Tracing Python expression evaluation step by step

Question

I\'m trying to write Python expression evaluation visualizer, that will show how Python expressions are evaluated step by step (for education purposes). Philip Guo\'s Python

Answer 1

Expression stepping is implemented in Thonny IDE.

It uses AST instrumentation, where each (sub)expression e is transformed into after(before(), e). Functions before and after are dummy functions for causing extra call-events in Python's tracing system. These extra calls notify when (sub)expression evaluation is about to start or has just ended. (Similar dummy functions are added to detect start and end of each statement.)

AST instrumentation and interpretation of these new events is done in thonny.backend.FancyTracer.

Python's AST nodes contain start position of corresponding text ranges, but they are sometimes incorrect. End positions are completely missing. thonny.ast_utils.mark_text_ranges tries to take care of this (but the solution is incomplete at the moment).

It would be nice if somebody extracted relevant functionality from Thonny to a more general package. Maybe even two packages -- one for computing location info for Python AST and other for detailed tracing of Python code. I'd be willing to help with this, if someone took the lead.