Overloading __instancecheck__ method of metaclass

[nicola-gigante]

Hi!

I need to overwrite the __instancecheck__ method for a C++ class exported using pybind11. AFAIK this needs to be done on the class' metaclass, but I'm not familiar with the concept.

Can I do that with pybind11?

My initial attempt:

py::class_<Pluto> plclass(m, "Pluto");
struct PippoMeta { };
py::class_<PippoMeta> pimeta(m, "PippoMeta");
py::class_<Pippo> piclass(m, "Pippo", py::metaclass(pimeta));

plclass.def(py::init());
piclass.def(py::init());
    
pimeta.def("__instancecheck__", [](PippoMeta &, Pluto) {
   py::print("Here");
   return true;
});

This does not work when I load the module:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    from my_module import *
ImportError: Pippo: PyType_Ready failed (AttributeError: mro)!

If I had to write this in Python, PippoMeta would have to derive from type. So maybe that's the missing bit, but how to define the py::class_ to derive from type?

Edit:

So looking at the pybind11 implementation it seems that registered classes must have pybind11 special default metaclass. So I tried to make my metaclass inherit from that, but this triggers an assert in pybind11:

py::class_<Pluto> plclass(m, "Pluto");
struct PippoMeta { };
py::class_<PippoMeta> pimeta(m, "PippoMeta", py::handle((PyObject *)py::detail::get_internals().default_metaclass));
py::class_<Pippo> piclass(m, "Pippo", py::metaclass(pimeta));

plclass.def(py::init());
piclass.def(py::init());
pimeta.def(py::init());
        
pimeta.def("__instancecheck__", [](PippoMeta &, Pluto) {
   py::print("Here");
   return true;
});

The output:

Assertion failed: (parent_tinfo != nullptr), function initialize, file pybind11.h, line 1353.
[1]    82705 abort      ./test.py

Looking at the failed assertion, it seems that pybind11 expects any registered class to have at least a registered base class. That's why get_type_info on PippoMeta's base class returns nullptr and the assert triggers. Is my conjecture right?

Note: cross-posted with StackOverflow