boost::python Export Custom Exception

Question

I am currently writing a C++ extension for Python using Boost.Python. A function in this extension may generate an exception containing information about the error (beyond

Answer 1

The solution is to create your exception class like any normal C++ class

class MyCPPException : public std::exception {...}

The trick is that all boost::python::class_ instances hold a reference to the object's type which is accessible through their ptr() function. You can get this as you register the class with boost::python like so:

class_ myCPPExceptionClass("MyCPPException"...);
PyObject *myCPPExceptionType=myCPPExceptionClass.ptr();
register_exception_translator(&translateFunc);

Finally, when you are translating the C++ exception to a Python exception, you do so as follows:

void translate(MyCPPException const &e)
{
    PyErr_SetObject(myCPPExceptionType, boost::python::object(e).ptr());
}

Here is a full working example:

#include 
#include 
#include 

class MyCPPException : public std::exception
{
private:
  std::string message;
  std::string extraData;
public:
  MyCPPException(std::string message, std::string extraData)
  {
    this->message = message;
    this->extraData = extraData;
  }
  const char *what() const throw()
  {
    return this->message.c_str();
  }
  ~MyCPPException() throw()
  {
  }
  std::string getMessage()
  {
    return this->message;
  }
  std::string getExtraData()
  {
    return this->extraData;
  }
};

void my_cpp_function(bool throwException)
{
  std::cout << "Called a C++ function." << std::endl;
  if (throwException)
    {
      throw MyCPPException("Throwing an exception as requested.",
               "This is the extra data.");
    }
}

PyObject *myCPPExceptionType = NULL;

void translateMyCPPException(MyCPPException const &e)
{
  assert(myCPPExceptionType != NULL);
  boost::python::object pythonExceptionInstance(e);
  PyErr_SetObject(myCPPExceptionType, pythonExceptionInstance.ptr());
}

BOOST_PYTHON_MODULE(my_cpp_extension)
{
  boost::python::class_
    myCPPExceptionClass("MyCPPException",
            boost::python::init());
  myCPPExceptionClass.add_property("message", &MyCPPException::getMessage)
    .add_property("extra_data", &MyCPPException::getExtraData);
  myCPPExceptionType = myCPPExceptionClass.ptr();
  boost::python::register_exception_translator
    (&translateMyCPPException);
  boost::python::def("my_cpp_function", &my_cpp_function);
}

Here is the Python code that calls the extension:

import my_cpp_extension
try:
    my_cpp_extension.my_cpp_function(False)
    print 'This line should be reached as no exception should be thrown.'
except my_cpp_extension.MyCPPException, e:
    print 'Message:', e.message
    print 'Extra data:',e.extra_data

try:
    my_cpp_extension.my_cpp_function(True)
    print ('This line should not be reached as an exception should have been' +
       'thrown by now.')
except my_cpp_extension.MyCPPException, e:
    print 'Message:', e.message
    print 'Extra data:',e.extra_data