Wrapping C++ code with PyUblas¶

Data members of type numpy_vector¶

If the piece of software you are wrapping contains numpy_vector structure or class members, note that you cannot use Boost.Python’s standard class_<>::def_readonly() or class_<>::def_readwrite() mechanism to expose these. This is again because these functions expect to be able to return an lvalue to Python when a member is read. Since there is no lvalue to-Python converter for numpy_vector, this will fail at runtime.

PyUblas contains a mechanism to circumvent this limitation:

struct my_vector_container
{
  const numpy_vector<double> my_ro_data_member;
  numpy_vector<double> my_rw_data_member;
};

BOOST_PYTHON_MODULE(memberdemo)
{
  class_<my_vector_container>("MyVectorContainer")
    .def(pyublas::by_value_ro_member(
         "my_ro_data_member",
         &my_vector_container::my_ro_data_member))
    .def(pyublas::by_value_rw_member(
         "my_rw_data_member",
         &my_vector_container::my_rw_data_member))
    ;
}

CAUTION: If you are exposing a member that is originally found in a base class of the class you are exposing, you will get errors when accessing that member from Python. (such as “Argument types ... did not match C++ signature”) This is because the C++ pointer-to-member type only records the type of the base class that Boost.Python knows nothing about. Therefore, the getters and setters implicitly used by the by_value_rw/ro_member code expect the base class as their first argument, and conversion fails. For regular members, Boost.Python contains sophisticated logic to fix this. Unfortunately, at this point, this logic is marked as private, and is therefore inaccessible to PyUblas.