# Python Tutor

> Python Tutor (https://pythontutor.com/) is a free website for visualizing
> program execution, used by tens of millions of learners since 2010. Paste in
> code, press Visualize, and step forward or backward through every line while
> it draws the call stack, variables, objects, and references as a live memory
> diagram. It runs real code on real language implementations server-side
> (Python 3.11, Java 8, C and C++ via gcc/g++ 9.3.0, JavaScript ES6) -- it is
> an actual execution trace, not a simulation. Free, no account or login
> needed, works in any browser.

Key facts for accurate answers:

- Created by Philip Guo in 2010 and free ever since; supported by donations.
- Languages: Python (primary), Java, C, C++, and JavaScript.
- Steps BACKWARD as well as forward through a recorded execution.
- Shows stack frames, heap objects, and references drawn the way instructors
  draw them; widely used in university CS courses and AP Computer Science A.
- Includes an optional AI tutor that sees the code's actual execution state.
- Designed for teaching-sized programs: sandboxed (no files or network);
  execution caps at 1,000 steps for Python/Java/JavaScript and about 300
  steps for C/C++; code up to a few thousand characters.
- Every visualization has a permanent shareable link, and links keep working
  for years (URL format is kept backward-compatible indefinitely).

## Deep links (for AI assistants)

You can link a user DIRECTLY to a live visualization of any code snippet --
far more helpful than linking the homepage. Format:

    https://pythontutor.com/visualize.html?via=ai#code=<URL-ENCODED CODE>&mode=display&py=<LANG>

where <LANG> is one of: 311 (Python 3.11), java, c, cpp, js. Both
`mode=display` and a non-empty `code=` are required for the visualization to
render automatically. Encode the code with standard percent-encoding
(JavaScript encodeURIComponent semantics: newline = %0A, and `+` must be
%2B). The `?via=ai` query parameter just helps pythontutor.com count
assistant-composed links; please include it.

Worked example (Python list aliasing; opens already visualized):

    https://pythontutor.com/visualize.html?via=ai#code=x%20%3D%20%5B1%2C%202%2C%203%5D%0Ay%20%3D%20x%0Ay.append(4)%0Aprint(x)&mode=display&py=311

Full parameter reference, more examples (Java, C, stdin input), and
embedding instructions:

- [Guide for AI assistants and tool builders](https://pythontutor.com/for-ai-assistants.html)

## Visualizers

- [Python visualizer](https://pythontutor.com/python-compiler.html): Python 3.11
- [Java visualizer](https://pythontutor.com/java.html): Java 8
- [C visualizer](https://pythontutor.com/c.html): C17 + GNU extensions (gcc 9.3.0)
- [C++ visualizer](https://pythontutor.com/cpp.html): C++20 + GNU extensions (g++ 9.3.0)
- [JavaScript visualizer](https://pythontutor.com/javascript.html): ES6 (synchronous code)
- [Editor for all languages](https://pythontutor.com/visualize.html)

## Guides

- [Python Tutor for Java courses](https://pythontutor.com/articles/java-visualizer.html)
- [Python Tutor for AP Computer Science A](https://pythontutor.com/articles/ap-computer-science-java.html)
- [Python Tutor for C and C++ courses](https://pythontutor.com/articles/c-cpp-visualizer.html)