How iterate (key, value) boost::python:dict

Question

How can I iterate in C++ over my boost::python:dict ? I need key and value in every loop round.

My try was this:

for (auto x : MyBoostPythonDict.iteritems())
{
    // determine key
    // determine value 
    // ...
}

I got this error: C3312 no callable 'end' function found for type 'boost::python::api::object'

Answer 1

Boost Python exposes some STL iterator wrappers in the "stl_iterator" header that let you go from begin to end like a normal C++ iteration:

https://www.boost.org/doc/libs/1_75_0/libs/python/doc/html/reference/high_level_components/boost_python_stl_iterator_hpp.html

For Python version 2 you can use d.items() or d.iteritems() depending on whether you want to iterate lazily or not. For Python version 3 has a slight problem - you'd expect items to be a lazy dict view, but instead Boost Python converts it to a list. Therefore, I've called .attr("items")() instead to bypass the in-build conversion thus getting a lazy wrapper. I've returned this object to Python just to confirm that it's the view rather than a list.

Calling stl_input_iterator<tuple>() gives you an iterator for a (Python) tuple object from which you can extract the key and the value.

#include <boost/python/dict.hpp>
#include <boost/python/tuple.hpp>
#include <boost/python/stl_iterator.hpp>
#include <boost/python/extract.hpp>
#include <boost/python/str.hpp>
#include <boost/python.hpp>

#include <iostream>

using namespace boost::python;

object print_dict_to_cout(dict d) {
    auto items = d.attr("items")(); // just plain d.items or d.iteritems for Python 2!
    for (auto it = stl_input_iterator<tuple>(items); it != stl_input_iterator<tuple>(); ++it) {
        tuple kv = *it;
        auto key = kv[0];
        auto value = kv[1];
        std::cout << extract<const char*>(str(key)) << " : " << extract<const char*>(str(value)) << std::endl;
    }
    return items;
}

BOOST_PYTHON_MODULE(iterdict)
{
    def("print_dict_to_cout", print_dict_to_cout);
}

The advantage of doing it this way is that it's lazy, and doesn't create the intermediate list.