Converting the 'dict' type to the 'nlohmann::json' type

[nyckmaia]

New User

Hi, I'm a Pybind11 user and I'm migrating my code to nanobind.

My Example

I have a C++ method exemplified below in which I have an optional json argument called config:

#include "nlohmann/json.hpp"

class A {
public:
    int myMethod(nlohmann::json config = {});
};

So, after my Pybind11 to nanobind ajustments, my wrapper code is:

#include <nanobind/nanobind.h>
#include <nanobind/stl/function.h>
#include <nanobind/stl/map.h>
#include <nanobind/stl/string.h>
#include <nanobind/stl/vector.h>

#include "myClasses/aClass.h"

#include "nlohmann/json.hpp"

namespace nb = nanobind;
using namespace nb::literals;

void AClass(nb::module_& m) {
 nb::class_<A> cls(m, "A");
 cls.def("myMethod", &A::myMethod, nb::arg("config") = nlohmann::json());
}

Here below is my compile-time error:

/home/nyck/.local/lib/python3.8/site-packages/nanobind/include/nanobind/nb_func.h:134:25: error: no match for ‘operator+’ (operand types are ‘nanobind::detail::descr<1>’ and ‘std::__cxx11::basic_string<char>’)
  134 |         const_name("(") +
      |         ~~~~~~~~~~~~~~~~^
  135 |         concat(type_descr(
      |         ~~~~~~~~~~~~~~~~~~
  136 |             make_caster<remove_opt_mono_t<intrinsic_t<Args>>>::Name)...) +
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/nyck/.local/lib/python3.8/site-packages/nanobind/include/nanobind/nanobind.h:44,
                 from /mnt/c/Users/nyck/Desktop/maialib/maiacore/src/maiacore/python_wrapper/py_score.cpp:1:
/home/nyck/.local/lib/python3.8/site-packages/nanobind/include/nanobind/nb_descr.h:43:42: note: candidate: ‘template<long unsigned int N1, long unsigned int N2, class ... Ts1, class ... Ts2> constexpr nanobind::detail::descr<(N1 + N2), Ts1 ..., Ts2 ...> nanobind::detail::operator+(const nanobind::detail::descr<N1, Ts1 ...>&, const nanobind::detail::descr<N2, Ts2 ...>&)’
   43 | constexpr descr<N1 + N2, Ts1..., Ts2...> operator+(const descr<N1, Ts1...> &a, const descr<N2, Ts2...> &b) {

Questions

1. What is wrong in my code? How can I fix it?
2. Pybind11 can convert C++ nlohmann::json type to Python dict type and vice versa. Is there any similar feature in nanobind to use json/dict data conversion?
3. I tried to use py::arg("config") = nb::dict() or py::arg("config") = nb::none(), but does't work too. How can I set the optional dict parameter in this case?

My System

• Ubuntu 22 LTS x64
• CMake 3.25.0
• Compiler: gcc 9.3.0
• C++ 17
• nanobind 2.0.0

[nyckmaia]

Solution

I fixed it!
My error was:

• The returning type of the method
• The conversion between dict and nlohmann::json

cls.def(
        "myMethod",
        [](A& self, nb::dict config) -> nlohmann::json {
            // From Python dict to JSON as std::string
            std::string jsonString = nb::cast<std::string>(nb::str(config));

            // From json string to nlohmann::json
            nlohmann::json jsonObj = nlohmann::json::parse(jsonString);

            return self.myMethod(jsonObj);
        },
        nb::arg("config") = nb::dict());