Boost.Python create handle from type

Question

In some places in exposing C++ code to python, I need to use a PyObject*. If I have an instance of a boost::python::class_ object, I can invoke ptr() on it. But what if I just have the type?

Basically, given the type list boost::python::bases<A, B, C>, I want to convert this to a boost::python::tuple of instances that I can pass into something like PyErr_NewExceptionWithDoc(). Is this possible?

Answer 1

Given a C++ type T, one can create a boost::python::type_id object, and then query into the Boost.Python registry for registration information. If an entry is found in the registry, then one can use it to obtain a handle to the Python class created for type T:

/// @brief Get the class object for a wrapped type that has been exposed
///        through Boost.Python.
template <typename T>
boost::python::object get_instance_class()
{
  // Query into the registry for type T.
  namespace python = boost::python;
  python::type_info type = python::type_id<T>();
  const python::converter::registration* registration =
    python::converter::registry::query(type);

  // If the class is not registered, return None.
  if (!registration) return python::object();

  python::handle<PyTypeObject> handle(python::borrowed(
    registration->get_class_object()));
  return python::object(handle);
}

---

Here is a complete example demonstrating locating a Python class object in the Boost.Python registry:

#include <boost/python.hpp>
#include <iostream>

/// @brief Get the class object for a wrapped type that has been exposed
///        through Boost.Python.
template <typename T>
boost::python::object get_instance_class()
{
  // Query into the registry for type T.
  namespace python = boost::python;
  python::type_info type = python::type_id<T>();
  const python::converter::registration* registration =
    python::converter::registry::query(type);

  // If the class is not registered, return None.
  if (!registration) return python::object();

  python::handle<PyTypeObject> handle(python::borrowed(
    registration->get_class_object()));
  return python::object(handle);
}

struct spam {};

int main()
{
  Py_Initialize();

  namespace python = boost::python;
  try
  {
    // Create the __main__ module.
    python::object main_module = python::import("__main__");
    python::object main_namespace = main_module.attr("__dict__");

    // Create `Spam` class.
    // >>> class Spam: pass
    auto spam_class_object = python::class_<spam>("Spam", python::no_init);
    // >>> print Spam
    main_module.attr("__builtins__").attr("print")(get_instance_class<spam>());
    // >>> assert(spam is spam)
    assert(spam_class_object.ptr() == get_instance_class<spam>().ptr());
  }
  catch (python::error_already_set&)
  {
    PyErr_Print();
    return 1;
  }
}

Output:

<class 'Spam'>

For more type related functionality, such as accepting type objects, is, and issubclass, see this answer.