Using boost::python, how to return vector of structs as list of dicts to Python?

Question

I have the following C++ code:

    struct MyType { int x, y; };
    struct A { 
        std::vector<MyType> get_data();
    };

which I want to interface to Python using Boost Python so that it can be used in the following way:

    a = A()
    ret = a.get_data();
    for r in ret:
        print('x=%d; y=%d;' % (r['x'], r['y']))

What I have now is a rather naive:

    BOOST_PYTHON_MODULE(pyA) { 
        class_<A>("A").def("get_data", &A::get_data);
    }

which gives me, as expected, the following error

     TypeError: No to_python (by-value) converter found for C++ type

when I try to call the get_data() function from the Python code.

I have seen posts on here (such as std::vector to boost::python::list) which describe how to use vector_indexing_suite to convert a std::vector<T> to a list for some types T (e.g. floats, strings), but I'm not sure how to extend this to deal with my struct -> dict conversion as well. Any help would be greatly appreciated.

doqtor · Accepted Answer · 2019-01-31 13:42:09Z

Below, how to expose your C++ code to Python. MyType needs 'equal to' comparison operator overloaded and also MyType itself needs to be exposed to Python.

#include <boost/python.hpp>
#include <boost/python/suite/indexing/vector_indexing_suite.hpp>
using namespace boost::python;

struct MyType { 
    int x, y;  
    bool operator==(const MyType& data) const {
        return this->x == data.x && this->y == data.y;
    }   
};
struct A { 
    std::vector<MyType> get_data() const { return {{1,2},{3,4}};};
};

BOOST_PYTHON_MODULE(pyA) {
    class_<MyType>("MyType")
        .def_readwrite("x", &MyType::x)
        .def_readwrite("y", &MyType::y);
    class_<std::vector<MyType>>("MyList")
        .def(vector_indexing_suite<std::vector<MyType>>());
    class_<A>("A").def("get_data", &A::get_data);
}

Below slightly modified Python script. get_data() return type is of a list so it needs to be accessed as such. If you want it to be a dict then convert it to dict in Python.

import pyA 

a = pyA.A()
ret = a.get_data();
for r in ret:
    print('x=%d; y=%d;' % (r.x, r.y))

Thank you very much for your reply. In the end I figured it out myself, and I chose my solution as it more directly implements what I wanted. But I'm really grateful that you took the time to help and reply to my post, it is much appreciated.

endtoendianess · Accepted Answer · 2019-02-18 10:57:47Z

In the end I adopted a solution like the following, which I post here in case it is useful to somebody else in the future. This can be improved by adding boost's "readonly" qualifier, but I haven't done so yet.

#include <boost/python.hpp>
using namespace boost::python;

struct Point { 
    int x, y;
};

using Points = std::vector<Point>;

struct Converter
{
    static PyObject* convert(const Points& v)
    {
        boost::python::list ret;
        for (const auto& c : v) {
            boost::python::dict *r = new boost::python::dict();
            (*r)["x"] = c.x;
            (*r)["y"] = c.y;
            ret.append(boost::python::object(*r));
         }
        return boost::python::incref(ret.ptr());
    }
};

BOOST_PYTHON_MODULE(mymodule)
{
    boost::python::to_python_converter<Points, Converter>();

    class_<MyClass, boost::noncopyable>("MyClass")
        .def("get_data", &MyClass::get_data);
}

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