How can I upcast a Python derived class to it's c++ base with Boost Python?

Question

I have a c++ class with a pure virtual function which I wrap in the following way using Boost Python:

class Model {
    virtual double Test() = 0;
};
class ModelWrapper : public Model, public boost::python::wrapper<Model> {
    double Test() {
        return this->get_override("Test")(); 
    }     
};
BOOST_PYTHON_MODULE(mymodule)
{
    class_<ModelWrapper, boost::noncopyable>("Model")
        .def("Test, pure_virtual(&Model::Test))
    ;
}

I also have a c++ function that's expecting a Model object as an argument:

double MyFunction(Model& m) { 
    return m.Test() 
}

Now what I would like to do is to make a derived class in Python that overrides the Model::Test function but can still be passed to MyFunction. Something like this:

from mymodule import *

class NewModel(Model):
    def Test(self):
        return 0.5

m = NewModel()
print(MyFunction(m))

which I would want to print out 0.5 in this case. Instead I get this error:

> ArgumentError: Python argument types in
>     mymodule.MyFunction(NewModel) did not match C++ signature:
>     MyFunction(Model {lvalue})

I'm not sure what step I'm missing in order for NewModel to be usable where Model is expected. Any insight would be greatly appreciated.

Answer 1

I needed to add 'Model.__init__(self)' to the NewModel __init__ function explicitly. After that it worked fine.