Wrapping Enums using Boost-Python

Question

I have problem wrapping an Enum for Python using Boost-Python.

Initially I intended to do something like the following in the try-catch (I've inserted my whole code below) statement:

main_namespace["Motion"] = enum_<TestClass::Motion>("Motion")
    .value("walk", TestClass::walk)
    .value("bike", TestClass::bike)
;

Everything was fine and compilation was done. At run time I got this error (which makes no sense to me):

AttributeError: 'NoneType' object has no attribute 'Motion'

Afterwards I decided to write a Python module using BOOST_PYTHON_MODULE in my code. After initializing Python interpreter I wanted to use this module right away but didn't know how(?). The following is my whole code:

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

using namespace std;
using namespace boost::python;

BOOST_PYTHON_MODULE(test)
{
    enum_<TestClass::Motion>("Motion")
        .value("walk", TestClass::walk)
        .value("bike", TestClass::bike)
    ;
}

int main()
{
    Py_Initialize();

    try
    {    
        object pyMainModule = import("__main__");
        object main_namespace = pyMainModule.attr("__dict__");

        //What previously I intended to do
        //main_namespace["Motion"] = enum_<TestClass::Motion>("Motion")
        //  .value("walk", TestClass::walk)
        //  .value("bike", TestClass::bike)
        //;

        //I want to use my enum here
        //I need something like line below which makes me able to use the enum!

        exec("print 'hello world'", main_namespace, main_namespace);
    }
    catch(error_already_set const&)
    {
        PyErr_Print();
    }

    Py_Finalize();
    return 0;
}

Anything useful to know about wrapping and using Enums in Python will be appreciated! Thanks in advance

Answer 1

The AttributeError is the result of trying to create a Python extension type without first setting scope. The boost::python::enum_ constructor states:

Constructs an enum_ object holding a Python extension type derived from int which is named name. The named attribute of the current scope is bound to the new extension type.

When embedding Python, to use a custom Python module, it is often easiest to use PyImport_AppendInittab, then import the module by name.

PyImport_AppendInittab("example", &initexample);
...
boost::python::object example = boost::python::import("example");

---

Here is a complete example showing two enumerations being exposed through Boost.Python. One is included in a separate module (example) that is imported by main, and the other is exposed directly in main.

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

/// @brief Mockup class with a nested enum.
struct TestClass
{
  /// @brief Mocked enum.
  enum Motion
  {
    walk,
    bike 
  };

  // @brief Mocked enum.
  enum Color
  {
    red,
    blue
  };
};

/// @brief Python example module.
BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  python::enum_<TestClass::Motion>("Motion")
    .value("walk", TestClass::walk)
    .value("bike", TestClass::bike)
    ;
}   

int main()
{
  PyImport_AppendInittab("example", &initexample); // Add example to built-in.
  Py_Initialize(); // Start interpreter.

  // Create the __main__ module.
  namespace python = boost::python;

  try
  {
    python::object main = python::import("__main__");
    python::object main_namespace = main.attr("__dict__");
    python::scope scope(main); // Force main scope

    // Expose TestClass::Color as Color
    python::enum_<TestClass::Color>("Color")
      .value("red", TestClass::red)
      .value("blue", TestClass::blue)
      ;

    // Print values of Color enumeration.
    python::exec(
      "print Color.values",
      main_namespace, main_namespace);

    // Get a handle to the Color enumeration.
    python::object color = main_namespace["Color"];
    python::object blue  = color.attr("blue");

    if (TestClass::blue == python::extract<TestClass::Color>(blue))
      std::cout << "blue enum values matched." << std::endl;

    // Import example module into main namespace.
    main_namespace["example"] = python::import("example");

    // Print the values of the Motion enumeration.
    python::exec(
      "print example.Motion.values",
      main_namespace, main_namespace);

    // Check if the Python enums match the C++ enum values.
    if (TestClass::bike == python::extract<TestClass::Motion>(
          main_namespace["example"].attr("Motion").attr("bike")))
      std::cout << "bike enum values matched." << std::endl;
  }
  catch (const python::error_already_set&)
  {
    PyErr_Print();
  }
}

Output:

{0: __main__.Color.red, 1: __main__.Color.blue}
blue enum values matched.
{0: example.Motion.walk, 1: example.Motion.bike}
bike enum values matched.