Boost Python No to_python converter found for std::string

Question

So, I am trying to create a to_python converter that will allow me to return a boost::optional from an exposed function and have it treated as T if the optional is set and None if not. Based on a post I found on C++Sig, I wrote the following code.

template<typename T>
struct optional_ : private boost::noncopyable {
  struct conversion {
    static PyObject* convert(boost::optional<T> const& value) {
      if (value) {
        return boost::python::to_python_value<T>()(*value);
      }
      Py_INCREF(Py_None);
      return Py_None;
    }
  };
  explicit optional_() {
    boost::python::to_python_converter<boost::optional<T>, conversion>();
  }
};

As far as I can tell, it works for converting the optionals, but python throws the following exception "TypeError: No to_python (by-value) converter found for C++ type: std::string". I know that C++ is able to convert strings to python since most of my exposed functions return strings. Why doesn't boost::python::to_python_value recognize it, and how can I utilize whatever converter it has?

Fixed by changing to the following (based on this article):

template<typename T>
struct optional_ : private boost::noncopyable {
  struct conversion {
    static PyObject* convert(boost::optional<T> const& value) {
      using namespace boost::python;
      return incref((value ? object(*value) : object()).ptr());
    }
  };
  explicit optional_() {
    boost::python::to_python_converter<boost::optional<T>, conversion>();
  }
};

Now to just do the other version so that it is cleaner and works better.

Please move your solution to an answer and accept it. – Mu Mind Jun 29 '11 at 00:22 — Mu Mind, Jun 29 '11 at 00:22

score 4 · Accepted Answer · edited Apr 10 '15 at 15:54

Ok here is the entire to and from optional converter based on the original C++ sig post but rewritten to use the high level boost.python API (sorry about the weird spacing).

template<typename T>
struct optional_ : private boost::noncopyable
{
  struct conversion :
    public boost::python::converter::expected_from_python_type<T>
  {
    static PyObject* convert(boost::optional<T> const& value) {
      using namespace boost::python;
      return incref((value ? object(*value) : object()).ptr());
    }
  };

  static void* convertible(PyObject *obj) {
    using namespace boost::python;
    return obj == Py_None || extract<T>(obj).check() ? obj : NULL;
  }

  static void constructor(PyObject *obj,
       boost::python::converter::rvalue_from_python_stage1_data *data)
  {
    using namespace boost::python;
    void *const storage =
      reinterpret_cast<
        converter::rvalue_from_python_storage<boost::optional<T> >*
      >(data)->storage.bytes;
    if(obj == Py_None) {
      new (storage) boost::optional<T>();
    } else {
      new (storage) boost::optional<T>(extract<T>(obj));
    }
    data->convertible = storage;
  }

  explicit optional_() {
    using namespace boost::python;
    if(!extract<boost::optional<T> >(object()).check()) {
      to_python_converter<boost::optional<T>, conversion, true>();
      converter::registry::push_back(
        &convertible,
        &constructor,
        type_id<boost::optional<T> >(),
        &conversion::get_pytype
      );
    }
  }
};

This works greats, thanks a lot! However, something must be missing: I can't use `[...].def("shortest", &automaton::shortest, (arg("num") = boost::none))` to declare optional arguments defaulting to `None`, I must write `[...].def("shortest", &automaton::shortest, (arg("num") = boost::optional()))`. Do you happen to know what is missing to have support for `boost::none`? — akim, Apr 10 '15 at 15:54

score 1 · Answer 2 · answered Feb 19 '15 at 10:59

There are a few typos in the above code - here is the corrected version:

#include <boost/noncopyable.hpp>
#include <boost/optional.hpp>
#include <boost/python.hpp>

template<typename T>
struct python_optional : private boost::noncopyable {
  struct conversion : public boost::python::converter::expected_from_python_type<T>
  {
    static PyObject* convert(boost::optional<T> const& value)
    {
      using namespace boost::python;
      return incref((value ? object(*value) : object()).ptr());
    }
  };

  static void* convertible(PyObject *obj) {
    using namespace boost::python;
    return obj == Py_None || extract<T>(obj).check() ? obj : NULL;
  }

  static void constructor(
    PyObject *obj,
    boost::python::converter::rvalue_from_python_stage1_data *data
  ) {
    using namespace boost::python;
    void *const storage =
      reinterpret_cast<
        converter::rvalue_from_python_storage<boost::optional<T> >*
      >(data)->storage.bytes;
    if(obj == Py_None) {
      new (storage) boost::optional<T>();
    } else {
      new (storage) boost::optional<T>(extract<T>(obj));
    }
    data->convertible = storage;
  }

  explicit python_optional() {
    using namespace boost::python;
    if(!extract<boost::optional<T> >(object()).check()) {
      to_python_converter<boost::optional<T>, conversion, true>();
      converter::registry::push_back(
        &convertible,
        &constructor,
        type_id<boost::optional<T> >(),
        &conversion::get_pytype
      );
    }
  }
};

Boost Python No to_python converter found for std::string

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