Change base parameter type in register_exception and exception constructor from PyObject* to handle

[YannickJadoul]

Follow-up of #2465. This should be the same, but is the pybind11 alternative, rather than a raw C API type.

[bstaletic]

/usr/include/python3.7m/pyerrors.h:326:24: note: candidate function not viable: no known conversion from 'pybind11::handle' to 'PyObject *' (aka '_object *') for 2nd argument

So this is a breaking change. py::handle can not be constructed from a PyObject* without also specifying whether you want to borrow or steal the reference.

On top of that, you mentioned using py::type instead of py::handle?

[YannickJadoul]

/usr/include/python3.7m/pyerrors.h:326:24: note: candidate function not viable: no known conversion from 'pybind11::handle' to 'PyObject *' (aka '_object *') for 2nd argument

So this is a breaking change. py::handle can not be constructed from a PyObject* without also specifying whether you want to borrow or steal the reference.

Huh, I thought you could. handle is only a wrapper around PyObject*, not doing any refcounting (borrow/steal are for object, when the refcount needs to be increased or not).

There's also an implicit conversion, so it's a bit weird that this doesn't work?

pybind11/include/pybind11/pytypes.h

Line 179 in 3bd0d7a

handle(PyObject *ptr) : m_ptr(ptr) { } // Allow implicit conversion from PyObject*

On top of that, you mentioned using py::type instead of py::handle?

Yes, but it's not there, yet. So indeed, let's link to #2388.

[YannickJadoul]

There's also an implicit conversion, so it's a bit weird that this doesn't work?

It's the opposite way that doesn't work implicitly, handle to PyObject*. Should've given the thing a compilation run before pushing "a harmless change" ;-)

[bstaletic]

Right, I was mixing things up with py::object(py::handle), which doesn't work.

Anyway, this looks good now.

[michalsustr]

Does this mean that now it is possible to do:

py::register_exception<ExceptionA>(m, "ExceptionA", PyExc_RuntimeError);
py::register_exception<ExceptionB>(m, "ExceptionB", ExceptionA);

i.e. make a hierarchy of exceptions, starting with python ones and continuing into the ones defined on the C++ side?

If not, how can this be achieved?

[YannickJadoul]

@michalsustr Not sure, actually. I would think it has always worked, but that before, you'd need to say py::register_exception<ExceptionB>(m, "ExceptionB", ExceptionA.ptr());, because handle is not implicitly convertible to a PyObject*.

[YannickJadoul]

@michalsustr (and others), I quickly checked, and this seems to already work:

#include <pybind11/pybind11.h>

namespace py = pybind11;

class X : public std::runtime_error {};
class Y : public std::runtime_error {};

PYBIND11_MODULE(example, m)
{
        auto exceptionA = py::register_exception<X>(m, "ExceptionA", PyExc_RuntimeError);
        py::register_exception<Y>(m, "ExceptionB", exceptionA.ptr());
}

So this PR just removes the need to add .ptr(), but that's all. So this PR shouldn't break/enable anything more?
It would of course still be nice to unify with py::class_, but that's a whole other expedition into unknown territory.