Advanced Flask/Backend: Blueprints, Error Handlers, Middleware, Async Tasks

🎯 Learning Objectives

• Structure Flask applications with Blueprints for modularity
• Implement custom error handlers and middleware
• Use Celery for background task processing
• Design RESTful APIs following best practices

📖 Core Content

1.1 Flask Blueprints

python
Run Inline
# runnable
from flask import Blueprint, jsonify
# Define blueprint
auth_bp = Blueprint('auth', __name__, url_prefix='/auth')
@auth_bp.route('/login', methods=['POST'])
def login():
    return jsonify({"message": "Login endpoint"})
@auth_bp.route('/register', methods=['POST'])
def register():
    return jsonify({"message": "Register endpoint"})
# Register in main app
# from flask import Flask
# app = Flask(__name__)
# app.register_blueprint(auth_bp)

1.2 Error Handlers

python
Run Inline
# runnable
from flask import jsonify
# Custom error handler
@app.errorhandler(404)
def not_found(error):
    return jsonify({
        "error": "Not Found",
        "message": "The requested resource was not found",
        "status_code": 404
    }), 404
@app.errorhandler(500)
def internal_error(error):
    return jsonify({
        "error": "Internal Server Error",
        "message": "An unexpected error occurred",
        "status_code": 500
    }), 500

1.3 Async Tasks with Celery

python
Run Inline
# runnable
from celery import Celery
celery = Celery('tasks', broker='redis://localhost:6379/0')
@celery.task
def process_image(image_path):
    """Background image processing - runs asynchronously."""
    # Heavy processing here
    result = {"status": "processed", "path": image_path}
    return result
# Trigger async task from Flask
# @app.route('/upload', methods=['POST'])
# def upload():
#     task = process_image.delay(file_path)
#     return jsonify({"task_id": task.id}), 202

1.4 Why This Matters

2. 📝 Practice Questions

Q1: Your Flask app has become a single file with 2000+ lines. Name three refactoring steps to improve maintainability.

1. Split into Blueprints: Create separate blueprints for auth, API, admin, etc. Each in its own file.
2. Extract models to separate module: Move all SQLAlchemy models to models.py.
3. Configuration in a separate file: Use config.py or environment variables for settings.
4. Services layer: Move business logic out of route handlers into service classes.
5. Celery tasks in a separate module: All async tasks in tasks.py.

This creates a structure like:

pseudo
app/
├── __init__.py       # App factory
├── config.py         # Configuration
├── models.py         # Database models
├── auth/             # Auth blueprint
├── api/              # API blueprint
├── tasks.py          # Celery tasks
└── utils.py          # Helper functions