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 a Python program in the Web browser and visualize what the computer is doing step-by-step as it executes the program.
Over 500,000 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. (http://pythontutor.com/)
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The vizualizations can also be shared with other people in live sessions, and they can also be embedded into any web page via a javascript snippet.