Exception Translation from C++ to Python with Boost.Python

Scenario

Say we have written a Python binding with Boost.Python. Something like following code.

// some.cpp
#include <stdexcept>
#include <boost/python.hpp>

struct my_exception : std::exception
{
    my_exception(int extra_info): extra_info(extra_info) {}
    char const *what() noexcept(true) { return "My exception"; }
    int const extra_info;
};

void do_throw()
{
   throw my_exception();
}

// Something should be done here ...

BOOST_PYTHON_MODULE(MyLib)
{
   using namespace boost::python;
   def("do_throw", &do_throw);
}

And we wish to catch a Python exception object in Python code, e.g.

import MyLib

try:
    MyLib.do_throw()
except MyLib.MyException as e:
    print e, e.extra_info
    pass

The situation is, we can’t directly pass the exception object, my_exception, to Python. Now the rescue is:

1. Define a Python MyException class
2. Create an unique instance of above class in your library
3. Translate my_exception to MyException

Define MyException Class

PyObject *createExceptionClass(std::string const &module, 
                               std::string const &exception_name, 
                               PyObject *base)
{
    namespace bp = boost::python
    std::string fullname = module + "." + exception_name;
    PyObject* typeObj = PyErr_NewException((char*)fullname.c_str(),
                                            base, 0);
    if (!typeObj) boost::python::throw_error_already_set();

    bp::scope().attr(exception_name.c_str()) =
            // semantically equal to make_shared(typeObj)
            bp::handle<>(bp::borrowed(typeObj));
    return typeObj;
}

Cited from StackOverflow

Create an Exception Instance

// static instance
PyObject *myexception_instanct_ = 0;

BOOST_PYTHON_MODULE(pyvddk)
{
    myexcdption_instance_ = 
        createExceptionClass("pyvddk", "MyException",
                             PyExc_RuntimeError);
}

Translate Exception

// static instance
PyObject *myexception_instanct_ = 0;

void translate(my_excpetion const &e)
{
    namespace bp = boost::python;
    bp::object exc(bp::handle<>(bp::borrowed(myexception_instance_)));
    // copy data from my_exception to MyException
    exc.attr("extra_info") = e.extra_info;
    PyErr_SetString(myexception_instance, e.what());
}

BOOST_PYTHON_MODULE(pyvddk)
{
    using namespace boost:python;
    
    myexcdption_instance_ = 
        createExceptionClass("pyvddk", "MyException",
                             PyExc_RuntimeError);

    register_exception_translator<my_exception>(&translate)
}

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Full Code Listing

PyObject *createExceptionClass(std::string const &module, 
                               std::string const &exception_name, 
                               PyObject *base)
{
    namespace bp = boost::python
    std::string fullname = module + "." + exception_name;
    PyObject* typeObj = PyErr_NewException((char*)fullname.c_str(),
                                            base, 0);
    if (!typeObj) boost::python::throw_error_already_set();

    bp::scope().attr(exception_name.c_str()) =
            // semantically equal to make_shared(typeObj)
            bp::handle<>(bp::borrowed(typeObj));
    return typeObj;
}

// static instance
PyObject *myexception_instanct_ = 0;

void translate(my_excpetion const &e)
{
    namespace bp = boost::python;
    bp::object exc(bp::handle<>(bp::borrowed(myexception_instance_)));
    // copy data from my_exception to MyException
    exc.attr("extra_info") = e.extra_info;
    PyErr_SetString(myexception_instance, e.what());
}

BOOST_PYTHON_MODULE(pyvddk)
{
    using namespace boost:python;
    
    myexcdption_instance_ = 
        createExceptionClass("pyvddk", "MyException",
                             PyExc_RuntimeError);

    register_exception_translator<my_exception>(&translate)
}

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