Flask Core Knowledge
Full Reference: See advanced.md for Flask-SocketIO WebSocket patterns, authentication, room management, namespaces, and broadcasting from external code.
Basic Setup
from flask import Flask, jsonify, request, abort
app = Flask(name)
@app.route('/users', methods=['GET']) def get_users(): return jsonify(users)
@app.route('/users/<int:user_id>', methods=['GET']) def get_user(user_id): user = find_user(user_id) if not user: abort(404) return jsonify(user)
@app.route('/users', methods=['POST']) def create_user(): data = request.get_json() user = create_user_in_db(data) return jsonify(user), 201
Blueprints
from flask import Blueprint
users_bp = Blueprint('users', name, url_prefix='/api/users')
@users_bp.route('/') def list_users(): return jsonify(get_all_users())
In app.py
app.register_blueprint(users_bp)
Key Extensions
Extension Purpose
Flask-SQLAlchemy Database ORM
Flask-Migrate Database migrations
Flask-JWT-Extended JWT auth
Flask-CORS CORS support
Flask-RESTful REST APIs
Application Factory
from flask import Flask from flask_sqlalchemy import SQLAlchemy from flask_cors import CORS
db = SQLAlchemy()
def create_app(config_name='production'): app = Flask(name) app.config.from_object(config[config_name])
db.init_app(app)
CORS(app, origins=app.config['CORS_ORIGINS'])
from .routes import users_bp, health_bp
app.register_blueprint(users_bp, url_prefix='/api/users')
app.register_blueprint(health_bp)
register_error_handlers(app)
return app
Configuration
import os
class Config: SECRET_KEY = os.environ.get('SECRET_KEY') SQLALCHEMY_TRACK_MODIFICATIONS = False SQLALCHEMY_ENGINE_OPTIONS = { 'pool_size': 10, 'pool_recycle': 3600, 'pool_pre_ping': True, }
class ProductionConfig(Config): DEBUG = False SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URL') SESSION_COOKIE_SECURE = True SESSION_COOKIE_HTTPONLY = True
Error Handling
from flask import jsonify
class APIError(Exception): def init(self, message, status_code=400): self.message = message self.status_code = status_code
def register_error_handlers(app): @app.errorhandler(APIError) def handle_api_error(error): return jsonify({'error': error.message}), error.status_code
@app.errorhandler(404)
def not_found(error):
return jsonify({'error': 'Resource not found'}), 404
@app.errorhandler(500)
def internal_error(error):
return jsonify({'error': 'Internal server error'}), 500
Health Checks
health_bp = Blueprint('health', name)
@health_bp.route('/health') def health(): return jsonify({'status': 'healthy'})
@health_bp.route('/ready') def ready(): try: db.engine.execute('SELECT 1') return jsonify({'status': 'ready', 'database': 'connected'}) except Exception as e: return jsonify({'status': 'not ready'}), 503
Testing
import pytest from app import create_app, db
@pytest.fixture def app(): app = create_app('testing') with app.app_context(): db.create_all() yield app db.drop_all()
@pytest.fixture def client(app): return app.test_client()
def test_create_user(client): response = client.post('/api/users/', json={ 'name': 'John', 'email': 'john@example.com' }) assert response.status_code == 201
When NOT to Use This Skill
-
Django projects - Django provides more batteries-included features
-
FastAPI with type validation - FastAPI has built-in Pydantic validation
-
Async-first applications - Flask is sync-based
-
Built-in admin required - Django admin is more comprehensive
Anti-Patterns
Anti-Pattern Why It's Bad Solution
Using global app instance Hard to test Use application factory pattern
No error handlers Generic 500 errors Register error handlers
Hardcoded config values Can't change per environment Use app.config.from_object()
Not using blueprints Monolithic code Split into blueprints
Missing CORS Frontend can't connect Use flask-cors extension
Quick Troubleshooting
Problem Diagnosis Fix
"Working outside application context" No app context Use with app.app_context():
Static files 404 Wrong path Check static_folder
CORS errors Not configured Install and configure flask-cors
Database pool exhausted No pooling Configure SQLALCHEMY_ENGINE_OPTIONS
Blueprint not found Not registered Call app.register_blueprint()
Production Checklist
-
Application factory pattern
-
Environment-based config
-
Structured JSON logging
-
Request ID tracing
-
Database connection pooling
-
Health/readiness endpoints
-
Global error handlers
-
Gunicorn with workers
-
pytest fixtures for testing
-
CORS properly configured
Monitoring Metrics
Metric Target
Response time (p99) < 200ms
Error rate (5xx) < 0.1%
Database pool usage < 80%
Reference Documentation
-
Blueprints
-
Extensions