Empty variant

[pschella]

Would be useful to cast between empty std::variant (i.e. std::monostate) and None. Something like this (in stl) should do the trick I think:

namespace pybind11 {
namespace detail {
    template <> struct type_caster<std::monostate> {
    public:
        PYBIND11_TYPE_CASTER(std::monostate, _("None"));

        bool load(handle src, bool) {
            return src.ptr() == Py_None;
        }

        static handle cast(std::monostate src, return_value_policy /* policy */, handle /* parent */) {
            Py_RETURN_NONE;
        }
    };
}
}

[SeanCurtis-TRI]

I completely agree. std::monostate is the preferred mechanism for indicating an empty variant, it would be nice to have it supported out of the box.

[Skylion007]

@SeanCurtis-TRI @pschella Would be happy to review a PR about this with relevant tests.

[gabrieldevillers]

Wasn't this fixed by #3818 ? @rwgk

[SeanCurtis-TRI]

I'm persuaded that the PR closes this issue. You have my vote.

[rwgk]

Wasn't this fixed by #3818 ? @rwgk

Yes, thanks for pointing out!