CS50 Fall 2025 - Lecture 9 - Flask (live, unedited) | CS50

In this CS50 Week 9 lecture, the speaker synthesizes the previous weeks' concepts, focusing on web programming using Python, SQL, and Flask. The lecture transitions from serving static content to creating dynamic web applications, emphasizing routes, URL parameters, and handling HTTP requests (GET and POST). The speaker introduces Flask as a micro-framework to streamline web development, covering topics such as rendering templates, taking user input, implementing forms, and validating data. The lecture progresses from basic "Hello, World" examples to a more complex Frosh IMs registration system, demonstrating practical applications of web programming principles, including database integration and session management using cookies. The lecture concludes by introducing the concept of APIs and JSON for data exchange, setting the stage for more advanced web development projects.

Outlines

Part 1: Introduction to Flask and Web Basics

Part 2: Handling User Input and Templates

Part 3: Form Methods and Logic

Part 4: Case Study: The Frosh IMs Application

Part 5: Validation and Database Integration

Part 6: Advanced Features and Security

Part 7: State Management: Cookies and Sessions

Part 8: APIs and Conclusion

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CS50 Fall 2025 - Lecture 9 - Flask (live, unedited)

CS50

Part 1: Introduction to Flask and Web Basics

Introduction to Web Programming with Flask

Setting Up a Basic Flask Application

Returning HTML from a Flask Application

Part 2: Handling User Input and Templates

Incorporating User Input via URL Parameters

Handling Missing URL Parameters and Refining the Code

Implementing a Web Form and Multiple Routes

Eliminating Redundancy with Template Inheritance

Part 3: Form Methods and Logic

Transitioning from GET to POST for Form Submission

Combining Routes and Adding Conditional Logic to Templates

Part 4: Case Study: The Frosh IMs Application

Introduction to a Real-World Example: Frosh IMs Website

Setting Up the Frosh IMs Application

Creating the Registration Form

Implementing the /register Route and Handling Form Submission

Part 5: Validation and Database Integration

Validating User Input on the Server Side

Dynamically Generating Form Options and Improving Error Messages

Storing Registration Data in a Database

Displaying Registration Data and Linking to the Registration List

Part 6: Advanced Features and Security

Implementing a Deregistration Feature

Security Considerations and the Importance of POST Requests

Adding an Image to the Error Page and Changing Radio Buttons to Checkboxes

Handling Multiple Sports and the MVC Paradigm

Part 7: State Management: Cookies and Sessions

Introduction to Cookies and Sessions

Implementing Login Functionality with Sessions

Exploring the Login Application Code

Implementing a Shopping Cart with Sessions

Exploring the Shopping Cart Application Code

Part 8: APIs and Conclusion

Creating an API with JSON and Conclusion

CS50 Fall 2025 - Lecture 9 - Flask (live, unedited)

CS50

Part 1: Introduction to Flask and Web Basics

18:07Introduction to Web Programming with Flask

Introduction to Web Programming with Flask

23:06Setting Up a Basic Flask Application

Setting Up a Basic Flask Application

29:55Returning HTML from a Flask Application

Returning HTML from a Flask Application

Part 2: Handling User Input and Templates

35:03Incorporating User Input via URL Parameters

Incorporating User Input via URL Parameters

40:33Handling Missing URL Parameters and Refining the Code

Handling Missing URL Parameters and Refining the Code

45:58Implementing a Web Form and Multiple Routes

Implementing a Web Form and Multiple Routes

53:32Eliminating Redundancy with Template Inheritance

Eliminating Redundancy with Template Inheritance

Part 3: Form Methods and Logic

1:01:10Transitioning from GET to POST for Form Submission

Transitioning from GET to POST for Form Submission

1:07:03Combining Routes and Adding Conditional Logic to Templates

Combining Routes and Adding Conditional Logic to Templates

Part 4: Case Study: The Frosh IMs Application

1:13:04Introduction to a Real-World Example: Frosh IMs Website

Introduction to a Real-World Example: Frosh IMs Website

1:26:50Setting Up the Frosh IMs Application

Setting Up the Frosh IMs Application

1:31:11Creating the Registration Form

Creating the Registration Form

1:37:05Implementing the /register Route and Handling Form Submission

Implementing the /register Route and Handling Form Submission

Part 5: Validation and Database Integration

1:41:11Validating User Input on the Server Side

Validating User Input on the Server Side

1:45:53Dynamically Generating Form Options and Improving Error Messages

Dynamically Generating Form Options and Improving Error Messages

1:52:26Storing Registration Data in a Database

Storing Registration Data in a Database

1:58:00Displaying Registration Data and Linking to the Registration List

Displaying Registration Data and Linking to the Registration List

Part 6: Advanced Features and Security

2:04:01Implementing a Deregistration Feature

Implementing a Deregistration Feature

2:11:14Security Considerations and the Importance of POST Requests

Security Considerations and the Importance of POST Requests

2:18:37Adding an Image to the Error Page and Changing Radio Buttons to Checkboxes

Adding an Image to the Error Page and Changing Radio Buttons to Checkboxes

2:23:35Handling Multiple Sports and the MVC Paradigm

Handling Multiple Sports and the MVC Paradigm

Part 7: State Management: Cookies and Sessions

2:27:27Introduction to Cookies and Sessions

Introduction to Cookies and Sessions

2:33:04Implementing Login Functionality with Sessions

Implementing Login Functionality with Sessions

2:38:11Exploring the Login Application Code

Exploring the Login Application Code

2:44:01Implementing a Shopping Cart with Sessions

Implementing a Shopping Cart with Sessions

2:47:19Exploring the Shopping Cart Application Code

Exploring the Shopping Cart Application Code

Part 8: APIs and Conclusion

2:55:04Creating an API with JSON and Conclusion

Creating an API with JSON and Conclusion