Question

I’m implementing a Python extension module using Boost.Python. The module should define its own custom exception classes that inherit Exception. How do I do that?

Answer 1

The following function creates a new Python exception class and adds it to the current scope. If it is called in a module initialization function, then it is added to the module.

The first argument is the name of the new exception class. The second argument is the type object for the base class of the new exception class; it defaults to the type object for Exception. The return value is the type object for the new exception class.

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

    string scopeName = bp::extract<string>(bp::scope().attr("__name__"));
    string qualifiedName0 = scopeName + "." + name;
    char* qualifiedName1 = const_cast<char*>(qualifiedName0.c_str());

    PyObject* typeObj = PyErr_NewException(qualifiedName1, baseTypeObj, 0);
    if(!typeObj) bp::throw_error_already_set();
    bp::scope().attr(name) = bp::handle<>(bp::borrowed(typeObj));
    return typeObj;
}

Use the function as follows:

Call the function in the module initialization function and store the return value in a global variable:

PyObject* myExceptionTypeObj = 0;

BOOST_PYTHON_MODULE(MyModule)
{
    ...
    myExceptionTypeObj = createExceptionClass("MyException");
    ...
}

Raise exception of type MyModule.MyException:

PyErr_SetString(myExceptionTypeObj, "Oh my!")