Philip Guo: One Developer, Two Decades, and Thirty Million Users Later: How Python Tutor Became a Long-Lasting and Widely-Used Academic Software Project

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CS547 Human-Computer Interaction Seminar (Seminar on People, Computers, and Design)

Fridays 11:30am-12:30pm PT · Gates B3 · Open to the public

Philip Guo

UC San Diego

One Developer, Two Decades, and Thirty Million Users Later: How Python Tutor Became a Long-Lasting and Widely-Used Academic Software Project

October 16, 2026

Since 2010 I have been developing Python Tutor (https://pythontutor.com/), a tool that lets users write code in a web browser and see what happens step-by-step as the computer runs it. The tool visualizes variables, objects, pointers, data structures, and stack frames. These visuals help students to debug their homework, teachers to explain concepts, and programmers to prepare for job interviews. Despite its name, this tool visualizes code written in Python, Java, C, C++, and JavaScript, which are five of the most popular languages used in both teaching and industry.

Throughout the past two decades, nearly 30 million students and teachers from over 180 countries have used Python Tutor to visualize nearly a billion pieces of code. In this talk I will reflect upon the design decisions (and non-decisions!) I made along the way to turn a one-off prototype into one of the most widely-used pieces of academic software today. I will cover three eras of Python Tutor's development: Act I - Inception, the unique conditions that birthed this idea as a grad student side project. Act II - Acceleration, sustaining a decade's worth of user growth as I began my career as a professor. Act III - Disruption, when Generative AI both threatens the health of this project and may also be what revitalizes it.

Philip Guo (https://pg.ucsd.edu/) started his Ph.D. in Computer Science at Stanford exactly twenty years ago in Fall 2006. He is now a Professor of Cognitive Science at UC San Diego. His research spans human-computer interaction, computing education, and human-centered AI for education. He studies how people learn computer programming and data science, and he builds tools to help people better understand code and data. For instance, his Python Tutor educational tool (https://pythontutor.com/) has been used by nearly 30 million students and teachers from over 180 countries throughout the past two decades. Python Tutor received a UIST Best Paper award and the ACM SIGCSE Test of Time award for long-term impact. His other lines of research have received 10-year long-term impact awards from VL/HCC and ISSTA, along with Best Paper awards at CHI, UIST, SIGCSE, VL/HCC, ICSE, and ISSTA.