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");
}