Python: Remove Attributable.__len__

For consistency because its only a mixin class. Write migration guide for breaking change.
openPMD · Sep 5, 2024 · fea1fda · fea1fda
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diff --git a/NEWS.rst b/NEWS.rst
@@ -15,6 +15,9 @@ Note that ADIOS2 does not support compression in BP3 files.
 
 pybind11 2.12.0 is now the minimally supported version for Python support.
 
+The ``len(...)`` of many classes has been reworked for consistency and returns now the number of entries (iterations, record components, etc.).
+Previously, this sporadically returned the number of attributes, which is better queried via ``len(<object>.attributes)``.
+
 
 0.15.0
 ------

diff --git a/src/binding/python/Attributable.cpp b/src/binding/python/Attributable.cpp
@@ -629,8 +629,6 @@ void init_Attributable(py::module &m)
         .def("delete_attribute", &Attributable::deleteAttribute)
         .def("contains_attribute", &Attributable::containsAttribute)
 
-        .def("__len__", &Attributable::numAttributes)
-
         // @todo _ipython_key_completions_ if we find a way to add a []
         // interface