How to get textual output when using exceptions in Python

Question

Hi all, my basic requirement is that I want a code of Python code to be embedded in a C++ application of mine. I am aware of the library called Boost and I am actively using it.

But there is a problem - Lee us say Python throws an exception and I want to handle that by maybe printing an error message in my app or let's say I want to get a detailed output inclusive of all concepts like the line number where the error occurred and such. 

My question is this - How do I achieve this? 

try {
module=import("MyModule"); //this line will throw excetion if MyModule contains an error
} catch ( error_already_set const & ) {
// HERE IS THE ERROR

std::cout << "error!" << std::endl;
}

All help appreciated, thanks!

Answer 1

Hi, the answer is pretty simple. 

Without the boost, this is the error snippet - It would be tedious to post the entire code from the traceback to post.

Check out the code:

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 this following piece of the code snippet is when I use the Boost. Check it out: 

try{
//use 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 here
    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"));
... //Do cleanup here

Hope this helps!