More information from SWIG::DirectorMethodException

[neworderofjamie]

These get thrown (and since #433 caught!) when errors happen in Python callbacks called from C++ using SWIG directors e.g. derived parameter calculation code. However, the code generated by SWIG to throw the exceptions looks like:

PyObject *error = PyErr_Occurred();
if (error) {
  Swig::DirectorMethodException::raise("Error detected when calling 'DerivedParamFunc.__call__'");
}

So the contents of the error i.e. the message is totally ignored (meaning that, until I dug into this I was worried something was really wrong with this code but infact I had just forgotten to import numpy...). Extra code can be monkey-patched into the exception throwing code with (from google):

%feature("director:except") {
    if( $error != NULL ) {
      PyObject *ptype, *pvalue, *ptraceback;
      PyErr_Fetch( &ptype, &pvalue, &ptraceback );
      PyErr_Restore( ptype, pvalue, ptraceback );
      PyErr_Print();
      Py_Exit(1);
    }
  }

which prints out a standard Python error message. However I'm not really sure what all the PyErr_Fetch and PyErr_Restore business does and I'd rather pipe the errors back into an exception message or something but, that is definitely beyond my Python C API understanding (@alexdewar any thoughts as a more experienced Python C API wrangler?)

[alexdewar]

I think the way to do it would be to convert pvalue to a Python string (which should include the type name, message and stack trace) and then C++-ify that:
https://stackoverflow.com/questions/37684153/convert-exception-error-to-string
https://stackoverflow.com/questions/5356773/python-get-string-representation-of-pyobject

If it turns out to be a faff I guess you could always dig into pybind11 and see what magic they're using.