C++ Iterators

Python iterator support a highly flexible interface allowing:

• Direct exposure of a class' begin() and end() functions:

    ...
    .def("__iter__", iterator<list_int>())

• Creation of iterators from member functions...

    ...
    .def("__iter__"
         , range(&my_class::x_begin, &my_class::x_end))

• ...and member data:

    ...
    .def("__iter__"
         , range(&std::pair<char*,char*>::first, &std::pair<char*,char*>::second))

• The ability to specify boost.python/CallPolicy, e.g. to prevent copying of heavyweight values:

    ...
    .def("__iter__"
         , range<return_value_policy<copy_non_const_reference> >(
               &my_sequence<heavy>::begin
             , &my_sequence<heavy>::end))

Custom Iterators

Suppose we have custom iterator class providing next() member function. To expose it let's take an approach from scitbx package:

inline object pass_through(object const& o) { return o; }

template<class Klass, class KlassIter>
struct iterator_wrappers
  {
    static Klass
    next(KlassIter& o)
    {
      Klass* result = o.next();
      if (!result) {
        PyErr_SetString(PyExc_StopIteration, "No more data.");
        boost::python::throw_error_already_set();
      }
      return *result;
    }

    static void
    wrap(const char* python_name)
    {
      //using namespace boost::python;
      class_<KlassIter>(python_name, no_init)
        .def("next", next)
        .def("__iter__", pass_through)
      ;
    }
  };

BOOST_PYTHON_MODULE(iter)
{
  ...
  iterator_wrappers<const MyClass,MyIter>().wrap("Iterator");
}