[C++-sig] Re: Hnadling Python exceptions in C++

[Berthold Höllmann]

"David Abrahams" <david.abrahams at rcn.com> writes:

> In Boost.Python v1, there's no special facility. You can use the
regular
> Python 'C' API, of course. We are planning some support in
Boost.Python v2.
> I'd like to try to serve your needs; what kind of facilities did you
have
> in mind?
> 
> Thanks,
> Dave
> 
> > ----- Original Message -----
> > From: "Berthold Hllmann" <hoel at germanlloyd.org>
> > Hello,
> > 
> > Is there any code for handling Python exceptions in C++? I only
found
> > report_ignored_exception in classes.cpp, but it's in anonymous
> > namespace and though inaccessable in my code.
> > 
> > Greetings
> > 
> > Berthold

OK, after thinking a bit about such a facility here is my idea. It
would be nice to have a C++ exception class that reads out PyErr_Fetch
when instantiated and can be converted back into a Python exception if
needed. I tried to start an implementation. The attached code is
untested, it is just posted to share the idea.

except.hpp:
--8<------------------------schnipp------------------------->8---
#ifndef EXCEPT_BH20020719_H_
#define EXCEPT_BH20020719_H_

#include <Python.h>
#include <string>

namespace boost {
  namespace python {
    namespace Error {
      class PyErrHandler {
      public:
	PyErrHandler();
	virtual void printOn(std::ostream&) const;
      private:
	PyObject *stype, *svalue, *straceback;
      };
      std::ostream& operator<<(std::ostream& os,
			       const PyErrHandler& el);
    }
  }
}

#endif // EXCEPT_BH20020719_H_
--8<------------------------schnapp------------------------->8---

and except.cpp
--8<------------------------schnipp------------------------->8---
#include <iostream>
#include "except.hpp"

boost::python::Error::PyErrHandler::PyErrHandler()
{
  PyObject *t, *v, *tb;
  PyErr_Fetch(&t, &v, &tb);
  if (t != NULL)
    stype = PyObject_Str(t);
  else
    stype = PyString_FromString("");

  if (v != NULL)
    svalue = PyObject_Str(v);
  else
    svalue = PyString_FromString("");

  if (tb != NULL)
    straceback = PyObject_Repr(tb);
  else
    straceback = PyString_FromString("");

  Py_XDECREF(t);
  Py_XDECREF(v);
  Py_XDECREF(tb);
}

void boost::python::Error::PyErrHandler::printOn(std::ostream& strm)
const
{
  strm << std::string(PyString_AsString(stype)) << "\n"
       << std::string(PyString_AsString(svalue)) << "\n"
       << std::string(PyString_AsString(straceback));
}

PyObject *boost::python::Error::PyErrHandler::operator() ()
{
  PyErr_Restore(t, v, tb)
  return NULL;
}

std::ostream&
boost::python::Error::operator<<(std::ostream& os,
	   const boost::python::Error::PyErrHandler& el)
{
  el.printOn(os);
  return os;
}
--8<------------------------schnapp------------------------->8---

Greetings

Berthold
-- 
Dipl.-Ing. Berthold H llmann   __   Address:
hoel at germanlloyd.org        G /  \ L Germanischer Lloyd
phone: +49-40-36149-7374    -+----+- Vorsetzen 32/35    P.O.Box 111606
fax  : +49-40-36149-7320      \__/   D-20459 Hamburg    D-20416 Hamburg



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