From bd25038da73bc4a01cd03efc5a806ef2fda96371 Mon Sep 17 00:00:00 2001
From: Axel Huebl <axel.huebl@plasma.ninja>
Date: Mon, 5 Aug 2024 11:58:44 -0700
Subject: [PATCH 1/2] Python Series/Iteration: Fix Length

If the `len(...)` intrinsic for series and iterations.

```py
import openpmd_api as io

s = io.Series("build/samples/git-sample/3d-bp4/example-3d-bp4_%T.bp", io.Access.read_only)

s
<openPMD.Series at 'build/samples/git-sample/3d-bp4/example-3d-bp4_%T.bp' with 2 iteration(s) and 10 attributes>

s.iterations
<openPMD.Iteration_Container with 2 entries and 0 attribute(s)>

len(s)
2

len(s.iterations)
2
```
---
 include/openPMD/binding/python/Container.H | 6 ++++++
 src/binding/python/Series.cpp              | 1 +
 2 files changed, 7 insertions(+)

diff --git a/include/openPMD/binding/python/Container.H b/include/openPMD/binding/python/Container.H
index a07847e600..cb42c89c00 100644
--- a/include/openPMD/binding/python/Container.H
+++ b/include/openPMD/binding/python/Container.H
@@ -85,6 +85,12 @@ declare_container(py::handle scope, std::string const &name)
         // keep container alive while iterator exists
         py::keep_alive<0, 1>());
 
+    // overwrite to avoid that the __len__ of Attributable is used
+    cl.def(
+        "__len__",
+        [](const Map &m) { return m.size(); },
+        "Number of elements in the container to iterate.");
+
     cl.def("__repr__", [name](Map const &m) {
         std::stringstream stream;
         stream << "<openPMD." << name << " with ";
diff --git a/src/binding/python/Series.cpp b/src/binding/python/Series.cpp
index 37de823f2a..59068c2728 100644
--- a/src/binding/python/Series.cpp
+++ b/src/binding/python/Series.cpp
@@ -252,6 +252,7 @@ For further details, refer to the non-MPI overload.
             )END")
 #endif
         .def("__bool__", &Series::operator bool)
+        .def("__len__", [](Series const &s) { return s.iterations.size(); })
         .def(
             "__repr__",
             [](Series const &s) {

From fea1fdaf979ae835de8dad7016d952180c2896f4 Mon Sep 17 00:00:00 2001
From: Axel Huebl <axel.huebl@plasma.ninja>
Date: Thu, 5 Sep 2024 10:34:02 -0700
Subject: [PATCH 2/2] Python: Remove `Attributable.__len__`

For consistency because its only a mixin class.
Write migration guide for breaking change.
---
 NEWS.rst                            | 3 +++
 src/binding/python/Attributable.cpp | 2 --
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/NEWS.rst b/NEWS.rst
index 89089391a4..5f95465cff 100644
--- 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
index 2383ceab50..806a57b666 100644
--- 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