VISUALIZE the execution of Python, Java, JavaScript, and TypeScript code

Online Python Tutor is a free educational tool created by Philip Guo that helps students overcome a fundamental barrier to learning programming: understanding what happens as the computer executes each line of a program's source code.

Using this tool, a teacher or student can write Python, Java, JavaScript, and TypeScript programs in the Web browser and visualize what the computer is doing step-by-step as it executes those programs.

Over one million people in over 165 countries have used Online Python Tutor to understand and debug their programs, often as a supplement to textbooks, lecture notes, and online programming tutorials.

For example, here is a visualization showing a Python program that recursively finds the sum of a linked list. Click the “Forward” button to see what happens as the computer executes each line of code.

Read the research paper – Philip J. Guo. Online Python Tutor: Embeddable Web-Based Program Visualization for CS Education. In Proceedings of the ACM Technical Symposium on Computer Science Education (SIGCSE), March 2013. [BibTeX]

SHARE live visualization sessions

Click the “Start a shared session” button to allow an instructor or friend to join your session. You can chat about your code and navigate the visualization together to get live, real-time tutoring. Watch this one-minute video demo:

Also, you can click the “Generate permanent link” button (at the bottom of this page) and paste that link in an email, social networking post, or forum question. When recipients click on your link, they will see the exact visualization you've created.

For example, clicking this link brings you directly to step 44 of 57 in a program that finds prime numbers using the Python for-else construct.

EMBED in any Web page

Using a single line of JavaScript code, you can embed an Online Python Tutor visualization within any Web page (as shown in the “Learn” box above). The screenshot below shows a visualization embedded within the online textbook for the introductory CS course at UC Berkeley (CS61A):

Details:

  • Online Python Tutor runs Python 2.7 and 3.3, hosted on any CGI-capable Web server.
  • Main technologies: Python with bdb for the Python backend, Node.js and Docker for the JavaScript and TypeScript backends; HTML/CSS/JavaScript with jQuery, D3.js, jsPlumb, TogetherJS, and Ace for the frontend
  • Free, open-source BSD-licensed code on GitHub
  • View the project documentation online.