Python Code Stepper / Debugger / Tutor for Jupyter Notebooks – nbtutor

Whilst reviewing / scoping* possible programming editor environments for the new level 1 courses, one of the things I was encouraged to look at was Philip Guo’s interactive Python Tutor.

According the the original writeup (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), the application has an HTML front end that calls on on a backend debugger: “the Online Python Tutor backend takes the source code of a Python program as input and produces an execution trace as output. The backend executes the input program under supervision of the standard Python debugger module (bdb), which stops execution after every executed line and records the program’s run-time state.”

The current version of the online tutor supports a wider range of languages – Python, Java, JavaScript, TypeScript, Ruby, C, and C++ – which presumably have their own backend interpreter and use a common trace response format?

The tutor itself allows you to step through code snippets a line at a time, displaying a trace of the current variable values.

Another nice feature of the Online Python Tutor, though it was a bit ropey when I first tried it out a few months ago, was the shared session support, that a learner and a tutor see, via a shared link, the same session, with an additional chat box allowing them to chat over the shared experience in realtime.

Whilst the Online Python Tutor allows URLs to saved programs (“tutorials”) to be generated and shared: link to the demo shown in the movie above. The code is actually passed via the URL.

One of the problems with the Online Python Tutor is that requires a network connection so that the code can be passed to the interpreter back end, executed to generate the code trace, and then passed back to the browser. It didn’t take long for folk to start embedding the tutor in an iframe to give a pseudo-traceability experience in the notebook context, but now the Online Python Tutor inspired nbtutor extension makes cell based tracing against the local python kernel possible**.

The nbtutor extension provides cell by cell tracing (when running a cell, all the code in the cell is executed, the trace returned, and then available for visualising. Note that all variables in scope are displayed in the trace, even if they have been set in other cells outside of the nbtutor magic. (I’m not sure if there’s a setting that allows you just to display the variables that are referenced within the cell?)  It is also possible to clear all variables in the global scope via a magic parameter, with a prompt to confirm that you really do want to clear out all those variable values.

I’m not sure that the best way would be to go about framing nbtutor exercises in a Jupyter notebook context, but I note that the notebooks used to support the MPR213 (Programming and Information Technology) course from the Department of Mechanical and Aeronautical Engineering in the Faculty of Engineering, Built Environment and Information Technology at the University of Pretoria now include nbtutor examples.

Footnotes:

* A cynic might say scoping in the sense not seriously considering anything other than the environments that had already been decided on before the course production process had really started… ;-) I also preferred BlockPy over Scratch, for example. My feeling was that if the OU was going to put developer effort in (the original claim was we wouldn’t have to put effort into Scratch, though of course we are because Scratch wasn’t quite right…) we could add more value to the OU and the community by getting involved with BlockPy, rather than a programming environment developed for primary school kids. Looking again at the “friendly” error messages that the BlockPy environment offers, I’m starting to wondering if elements of that could be reused for some IPython notebook magic…

** Again, I’m of the mind that were it 20 years ago, porting the Online Python Tutor to the Jupyter notebook context might have been something we’d have considered doing in the OU…

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