Rudimentary Python file-like -> iostream adapter for Pybind11.

#include <iosfwd>
#include <iostream>
#include <sstream>

#include <boost/iostreams/categories.hpp>
#include <boost/iostreams/stream.hpp>

#include <pybind11/pybind11.h>
#include <pybind11/embed.h>

namespace py = pybind11;


class pyobject_source
{
public:
    typedef char char_type;
    typedef boost::iostreams::source_tag category;
    pyobject_source(py::object& source_obj)
        : source_obj_(source_obj)
        , pos_(0)
    {
    }

    std::streamsize read(char_type* s, std::streamsize n)
    {
        if (n <= 0) {
            return 0;
        }
        py::object result = source_obj_.attr("read")(n);
        auto const payload = result.cast<std::string>();
        if (payload.empty()) {
            return -1; // EOF
        }
        std::copy(payload.begin(), payload.end(), s);
        return payload.size();
    }

    py::object& source_obj()
    {
        return source_obj_;
    }
private:
    py::object& source_obj_;
    size_t pos_;
};

std::string test_read_stream(std::istream& stream)
{
    std::ostringstream result;
    std::string line;

    while (std::getline(stream, line)) {
        result << line << '\n';
    }

    return result.str();
}

PYBIND11_EMBEDDED_MODULE(testmodule, m)
{
    m.def("test_read_stream", [](py::object file_like) -> std::string {
        boost::iostreams::stream<pyobject_source> wrapper_stream(file_like);
        return test_read_stream(wrapper_stream);
    });
}

int main()
{
    py::scoped_interpreter guard{};

    try {
        py::exec(R"(\
import testmodule
import io

s = io.BytesIO(b"One\nTwo\nThree")
print(testmodule.test_read_stream(s))
)");
    } catch (py::error_already_set& e) {
        std::cerr << e.what() << "\n";
    }
}