2012-01-16

Passing a user defined python object to C++ code(3)


How to access to the self.vertex_list.

This time I will explain how to access to the self.vertex_list of TriMesh. This is a list of length three numpy.array. We have already seen how to get the python list (dict's keylist) in my former article http://shitohichiumaya.blogspot.com/2010/08/boostpython-how-to-pass-python-object.html.

How to access to the float numpy.array element is the following.

void print_float32_3(boost::python::object const & float32_3_obj)
{
    // check sequence length is 3 or not.
    if(boost::python::len(float32_3_obj) != 3){
        std::string const objstr =
            boost::python::extract< std::string >(
                boost::python::str(float32_3_obj));
        std::cerr << "print_float32_3: arg is not a float[3] obj ["
                  << objstr << "]" << std::endl;
        return;
    }


    float vec[] = {0.0f, 0.0f, 0.0f};
    for(int i = 0; i < 3; ++i){
        vec[i] = boost::python::extract< float >(
            float32_3_obj.attr("__getitem__")(i));
    }
    std::cout << "float[3] = " << vec[0] << " " << vec[1] << " "
              << vec[2] << std::endl;
}

First, numpy.array is a sequence, therefore, we can ask length of it by boost::python::len(). We check the length is 3 or not since I defined it is three dimensional vector in the python code. Then we can access to each element through the attribute __getitem__. The python code puts float values to numpy.array, thus we can extract float value with extract< float >. Strictly speaking, numpy.array contains numpy.float64 instead of float, but, extract< float > still works here.

The last member of TriMesh, the triangle face index, should be done in the same way. Actually I would like to say that, but, there is a glitch here in numpy. We can not access this elements in the same way. We will get an errer:
TypeError: No registered converter was able to produce a C++ rvalue of type int from this Python object of type numpy.int64(). 
I would like to talk about this issue in the next blog.

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