Bind nlohmann::json::array_t is possible?

[pybind]

Bind nlohmann::json::array_t is possible?

[martinRenou]

Mmh, it might be a good idea to add support for array_t in this extension indeed.

You might be able to make it work with something like:

pybind11::object test(pybind11::list& a) const
{
    nlohmann::json j = a;

    nlohmann::json node;
    node["my key"] = "my string";

    j.push_back(node);

    return j;
}

[martinRenou]

Sure. Let's turn it into a py::object first then.

pybind11::object test(pybind11::list& a) const
{
    nlohmann::json j = py::object(a);

    nlohmann::json node;
    node["my key"] = "my string";

    j.push_back(node);

    return j;
}

This is really not ideal, but it should work.

[martinRenou]

I think py::list should work

[nyckmaia]

Thank you @martinRenou for your quick response.

Could you please look this example below?

C++ side:

nlohmann::json::array_t my_array;

nlohmann::json firstElement;
firstElement["first"] = 1;

my_array.push_back(firstElement);

// Do another stuff ...

test(my_array);

C++ function:

pybind11::list test(pybind11::list& input)
{
    // Try to bind:
    nlohmann::json::array_t out = input;

    // New JSON field:
    nlohmann::json node;
    node["my key"] = "my string";

     // Insert JSON element:
     out.push_back(node);

    return out;
}

I try to compile but, t I got these 2 errors below:

error: conversion from ‘pybind11::list’ to non-scalar type ‘nlohmann::basic_json<>::array_t’ {aka ‘std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >’} requested
     nlohmann::json::array_t j = a;
                                 ^
error: could not convert ‘j’ from ‘nlohmann::basic_json<>::array_t’ {aka ‘std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >’} to ‘pybind11::list’
     return j;

Could you help me again?

[martinRenou]

Actually, I think if you return a pybind11::object you will get a Python list when executing your function Python side, did you try?

[nyckmaia]

Yes, I tried...

pybind11::object test(pybind11::list& a) const
{
    nlohmann::json::array_t j = a;

    nlohmann::json node;
    node["my key"] = "my string";

    j.push_back(node);

    return j;
}

C++ Compile time error:

error: conversion from ‘pybind11::list’ to non-scalar type ‘nlohmann::basic_json<>::array_t’ {aka ‘std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >’} requested
     nlohmann::json::array_t j = a;
                                 ^
error: could not convert ‘j’ from ‘nlohmann::basic_json<>::array_t’ {aka ‘std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >’} to ‘pybind11::object’
     return j;

[nyckmaia]

I tried it too, but I got 1 compile time error:

error: conversion from ‘pybind11::list’ to non-scalar type ‘nlohmann::json’ {aka ‘nlohmann::basic_json<>’} requested
     nlohmann::json j = a;

[nyckmaia]

Ok...now it worked! Thank you @martinRenou !

[martinRenou]

Sure!

[martinRenou]

This will change in the next release. The py::object to nlohmann::json conversion will be done automatically.