CS50x 2026 - Lecture 9 - Flask | CS50

The lecture synthesizes web development concepts, emphasizing the creation of dynamic web applications using Python, SQL, and Flask. It transitions from serving static content to handling dynamic routes and user inputs, illustrating how URLs can drive server-side logic. The discussion covers using Flask to manage GET and POST requests, incorporating Jinja for templating, and structuring projects following the MVC paradigm. The lecture also addresses practical considerations such as sanitizing user inputs to prevent security vulnerabilities and using global variables. It culminates in re-implementing a basic registration website, highlighting the use of databases for persistent data storage and demonstrating techniques for managing user sessions with cookies.

Outlines

Part 1: Introduction to Web Programming

Part 2: Flask Fundamentals

Part 3: Interactive Forms and Templates

Part 4: Case Study: Frosh IMs Registration

Part 5: Validation and Data Persistence

Part 6: Advanced Features and Security

Part 7: Architecture, Sessions, and APIs

Part 8: Conclusion

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CS50x 2026 - Lecture 9 - Flask

CS50

Part 1: Introduction to Web Programming

00:00CS50 Week 9: Synthesizing Concepts for Web Programming

CS50 Week 9: Synthesizing Concepts for Web Programming

00:14From File Paths to Web Routes: Gaining Control Over URLs

From File Paths to Web Routes: Gaining Control Over URLs

00:49Introducing Flask: A Python Microframework for Web Applications

Introducing Flask: A Python Microframework for Web Applications

Part 2: Flask Fundamentals

05:43Setting Up Flask: Commands, App Files, and Python Decorators

Setting Up Flask: Commands, App Files, and Python Decorators

09:40Creating a Basic Web Application: Returning HTML from Python

Creating a Basic Web Application: Returning HTML from Python

14:02Factoring Out HTML: Using Templates for Dynamic Web Pages

Factoring Out HTML: Using Templates for Dynamic Web Pages

17:06Taking User Input: Passing Parameters via URLs and Jinja Templating

Taking User Input: Passing Parameters via URLs and Jinja Templating

22:46Handling Missing Parameters: Conditional Logic and Default Values

Handling Missing Parameters: Conditional Logic and Default Values

Part 3: Interactive Forms and Templates

25:08Introducing Forms: Creating a Web Form for User Input

Introducing Forms: Creating a Web Form for User Input

30:04Debugging and Template Creation: Addressing Server Errors

Debugging and Template Creation: Addressing Server Errors

35:39Template Inheritance: Factoring Out Common HTML with Layouts

Template Inheritance: Factoring Out Common HTML with Layouts

42:37Enhancing Privacy: Switching from GET to POST Methods

Enhancing Privacy: Switching from GET to POST Methods

47:00Consolidating Routes: Combining Form Display and Processing

Consolidating Routes: Combining Form Display and Processing

Part 4: Case Study: Frosh IMs Registration

50:53Template Logic and Real-World Motivation: Frosh IMs Website

Template Logic and Real-World Motivation: Frosh IMs Website

56:00Building the Frosh IMs Registration Form: HTML Structure and Dropdown Menus

Building the Frosh IMs Registration Form: HTML Structure and Dropdown Menus

1:02:12Implementing the Registration Route: Handling Form Submissions

Implementing the Registration Route: Handling Form Submissions

1:07:11Client-Side Vulnerabilities: Exploiting HTML for Unauthorized Registration

Client-Side Vulnerabilities: Exploiting HTML for Unauthorized Registration

Part 5: Validation and Data Persistence

1:09:41Server-Side Validation: Preventing Unauthorized Sport Registrations

Server-Side Validation: Preventing Unauthorized Sport Registrations

1:14:13Dynamic Dropdowns: Generating Options from a Global Variable

Dynamic Dropdowns: Generating Options from a Global Variable

1:17:23Improving User Experience: Specific Error Messages and Data Storage

Improving User Experience: Specific Error Messages and Data Storage

1:22:00Implementing Data Persistence: Storing Registrations in a Database

Implementing Data Persistence: Storing Registrations in a Database

1:27:23Enhancing Data Display: Linking to the Registration List

Enhancing Data Display: Linking to the Registration List

1:31:11Data Integrity: Storing Registrations in a Database

Data Integrity: Storing Registrations in a Database

Part 6: Advanced Features and Security

1:37:37Implementing Deregistration: Removing Users from the Database

Implementing Deregistration: Removing Users from the Database

1:44:28Security Considerations: Preventing Malicious Deregistrations

Security Considerations: Preventing Malicious Deregistrations

1:47:12Improving Error Messages: Adding Images to Error Pages

Improving Error Messages: Adding Images to Error Pages

1:50:31Modernizing the Registration Form: Multiple Sport Selection

Modernizing the Registration Form: Multiple Sport Selection

Part 7: Architecture, Sessions, and APIs

1:55:15MVC Paradigm: Model, View, Controller

MVC Paradigm: Model, View, Controller

2:02:21Implementing Login: Cookies and Sessions

Implementing Login: Cookies and Sessions

2:06:03Login Implementation: Code Walkthrough

Login Implementation: Code Walkthrough

2:11:16Implementing a Shopping Cart: Sessions for E-Commerce

Implementing a Shopping Cart: Sessions for E-Commerce

2:15:49Building an API: Returning Data in JSON Format

Building an API: Returning Data in JSON Format

Part 8: Conclusion

2:25:28Conclusion: Building Blocks for Web and Mobile Applications

Conclusion: Building Blocks for Web and Mobile Applications