cwick cwick - 1 month ago 6
Python Question

Accessing a Python traceback from the C API

I'm having some trouble figuring out the proper way to walk a Python traceback using the C API. I'm writing an application that embeds the Python interpreter. I want to be able to execute arbitrary Python code, and if it raises an exception, to translate it to my own application-specific C++ exception. For now, it is sufficient to extract just the file name and line number where the Python exception was raised. This is what I have so far:

PyObject* pyresult = PyObject_CallObject(someCallablePythonObject, someArgs);
if (!pyresult)
{
PyObject* excType, *excValue, *excTraceback;
PyErr_Fetch(&excType, &excValue, &excTraceback);
PyErr_NormalizeException(&excType, &excValue, &excTraceback);

PyTracebackObject* traceback = (PyTracebackObject*)traceback;
// Advance to the last frame (python puts the most-recent call at the end)
while (traceback->tb_next != NULL)
traceback = traceback->tb_next;

// At this point I have access to the line number via traceback->tb_lineno,
// but where do I get the file name from?

// ...
}


Digging around in the Python source code, I see they access both the filename and module name of the current frame via the
_frame
structure, which looks like it is a privately-defined struct. My next idea was to programmatically load the Python 'traceback' module and call its functions with the C API. Is this sane? Is there a better way to access a Python traceback from C?

Answer

I've discovered that _frame is actually defined in the frameobject.h header included with Python. Armed with this plus looking at traceback.c in the Python C implementation, we have:

#include <Python.h>
#include <frameobject.h>

PyTracebackObject* traceback = get_the_traceback();

int line = traceback->tb_lineno;
const char* filename = PyString_AsString(traceback->tb_frame->f_code->co_filename);

But this still seems really dirty to me.

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