From fbbf3b9b3e476f2e6e0f0c317635eafb13b321d4 Mon Sep 17 00:00:00 2001
From: Daniel Yeh <46629671+Dan-Yeh@users.noreply.github.com>
Date: Tue, 23 Sep 2025 05:12:15 +0200
Subject: [PATCH] Support pickling array for bfloat16 (#2586)

* add bfloat16 pickling

* Improvements

* improve

---------

Co-authored-by: Chen-Chen Yeh <ge96noj@mytum.de>
---
 python/src/array.cpp       | 33 +++++++++++++++++++++++++++++----
 python/src/convert.cpp     |  3 +--
 python/src/convert.h       |  4 ++++
 python/tests/test_array.py |  8 ++------
 4 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/python/src/array.cpp b/python/src/array.cpp
index ae38fa211..9367d4e09 100644
--- a/python/src/array.cpp
+++ b/python/src/array.cpp
@@ -466,12 +466,37 @@ void init_array(nb::module_& m) {
           })
       .def(
           "__iter__", [](const mx::array& a) { return ArrayPythonIterator(a); })
-      .def("__getstate__", &mlx_to_np_array)
+      .def(
+          "__getstate__",
+          [](const mx::array& a) {
+            auto nd = (a.dtype() == mx::bfloat16)
+                ? mlx_to_np_array(mx::view(a, mx::uint16))
+                : mlx_to_np_array(a);
+            return nb::make_tuple(nd, static_cast<uint8_t>(a.dtype().val()));
+          })
       .def(
           "__setstate__",
-          [](mx::array& arr,
-             const nb::ndarray<nb::ro, nb::c_contig, nb::device::cpu>& state) {
-            new (&arr) mx::array(nd_array_to_mlx(state, std::nullopt));
+          [](mx::array& arr, const nb::tuple& state) {
+            if (nb::len(state) != 2) {
+              throw std::invalid_argument(
+                  "Invalid pickle state: expected (ndarray, Dtype::Val)");
+            }
+            using ND = nb::ndarray<nb::ro, nb::c_contig, nb::device::cpu>;
+            ND nd = nb::cast<ND>(state[0]);
+            auto val = static_cast<mx::Dtype::Val>(nb::cast<uint8_t>(state[1]));
+            if (val == mx::Dtype::Val::bfloat16) {
+              auto owner = nb::handle(state[0].ptr());
+              new (&arr) mx::array(nd_array_to_mlx(
+                  ND(nd.data(),
+                     nd.ndim(),
+                     reinterpret_cast<const size_t*>(nd.shape_ptr()),
+                     owner,
+                     nullptr,
+                     nb::bfloat16),
+                  mx::bfloat16));
+            } else {
+              new (&arr) mx::array(nd_array_to_mlx(nd, std::nullopt));
+            }
           })
       .def("__dlpack__", [](const mx::array& a) { return mlx_to_dlpack(a); })
       .def(
diff --git a/python/src/convert.cpp b/python/src/convert.cpp
index 1340b663a..88da37103 100644
--- a/python/src/convert.cpp
+++ b/python/src/convert.cpp
@@ -23,8 +23,6 @@ struct ndarray_traits<mx::float16_t> {
   static constexpr bool is_int = false;
   static constexpr bool is_signed = true;
 };
-
-static constexpr dlpack::dtype bfloat16{4, 16, 1};
 }; // namespace nanobind
 
 int check_shape_dim(int64_t dim) {
@@ -51,6 +49,7 @@ mx::array nd_array_to_mlx(
     std::optional<mx::Dtype> dtype) {
   // Compute the shape and size
   mx::Shape shape;
+  shape.reserve(nd_array.ndim());
   for (int i = 0; i < nd_array.ndim(); i++) {
     shape.push_back(check_shape_dim(nd_array.shape(i)));
   }
diff --git a/python/src/convert.h b/python/src/convert.h
index f5016c8af..4f8a77abe 100644
--- a/python/src/convert.h
+++ b/python/src/convert.h
@@ -12,6 +12,10 @@
 namespace mx = mlx::core;
 namespace nb = nanobind;
 
+namespace nanobind {
+static constexpr dlpack::dtype bfloat16{4, 16, 1};
+}; // namespace nanobind
+
 struct ArrayLike {
   ArrayLike(nb::object obj) : obj(obj) {};
   nb::object obj;
diff --git a/python/tests/test_array.py b/python/tests/test_array.py
index ae1cb784f..e932382b1 100644
--- a/python/tests/test_array.py
+++ b/python/tests/test_array.py
@@ -532,7 +532,7 @@ class TestArray(mlx_tests.MLXTestCase):
         self.assertEqual(str(x), expected)
 
         x = mx.array([[1, 2], [1, 2], [1, 2]])
-        expected = "array([[1, 2],\n" "       [1, 2],\n" "       [1, 2]], dtype=int32)"
+        expected = "array([[1, 2],\n       [1, 2],\n       [1, 2]], dtype=int32)"
         self.assertEqual(str(x), expected)
 
         x = mx.array([[[1, 2], [1, 2]], [[1, 2], [1, 2]]])
@@ -886,6 +886,7 @@ class TestArray(mlx_tests.MLXTestCase):
             mx.uint64,
             mx.float16,
             mx.float32,
+            mx.bfloat16,
             mx.complex64,
         ]
 
@@ -895,11 +896,6 @@ class TestArray(mlx_tests.MLXTestCase):
             y = pickle.loads(state)
             self.assertEqualArray(y, x)
 
-        # check if it throws an error when dtype is not supported (bfloat16)
-        x = mx.array([[[1, 2], [3, 4]], [[5, 6], [7, 8]]], dtype=mx.bfloat16)
-        with self.assertRaises(TypeError):
-            pickle.dumps(x)
-
     def test_array_copy(self):
         dtypes = [
             mx.int8,