CPython – print traceback

Question

How can I convert a traceback object to its string representation in CPython?
I tried this but the output is not very useful.

    PyObject *type, *value, *traceback;
    PyErr_Fetch(&type, &value, &traceback);

    PyObject* strTrace = PyObject_Repr(traceback);
    pyStrTrace = PyUnicode_AsEncodedString(strTrace, "utf-8", "strict");
    Py_XDECREF(strTrace);
    const char* sTrace = PyBytes_AS_STRING(pyStrTrace);

Content of sTrace:

 <traceback object at 0x000002594A5CC598>

Asked By: JeFf

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Source

Answer 1

One comment suggested to import traceback python module which would most certainly work well but I prefer a C++ only solution so I came up with this snippet. Hopefully, someone finds it useful.

First include Python’s <frameobject.h>

if (PyTraceBack_Check(traceback))
{
    PyTracebackObject* traceRoot = (PyTracebackObject*)traceback;
    PyTracebackObject* pTrace = traceRoot;
    
    while (pTrace != NULL)
    {
        PyFrameObject* frame = pTrace->tb_frame;
        PyCodeObject* code = frame->f_code;

        int lineNr = PyFrame_GetLineNumber(frame);
        const char *sCodeName = PyUnicode_AsUTF8(code->co_name);
        const char *sFileName = PyUnicode_AsUTF8(code->co_filename);

        printf("at %s (%s:%d); ", sCodeName, sFileName, lineNr);
        pTrace = pTrace->tb_next;
    }        
}

As of Python 3.11 frame->f_code is nolonger possible (PyFrameObject was moved private). It’s suggested in the changelog that PyFrame_GetCode(frame) is used instead.

Likewise, this removes the need to include <frameobject.h>

For backwards compatibility you can implement this function yourself:

#if PY_VERSION_HEX < 0x030900B1
#include <frameobject.h>
static inline PyCodeObject* PyFrame_GetCode(PyFrameObject *frame)
{
    Py_INCREF(frame->f_code);
    return frame->f_code;
}
#endif

Answered By: JeFf

CPython – print traceback

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