11.1 Review of Key Concepts

Key Concepts

Flask Basics
Routing and Views
Templates and Jinja2
Request and Response Objects
Static Files and Forms
Database Integration
Authentication and Authorization
Error Handling
Testing Flask Applications
Deployment
Advanced Topics

Flask Basics

Flask is a micro web framework written in Python. It is designed to be simple and easy to use, yet powerful enough to build complex web applications. Flask is often referred to as a "micro" framework because it does not require particular tools or libraries.

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello, World!'

if __name__ == '__main__':
    app.run()

Routing and Views

Routing in Flask involves mapping URLs to specific functions that handle the request. These functions are called views. The @app.route decorator is used to define routes.

@app.route('/about')
def about():
    return 'About Page'

Templates and Jinja2

Templates allow you to separate the presentation logic from the business logic. Flask uses the Jinja2 templating engine, which allows for dynamic content and control structures.

<html>
<body>
    <h1>Hello, {{ name }}!</h1>
</body>
</html>

Request and Response Objects

The request object contains all the data sent by the client, such as form data or query parameters. The response object is used to send data back to the client.

from flask import request, jsonify

@app.route('/data')
def get_data():
    data = {'name': request.args.get('name')}
    return jsonify(data)

Static Files and Forms

Static files, such as CSS, JavaScript, and images, are served directly by Flask. Forms are used to collect user input, and Flask provides utilities to handle form data securely.

<form method="post" action="/submit">
    <input type="text" name="username">
    <input type="submit" value="Submit">
</form>

Database Integration

Flask can be integrated with various databases using extensions like Flask-SQLAlchemy. This allows you to interact with databases using Python objects.

from flask_sqlalchemy import SQLAlchemy

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)

Authentication and Authorization

Authentication verifies the identity of a user, while authorization determines what actions a user is allowed to perform. Flask-Login and Flask-Principal are popular extensions for handling these tasks.

from flask_login import LoginManager, UserMixin, login_required

login_manager = LoginManager(app)

class User(UserMixin):
    pass

@login_manager.user_loader
def load_user(user_id):
    return User()

@app.route('/protected')
@login_required
def protected():
    return 'This is a protected page'

Error Handling

Error handling in Flask involves defining custom error pages and handling exceptions. The @app.errorhandler decorator is used to define error handlers.

@app.errorhandler(404)
def page_not_found(e):
    return 'Page Not Found', 404

Testing Flask Applications

Testing ensures that your application behaves as expected. Flask-Testing is an extension that provides utilities for writing unit and integration tests.

from flask_testing import TestCase

class MyTest(TestCase):
    def create_app(self):
        app.config['TESTING'] = True
        return app

    def test_home_page(self):
        response = self.client.get('/')
        self.assert200(response)

Deployment

Deployment involves making your Flask application available on the internet. Common deployment options include Heroku, AWS, and Docker.

# Dockerfile
FROM python:3.8-slim
WORKDIR /app
COPY . /app
RUN pip install -r requirements.txt
CMD ["python", "app.py"]

Advanced Topics

Advanced topics include Blueprints, RESTful APIs, Websockets, and background tasks. These topics allow you to build more complex and scalable applications.

from flask import Blueprint

auth = Blueprint('auth', __name__, url_prefix='/auth')

@auth.route('/login')
def login():
    return 'Login Page'