GitLab

Created by: laurentes

Our current python bindings that relies on C++ methods/functions that take blitz arrays as arguments are quite heterogeneous. Ideally, we should follow this way:

Given a class:

class Myclass {
  public:
    void setW(const blitz::Array<double,1>& w) { m_w = w; }
    const blitz::Array<double,1>& getW() { return m_w; }
  private:
    blitz::Array<double,1> m_w;
};

The python binding could be done as follows:

static void py_setW(bob::Myclass& m, bob::python::const_ndarray w) {
  machine.setW(w.bz<double,1>());
}

class_<bob::Myclass, boost::shared_ptr<bob::Myclass> >("Myclass",
    "This class implements ...", init<>())
    .add_property("w", make_function(&bob::Myclass::getW, return_value_policy<copy_const_reference>()), &py_setW, "Paramaters for ...")

For the getter, this will make a copy of the array and cast it into a NumPy array. For the setter, this allows various type (NumPy array, Python list) to be supported, and exceptions are managed by the bz<>() method.

For std::vector of blitz::Array's, the following could be done: Given a class:

class Myclass {
  public:
    void setW(const std::vector<blitz::Array<double,1> >& w) { m_w = ... }
    const std::vector<blitz::Array<double,1> >& getW() { return m_w; }
  private:
    std::vector<blitz::Array<double,1> > m_w;
};

The python binding could be done as follows:

static void py_setW(bob::Myclass& m, object w) {
  stl_input_iterator<bob::python::const_ndarray> dbegin(w), dend;
  std::vector<bob::python::const_ndarray> wdata(dbegin, dend); 
  std::vector<blitz::Array<double,1> > wb;
  for(size_t i=0; i<wdata.size(); ++i)
    wb.push_back(wdata[i].bz<double,1>());
  machine.setW(wb);
}

static object py_getW(bob::Myclass& m) {
  boost::python::list l;
  const std::vector<double,1>& w = m.getW();
  for(size_t i=0; i<w.size(); ++i)
    l.append(w[i]);
  return boost::python::tuple(l);
}

class_<bob::Myclass, boost::shared_ptr<bob::Myclass> >("Myclass",
    "This class implements ...", init<>())
    .add_property("w", &py_getW, &py_setW, "Paramaters for ...")

This way, the setter allows heterogeneous python type (NumPy array, python list) and the getters relies on the copy automagically done from blitz to NumPy.