Aivar Aivar - 8 days ago 5
Python Question

How to get calling expression when tracing a Python function?

When inside tracing function, debugging a function call, is it possible to somehow retrieve the calling expression?

I can get calling line number from traceback object but if there are several function calls (possibly to the same function) on that line (eg. as subexpression in a bigger expression) then how could I learn where this call came from? I would be happy even with the offset from start of the source line.

traceback.tb_lasti
seems to give more granual context (index of last bytecode tried) -- is it somehow possible to connect a bytecode to its exact source range?

EDIT: Just to clarify -- I need to extract specific (sub)expression (the callsite) from the calling source line.

Answer

That's how I finally solved the problem: I instrumented each function call in the original program by wrapping it in a call to a helper function together with information about the source location of the original call. Actually I was interested in controlling the evaluation of each subexpression in the program, so I wrapped each subexpression.

More precisely: when I had an expression e in the original program, it became

_after(_before(location_info), e)

in the instrumented program. The helpers were defined like this:

def _before(location_info):
    return location_info

def _after(location_info, value):
    return value

When tracer reported the call to _before, I knew that it's about to evaluate the expression at location represented by location_info (tracing system gives me access to local variables/parameters, that's how I got to know the value of location_info). When tracer reported the call to _after, I knew that the expession indicated by location_info was just evaluated and the value is in value.

I could have written the execution "event handling" right into those helper functions and bypass the tracing system altogether, but I needed it for other reasons as well, so I used those helpers only for triggering a "call" event in tracing system.

The result can be seen here: http://thonny.org