How to get Python exception text

This thread has been very useful for me, but I had problems with the Python C API when I tried to extract the error message itself with no traceback. I found plenty of ways to do that in Python, but I couldn't find any way to do this in C++. I finally came up with the following version, which uses the C API as little as possible and instead relies much more on boost python.

PyErr_Print();

using namespace boost::python;

exec("import traceback, sys", mainNamespace_);
auto pyErr = eval("str(sys.last_value)", mainNamespace_);
auto pyStackTrace = eval("'\\n'.join(traceback.format_exception(sys.last_type, sys.last_value, sys.last_traceback))", mainNamespace_);

stackTraceString_ = extract<std::string>(pyStackTrace);
errorSummary_ = extract<std::string>(pyErr);

The reason this works is because PyErr_Print() also sets the value for sys.last_value, sys.last_type, and sys.last_traceback. Those are set to the same values as sys.exc_info would give, so this is functionally similar to the following python code:

import traceback
import sys

try:
    raise RuntimeError("This is a test")
except:
    err_type = sys.exc_info()[0]
    value = sys.exc_info()[1]
    tb = sys.exc_info()[2]

    stack_trace = "\n".join(traceback.format_exception(err_type, value, tb))
    error_summary = str(value)


print(stack_trace)
print(error_summary)

I hope someone finds this useful!

Well, I found out how to do it.

Without boost (only error message, because code to extract info from traceback is too heavy to post it here):

PyObject *ptype, *pvalue, *ptraceback;
PyErr_Fetch(&ptype, &pvalue, &ptraceback);
//pvalue contains error message
//ptraceback contains stack snapshot and many other information
//(see python traceback structure)

//Get error message
char *pStrErrorMessage = PyString_AsString(pvalue);

And BOOST version

try{
//some code that throws an error
}catch(error_already_set &){

    PyObject *ptype, *pvalue, *ptraceback;
    PyErr_Fetch(&ptype, &pvalue, &ptraceback);

    handle<> hType(ptype);
    object extype(hType);
    handle<> hTraceback(ptraceback);
    object traceback(hTraceback);

    //Extract error message
    string strErrorMessage = extract<string>(pvalue);

    //Extract line number (top entry of call stack)
    // if you want to extract another levels of call stack
    // also process traceback.attr("tb_next") recurently
    long lineno = extract<long> (traceback.attr("tb_lineno"));
    string filename = extract<string>(traceback.attr("tb_frame").attr("f_code").attr("co_filename"));
    string funcname = extract<string>(traceback.attr("tb_frame").attr("f_code").attr("co_name"));
... //cleanup here

In the Python C API, PyObject_Str returns a new reference to a Python string object with the string form of the Python object you're passing as the argument -- just like str(o) in Python code. Note that the exception object does not have "information like line number" -- that's in the traceback object (you can use PyErr_Fetch to get both the exception object and the traceback object). Don't know what (if anything) Boost provides to make these specific C API functions easier to use, but, worst case, you could always resort to these functions as they are offered in the C API itself.

This is the most robust method I've been able to come up so far:

    try {
        ...
    }
    catch (bp::error_already_set) {
        if (PyErr_Occurred()) {
            msg = handle_pyerror(); 
        }
        py_exception = true;
        bp::handle_exception();
        PyErr_Clear();
    }
    if (py_exception) 
    ....


// decode a Python exception into a string
std::string handle_pyerror()
{
    using namespace boost::python;
    using namespace boost;

    PyObject *exc,*val,*tb;
    object formatted_list, formatted;
    PyErr_Fetch(&exc,&val,&tb);
    handle<> hexc(exc),hval(allow_null(val)),htb(allow_null(tb)); 
    object traceback(import("traceback"));
    if (!tb) {
        object format_exception_only(traceback.attr("format_exception_only"));
        formatted_list = format_exception_only(hexc,hval);
    } else {
        object format_exception(traceback.attr("format_exception"));
        formatted_list = format_exception(hexc,hval,htb);
    }
    formatted = str("\n").join(formatted_list);
    return extract<std::string>(formatted);
}