Hashing enums

[mnijhuis-tos]

While converting pybind11 bindings for an enumeration type, I noticed the following:

• Simply replacing py:: by nb:: was enough. The existing value() and export_values() calls still worked. Great!
• When using a set with enumeration values, I got an error that the type is not hashable. Unlike pybind11, nanobind does not add a __hash__ function to the type.

When adding a __hash__ function myself, I noticed the following:

• Since nb::enum_ is derived from nb::class_, I can use the def(__hash__, ...). However, the return value of def() is an nb::class_, and not an nb::enum_, so I had to put this def() after all the value() calls.
• Adding the def("__hash__", ...) indeed removes the error that the type is not hashable.
• Instead of the not-hashable error, I'm now getting a "RuntimeError: nanobind::detail::nb_func_vectorcall_simple(): the 'self' argument of a method call should be a nanobind class." My function looks as follows:
  .def("__hash__", [](const Enum& e) { return static_cast<int>(e); })
Enum is the same type as in the nb::enum_<Enum>() call.

Can you tell me if you plan to support hashing enums, like pybind11 does, and/or tell me if and how I can add my own __hash__ function to an nb::enum_ binding?

[wjakob]

Hi @mnijhuis-tos -- that's a good catch. Could you create a PR that implements the hashing function in the libnanobind component? (specifically, that would be in the file src/nb_enum.cpp). Instead of using .def("__hash__", ...) would do this at a lower level by providing the Py_tp_hash type slot in this file. Hopefully it's clear enough if you look at the other code there.

[mnijhuis-tos]

I've written an nb_enum_hash function, based on nb_enum_int. See #106 .