This free service is maintained by one volunteer in my spare time, so I'm unable to respond to most bug reports and feature requests. Your issue is likely listed here. If you're sure it's not here, email me and use the "Generate permanent link" button to include a URL of your code.
- If you don't receive a reply from me, assume that your issue will NOT be addressed. Please do not email me multiple times.
- I can't provide technical support for users who want to install Python Tutor locally on their own computers or host it on their own servers.
- I can't provide support for Python Tutor code that's embedded within other people's websites, such as class websites or online tutorials. Contact the people in charge of those websites for help.
- I'm not accepting any code contributions or GitHub pull requests at this time. Feel free to fork the code and do whatever you like with it, as long as you abide by its software licenses.
Python Tutor is designed to imitate what an instructor in an introductory programming class (e.g., CS1, CS2) draws on the blackboard:
(source: UC Berkeley's CS61B course)
It is not meant to be a replacement for professional code debuggers or IDEs. This means that you should only use it to write small pieces of self-contained code that runs for not too many execution steps. After all, an instructor can't draw hundreds of lines of code, hundreds of data structures and pointers, or hundreds of execution steps on the blackboard! Also, code in introductory classes usually don't access external libraries.
Due to this ultra-focused design, the following features are not supported and will likely never be supported:
- Code that is too large in size. Shorten your code to what fits on a blackboard!
- Code that runs for too many steps (e.g., > 1,000 steps) or for a long time
- shorten your code to isolate exactly where you want to visualize and debug
- or set Python breakpoints using
#breakcomments
- Reading data from external files (you can use strings to emulate files: examples for Python3 and Python2)
- Interfacing with databases, network, filesystem, or other external resources
- Anything involving GUI programming or GUI/webpage components
- Multi-threaded, concurrent, or asynchronous code; Python Tutor is only for regular single-threaded execution
- Importing external libraries (try "Python 3.6 with Anaconda (experimental)" if you want to use more libraries)
- Editing multiple source code files (Python Tutor is not an IDE)
- User accounts, saving code as files in the cloud, or integrating with online services like GitHub
- Integrating with programming environments like Jupyter Notebooks, IDEs, or text editors
- for strings and numbers, you can't rely on the behaviors of
id()orismatching CPython on your computer; when teaching beginners, you shouldn't rely on these behaviors since they are implementation-specific optimizations. - some infinite loops: the server times out without showing partial results or good error messages
- to cut down execution times, set Python breakpoints using
#breakcomments
- to cut down execution times, set Python breakpoints using
- random number generators and user input (via input() or raw_input()) sometimes don't work well together
- raw_input/input might not work in iframe embeds
- doesn't show when function parameters get mutated (make a copy to a new local variable to visualize)
- function return values are not visualized (add a temporary variable to visualize)
- unions
- some complex typedefs
- taking text input from the user using scanf(), cin >>, etc.
- code with memory-related errors: it will fail-fast using Valgrind Memcheck
- C++ STL and string objects aren't visualized nicely (see GitHub issue)
- stack arrays without compile-time sizes
- read-only memory isn't visualized separately from the heap
- asynchronous and event-driven code
- including setTimeout, setInterval, etc.
- promises, async/await
- anything that operates on webpages, such as DOM manipulation, alert(), prompt(), confirm(), etc.
- this includes trying to import frontend libraries or frameworks (e.g., jQuery, React)
- Date() object
- some data structures like ArrayList aren't visualized properly (see GitHub issue)
- Python Tutor is meant for desktop/laptop computers, not for mobile devices. Some features such as live help mode simply don't work on mobile devices. The UI also looks cluttered and can be buggy on small screens.
- Stepping within a line of code to show how subexpressions get evaluated within that line; the best workaround is to split complex expressions into multiple lines and assign temporary variables on each line (example).
- Unicode doesn't well, especially for Ruby: #134, and Python 2: #77, #124, #194 (use ASCII characters when possible)
- Passing in command-line arguments via argv[] array (use hard-coded strings instead)
- If you're behind some kinds of firewalls or proxy servers, the visualizer or live chat may not work
- URL shortening (you should use your own third-party URL shortener service)
- https iframe embedding with non-Python languages (iframe embed should work for Python if you use
https://for URL) - Standalone application or offline mode (you can download the code and install it yourself but I don't have time to provide tech support for local installations)
Look through these issue trackers for more known bugs and unsupported features:
- https://github.com/pgbovine/OnlinePythonTutor/issues
- https://github.com/pgbovine/opt-cpp-backend/issues
I thought all objects in Python are (conceptually) on the heap; why does Python Tutor render primitive values (e.g., numbers, strings) inside of stack frames?
This was a design decision made to keep the display less cluttered; if we were truly faithful to Python's semantics, that would result in far too many arrows (pointers) being drawn. However, note that since primitives are immutable and thus behave identically regardless of aliasing, it doesn't matter whether they're rendered in the stack or heap.
Update on 2013-01-06: I've added a drop-down menu option with two choices: "inline primitives and nested objects" versus "render all objects on the heap". If you want to render all objects on the heap, select the latter option. To avoid too many arrows being drawn, also toggle the default "draw references using arrows" option to "use text labels for references". Here is a direct link to activate those two settings:
http://pythontutor.com/visualize.html#heapPrimitives=true&textReferences=true
Of course! Toggle options are customizable via the query string. Here are the default settings:
For example, if you want to default to C, visit: http://pythontutor.com/visualize.html#&py=c
Or Java: http://pythontutor.com/visualize.html#&py=java
Or if you want to render all objects on the heap and use text label references, visit: http://pythontutor.com/visualize.html#heapPrimitives=true&textReferences=true
Yes, only for Python, though. Change the embed URL from http:// to https:// and it should hopefully work.
First check out this wishlist doc, then feel free to email me if you have ideas that haven't yet been mentioned there.