Boost.Python constructor with optional arguments

Question

I want to use Boost.Python to create a Python wrapper for a C++ constructor with optional arguments. I want the Python wrapper to act like this:

class Foo():
  def __init__(self, filename, phase, stages=None, level=0):
    """
    filename -- string
    phase -- int
    stages -- optional list of strings
    level -- optional int
    """
    if stages is None:
      stages = []
    # ...

How do I do this with Boost.Python? I don't see how to do it with make_constructor, and I don't know how to make a constructor with raw_function. Is there some better documentation than this out there?

My specific problem is trying to add two optional arguments (stages and level) to these two constructors:

https://github.com/BVLC/caffe/blob/rc3/python/caffe/_caffe.cpp#L76-L96

Could you use the args class in you constructor def? That should let you generate the keyword-expression that make_constructor takes as well. — Dan Mašek
– Dan Mašek, Commented Mar 21, 2016 at 18:37
Some notes about how to make raw constructor are on the python wiki. — Dan Mašek
– Dan Mašek, Commented Mar 21, 2016 at 18:52
as in make_constructor(&Net_Init, default_call_policies(), (arg("param_file"), arg("phase"), arg("stages")=object(), arg("level")=0)) -- stages being object, so that you can deal with None. — Dan Mašek
– Dan Mašek, Commented Mar 21, 2016 at 19:05
Thanks @DanMašek - I think I've got something working! I'll post my solution after I double-check it ... — Luke Yeager
– Luke Yeager, Commented Mar 21, 2016 at 21:04

Luke Yeager · Accepted Answer · 2016-03-21 22:01:14Z

Thanks to Dan's comments, I found a solution that works. I'll copy most of it here since there are some interesting tidbits about how to extract objects from bp::object, etc.

// Net constructor
shared_ptr<Net<Dtype> > Net_Init(string param_file, int phase,
    const int level, const bp::object& stages,
    const bp::object& weights_file) {
  CheckFile(param_file);

  // Convert stages from list to vector
  vector<string> stages_vector;
  if (!stages.is_none()) {
      for (int i = 0; i < len(stages); i++) {
        stages_vector.push_back(bp::extract<string>(stages[i]));
      }   
  }   

  // Initialize net 
  shared_ptr<Net<Dtype> > net(new Net<Dtype>(param_file,
      static_cast<Phase>(phase), level, &stages_vector));

  // Load weights
  if (!weights_file.is_none()) {
      std::string weights_file_str = bp::extract<std::string>(weights_file);
      CheckFile(weights_file_str);
      net->CopyTrainedLayersFrom(weights_file_str);
  }   

  return net;
}   

BOOST_PYTHON_MODULE(_caffe) {
  bp::class_<Net<Dtype>, shared_ptr<Net<Dtype> >, boost::noncopyable >("Net",
    bp::no_init)
    .def("__init__", bp::make_constructor(&Net_Init,
          bp::default_call_policies(), (bp::arg("network_file"), "phase",
            bp::arg("level")=0, bp::arg("stages")=bp::object(),
            bp::arg("weights_file")=bp::object())))
}

The generated signature is:

__init__(boost::python::api::object, std::string network_file, int phase, 
   int level=0, boost::python::api::object stages=None,
   boost::python::api::object weights_file=None)

And I can use it like:

net = caffe.Net('network.prototxt', weights_file='weights.caffemodel',
  phase=caffe.TEST, level=1, stages=['deploy'])

Full code available in pull request here: https://github.com/BVLC/caffe/pull/3863

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