📘 Overview

• What is it?: Flask is a lightweight WSGI web application framework in Python. Designed to make getting started quick and easy, it is a “microframework” because it does not require particular tools or libraries (such as a database abstraction layer or validation form).
• Key Features:
  • Micro & Pluggable: Core features are minimal, but it is highly extensible, allowing developers to add SQLAlchemy, Web Forms, User Auth, etc., as needed.
  • Routing: Clean decorator-based URL mapping.
  • Jinja2 Templating: Powerful layout building with template inheritance, loops, and variable binding.
  • Built-in Dev Server: Fast reload and debugger in development.
• Installation:

  pip install Flask

🧾 Core Concepts

• Routing: Mapping URL paths to Python view functions (e.g. @app.route('/user/<username>')).
• Blueprints: Modules used to organize application routes, assets, and templates into clean components (crucial for scaling Flask apps).
• Request/Response: Context objects representing incoming client requests (request.args, request.form, request.json) and the outgoing response structure.
• Application Factory Pattern: A design pattern where the application instance is created inside a function (e.g., create_app()), allowing multiple configurations and better testing.

💻 Common Code Patterns & Cheat Sheet

• Basic Application:

  from flask import Flask, jsonify, request
   
  app = Flask(__name__)
   
  @app.route("/", methods=["GET"])
  def home():
      return jsonify({"message": "Hello from Flask!"})
   
  if __name__ == "__main__":
      app.run(debug=True)

• Dynamic Routing & Request Handling:

  @app.route("/user/<int:user_id>", methods=["POST"])
  def update_user(user_id):
      # Extract JSON payload
      data = request.get_json()
      username = data.get("username")
      
      # Process data...
      return jsonify({"id": user_id, "updated_to": username}), 200

• Using Blueprints (Modularity):

  # routes/auth.py
  from flask import Blueprint
   
  auth_bp = Blueprint("auth", __name__, url_prefix="/auth")
   
  @auth_bp.route("/login")
  def login():
      return "Login Page"
   
  # main.py
  from flask import Flask
  from routes.auth import auth_bp
   
  app = Flask(__name__)
  app.register_blueprint(auth_bp)

💡 Best Practices & Tips

• Factory Pattern: Always use the application factory pattern (def create_app(): ...) for production apps to avoid global state issues and make testing easier.
• Config Management: Store sensitive configs in a class (like ProductionConfig or DevelopmentConfig) loaded from environment variables.
• Error Handling: Register global error handlers using @app.errorhandler(404) or @app.errorhandler(Exception) to return clean JSON error payloads.

🔗 Navigation & Internal Links

• Parent: Python
• Related Notes: Web Development | FastAPI | SQLAlchemy