Online Python Tutor is a free educational tool created by Philip Guo that helps people overcome a fundamental barrier to learning programming: understanding what happens as the computer executes each line of a program's source code.
So far, over 1.2 million people in over 165 countries have used Online Python Tutor to visualize over 11 million pieces of code, 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.