.info("Flask app initialized")
    return app

if __name__ == '__main__':
    port = int(os.environ.get('PORT', {port_number}))
    app = create_app()
    logger.info(f"Swagger UI: http://localhost:{port}/swagger")
    app.run(host='0.0.0.0', port=port)

CRITICAL: Replace:

• {Project Name} → Human-readable project name (e.g., "Materia Server")
• {project_name} → Snake case project name (e.g., "materia_server")
• {port_number} → Actual port number (e.g., 5151)
• {feature} → Feature name from user's response

Step 4: Create Blueprint and Schema Files

For each feature/endpoint, create two files:

File: blueprints/{feature}.py

from flask.views import MethodView
from flask_smorest import Blueprint, abort
from .{feature}_schemas import {Feature}QuerySchema, {Feature}ResponseSchema, ErrorResponseSchema

blp = Blueprint('{feature}', __name__, url_prefix='/api', description='{Feature} API')

@blp.route('/{feature}')
class {Feature}Resource(MethodView):
    @blp.arguments({Feature}QuerySchema, location='query')
    @blp.response(200, {Feature}ResponseSchema)
    @blp.alt_response(400, schema=ErrorResponseSchema)
    @blp.alt_response(500, schema=ErrorResponseSchema)
    def get(self, query_args):
        try:
            # TODO: Implement logic
            return {"message": "Success", "data": []}
        except ValueError as e:
            abort(400, message=str(e))
        except Exception as e:
            abort(500, message=str(e))

File: blueprints/{feature}_schemas.py

from marshmallow import Schema, fields

class {Feature}QuerySchema(Schema):
    limit = fields.Integer(load_default=100, metadata={'description': 'Max results'})
    offset = fields.Integer(load_default=0, metadata={'description': 'Skip count'})

class {Feature}ResponseSchema(Schema):
    message = fields.String(required=True)
    data = fields.List(fields.Dict(), required=True)

class ErrorResponseSchema(Schema):
    code = fields.Integer(required=True)
    status = fields.String(required=True)
    message = fields.String()

CRITICAL: Replace:

• {Feature} → PascalCase feature name (e.g., "TradableTokens")
• {feature} → Snake case feature name (e.g., "tradable_tokens")

Step 5: Create Common Singleton Manager (If Needed)

If the project needs shared services (database, API clients, etc.), create a singleton manager:

File: common.py

"""
Singleton manager for shared service instances.

Provides centralized initialization of database connections, API clients,
and other shared resources.
"""

import os
import logging


logger = logging.getLogger(__name__)


class ServiceManager:
    """
    Singleton manager for shared service instances.

    Ensures only one instance of each service is created and reused
    across all blueprints.
    """

    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super(ServiceManager, cls).__new__(cls)
            cls._instance._initialized = False
        return cls._instance

    def __init__(self):
        if self._initialized:
            return

        # Initialize services
        self._db = None
        self._initialized = True
        logger.info("ServiceManager initialized")

    def get_database(self):
        """
        Get database connection instance.

        Returns:
            Database connection instance (lazy initialization)
        """
        if self._db is None:
            # Import database driver
            from src.{project_name}.database import Database

            # Get connection parameters from environment
            db_host = os.environ.get('{PROJECT_NAME}_DB_HOST', 'localhost')
            db_name = os.environ.get('{PROJECT_NAME}_DB_NAME', '{project_name}')
            db_user = os.environ.get('{PROJECT_NAME}_DB_USER', '{project_name}')
            db_passwd = os.environ.get('{PROJECT_NAME}_DB_PASSWORD')

            if not db_passwd:
                raise ValueError("{PROJECT_NAME}_DB_PASSWORD environment variable required")

            self._db = Database(db_host, db_name, db_user, db_passwd)
            logger.info("Database connection initialized")

        return self._db


# Global singleton instance
service_manager = ServiceManager()

CRITICAL: Replace:

• {PROJECT_NAME} → Uppercase project name (e.g., "MATERIA_SERVER")
• {project_name} → Snake case project name (e.g., "materia_server")

Step 6: Create Requirements File

Create requirements.txt with flask-smorest dependencies:

# Flask and API framework
Flask==3.0.0
flask-smorest==0.44.0
flask-cors==4.0.0

# Schema validation and serialization
marshmallow==3.20.1

# Production server (optional but recommended)
gunicorn==21.2.0

# Database (if needed)
psycopg2-binary==2.9.9

Step 7: Create Blueprints init.py

Create blueprints/__init__.py:

"""
Blueprint modules for {Project Name} API.

Each blueprint represents a distinct feature or resource endpoint.
"""

Step 8: Document Usage

Create or update README.md with:

Running the Server

# Development mode
python {project_name}.py

# Production mode with Gunicorn
gunicorn -w 4 -b 0.0.0.0:{port_number} '{project_name}:create_app()'

Environment Variables

Server Configuration:

• PORT - Server port (default: {port_number})
• DEBUG - Enable debug mode (default: False)

Database (if applicable):

• {PROJECT_NAME}_DB_HOST - Database host (default: localhost)
• {PROJECT_NAME}_DB_NAME - Database name (default: {project_name})
• {PROJECT_NAME}_DB_USER - Database user (default: {project_name})
• {PROJECT_NAME}_DB_PASSWORD - Database password (REQUIRED)

API Documentation

Once running, access Swagger UI at:

http://localhost:{port_number}/swagger

Design Principles

This pattern follows these principles:

Architecture:

1. Blueprint Organization - Modular endpoint organization, one blueprint per feature
2. MethodView Classes - Class-based views for HTTP methods (get, post, put, delete)
3. Separation of Concerns - Routes, schemas, and business logic separated
4. Singleton Manager - Centralized service initialization prevents duplicate connections
5. Application Factory - create_app() pattern for testing and flexibility

API Design:

1. OpenAPI/Swagger - Automatic documentation via flask-smorest
2. Schema-Driven - Marshmallow schemas for validation and serialization
3. Type Safety - @blp.arguments() and @blp.response() decorators
4. Error Handling - Consistent error responses with proper HTTP status codes
5. CORS Support - Cross-origin requests for frontend consumption

Best Practices:

1. Environment-Based Config - All secrets via environment variables
2. Logging - Structured logging throughout
3. Idempotent Operations - Safe to call multiple times
4. Production Ready - Gunicorn support out of the box
5. Testing Friendly - Application factory enables easy testing

Integration with Other Skills

Database Integration

If database is needed, use postgres-setup skill first:

User: "Set up postgres database for my project"

Then reference the database in your blueprints via the singleton manager:

from common import service_manager

db = service_manager.get_database()

Package Structure

If publishing as a package, use python-pypi-setup skill:

User: "Set up Python package for PyPI"

Example Usage in Claude Code

User: "Set up Flask API server for my project"

Claude: "What is your project name?"

User: "crypto-tracker"

Claude: "What features/endpoints do you need?"

User: "prices, tokens, portfolio"

Claude: "Do you need database integration?"

User: "yes"

Claude: "What port should the server run on?"

User: "8080"

Claude:

1. Creates crypto_tracker.py with Flask app
2. Creates blueprints/ directory with:
   • prices.py and prices_schemas.py
   • tokens.py and tokens_schemas.py
   • portfolio.py and portfolio_schemas.py
3. Creates common.py with ServiceManager singleton
4. Creates requirements.txt with dependencies
5. Documents environment variables needed
6. Provides startup instructions

Optional: Docker Support

If user requests Docker, create Dockerfile:

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
EXPOSE {port_number}
CMD ["gunicorn", "-w", "4", "-b", "0.0.0.0:{port_number}", "{project_name}:create_app()"]