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

Here's the solution from Jack Edmonds, ported to Python 3, using advice from here which itself uses code from here. Assembling it all together (and modernizing the C++ code a little bit) gives:

#include 
#include 
#include 

class MyCPPException : public std::exception
{
public:
    MyCPPException(const std::string &message, const std::string &extraData)
        : message(message), extraData(extraData)
    {
    }
    const char *what() const noexcept override
    {
        return message.c_str();
    }
    std::string getMessage() const
    {
        return message;
    }
    std::string getExtraData() const
    {
        return extraData;
    }
private:
    std::string message;
    std::string 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.");
    }
}

static PyObject* createExceptionClass(const char* name, PyObject* baseTypeObj = PyExc_Exception)
{
    using std::string;
    namespace bp = boost::python;

    const string scopeName = bp::extract(bp::scope().attr("__name__"));
    const string qualifiedName0 = scopeName + "." + name;
    PyObject* typeObj = PyErr_NewException(qualifiedName0.c_str(), baseTypeObj, 0);
    if (!typeObj) bp::throw_error_already_set();
    bp::scope().attr(name) = bp::handle<>(bp::borrowed(typeObj));
    return typeObj;
}

static PyObject *pythonExceptionType = NULL;

static void translateMyCPPException(MyCPPException const &e)
{
    using namespace boost;
    python::object exc_t(python::handle<>(python::borrowed(pythonExceptionType)));
    exc_t.attr("cause") = python::object(e); // add the wrapped exception to the Python exception
    exc_t.attr("what") = python::object(e.what()); // for convenience
    PyErr_SetString(pythonExceptionType, e.what()); // the string is used by print(exception) in python
}

BOOST_PYTHON_MODULE(my_cpp_extension)
{
    using namespace boost;
    python::class_
            myCPPExceptionClass("MyCPPException",
                                python::init());
    myCPPExceptionClass.add_property("message", &MyCPPException::getMessage)
            .add_property("extra_data", &MyCPPException::getExtraData);

    pythonExceptionType = createExceptionClass("MyPythonException");
    python::register_exception_translator(&translateMyCPPException);
    python::def("my_cpp_function", &my_cpp_function);
}

and the python file to test it:

#!/usr/bin/env python3

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.MyPythonException as e:
    print('Message:', e.what)
    print('Extra data:',e.cause.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.MyPythonException as e:
    print('Message:', e.what)
    print('Extra data:',e.cause.extra_data)

And catching it as a standard python Exception works too:

except Exception as e:
    print('Exception: ',e)