How to support len() method for pybind11::enum_ objects

[hsalimi]

Suppose that I have a C++ enumeration like this:

enum class Kind {Kind1 = 1, Kind2, Kind3};

to bind this enumeration into a Python enumeration using Pybind11, I am doing something like this:

py::enum_<Kind>(py_module, "Kind")
   .value("Kind1", Kind::Kind1)
   .value("Kind2", Kind::Kind2)
   .value("Kind3", Kind::Kind3)
   .def("__len__", 
      [](Kind p) {
         return 3;
      });

After compiling the code, if I ask for the length of the enumeration, I will get this error:

>>> len(Kind)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'pybind11_type' has no len()

Any ideas how I can allow my python users to have len() for the exposed enumerations? I am using Pybind11 version 2.10.1 on Visual Studio 2019 (C++17).

Update 1: Even if I set the the magic method as an attribute like this (as proposed by other solutions), the problem is still the same:

   auto k = py::enum_<Kind>(py_module, "Kind")
      .value("Kind1", Kind::Kind1)
      .value("Kind2", Kind::Kind2)
      .value("Kind3", Kind::Kind3)
      .def("__len__",                        
         [](const Kind& p) {
            return 3;
         });

   k.attr("__len__") = py::cpp_function([](const py::object& arg) -> py::int_
      { return py::int_(3); },
      py::name("__len__"), py::is_method(py_module));