nb::enum_<...>.def_prop_ro(...) after commit d25bde8f

[francesco-ballarin]

Hi,
in basix, one of the components of FEniCS, we have a few enums with a readonly property, e.g.

  nb::enum_<lattice::type>(m, "LatticeType")
      .value("equispaced", lattice::type::equispaced)
      .value("gll", lattice::type::gll)
      .value("chebyshev", lattice::type::chebyshev)
      .value("gl", lattice::type::gl)
      .def_prop_ro("name",
                   [](nb::object obj) { return nb::getattr(obj, "__name__"); });

at https://github.com/FEniCS/basix/blob/baff38a39049369b382752669b1a4667434b03a0/python/wrapper.cpp#L423-L428

Compiling with current nanobind main branch, we get errors like

      /tmp/basix-src/python/wrapper.cpp:428:8: error: ‘class nanobind::enum_<basix::lattice::type>’ has no member named ‘def_prop_ro’
        428 |       .def_prop_ro("name",
            |        ^~~~~~~~~~~

which, by looking at recent git history, may be related to the change in d25bde8#diff-92b184679f711c1aabb5bcd64d973335fac346586e7027ffa3d12b5f5ccb765c

Can you suggest us how to obtain something equivalent to nb::enum_<...>.def_prop_ro(...) in nanobind 2.x?

Thanks,
Francesco

[wjakob]

As a workaround, you could assign the enum binding to a variable and then write:

my_enum.attr("name") = nb::cpp_function(..) where you copy-paste your previous lambda into the .. part.

[francesco-ballarin]

my_enum.attr("name") = nb::cpp_function(..)

I tried, but it seems to me that it defines a method, rather than a property. Is there any way I can wrap the nb::cpp_function into a property?

FYI, the modified snippet above reads

void attach_name(nb::module_& m, std::string enum_type)
{
  m.attr(enum_type.c_str()).attr("name") = nb::cpp_function(
    [](nb::object obj) { return nb::getattr(obj, "__name__"); });
}

NB_MODULE(..., m)
{
  nb::enum_<lattice::type>(m, "LatticeType")
      .value("equispaced", lattice::type::equispaced)
      .value("gll", lattice::type::gll)
      .value("chebyshev", lattice::type::chebyshev)
      .value("gl", lattice::type::gl);
  attach_name(m, "LatticeType");
}

[francesco-ballarin]

Hi @wjakob,

I tried, but it seems to me that it defines a method, rather than a property. Is there any way I can wrap the nb::cpp_function into a property?

do you have any suggestion for us on how to do this?

If you feel that this is not the right place for this discussion (e.g., this should be reported as an issue) please let me know.

Thanks,
Francesco & the FEniCS team

[francesco-ballarin]

Thanks, I will try that.

As a workaround

Do you expect this workaround to be the way to go in future, or do you expect def_prop_ro to come back at some point before 2.0 is released?
(No big deal either way, just to understand how to phrase the change to the other developers in the downstream project when creating the associated PR).

[wjakob]

See 2889d77

[wjakob]

Please let me know if that fixes the issues.

[francesco-ballarin]

Thanks @wjakob , I can confirm that the commit does fix the issue for us!