Broken mro when using multiple inheritance with py::metaclass

[pschella]

When a derived class MroDerived needs to expose a static attribute (which necessitates using py::metaclass), but also inherits from multiple bases Python raises the following error:

ImportError: MroDerived: PyType_Ready failed (TypeError: mro() returned base with unsuitable layout ('pybind11_tests.MroBaseB'))!

For a complete example see https://github.com/lsst-dm/pybind11/tree/broken_mro

[pschella]

This is on Python 3.5.2.

[r-owen]

I will also say that the need to add py::metaclass to all our code is a bit of a nuisance. It appears that once it has been specified for a given class it is needed for all subclasses of that class, as well, so it feels a bit fragile. What benefit does one derive by omitting it? If it's not a big drawback to use it then we'll probably just add it to all our code to avoid problems (and wish there was some way to make it the default).

[pschella]

While I agree with it being a nuisance, this was already ruled upon by the lead developer.
However, adding py::metaclass to all classes does not resolve this problem.