From 5845809d82d9f4895954a871e27c9b2e39acd79e Mon Sep 17 00:00:00 2001
From: Bryn Pickering <17178478+brynpickering@users.noreply.github.com>
Date: Tue, 20 May 2025 15:50:48 +0100
Subject: [PATCH 1/3] Coerce unknown types to O dtype

---
 xarray/compat/array_api_compat.py | 13 +++++++++----
 xarray/tests/test_dtypes.py       |  4 ++++
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/xarray/compat/array_api_compat.py b/xarray/compat/array_api_compat.py
index e1e5d5c5bdc..1b6c440cc7a 100644
--- a/xarray/compat/array_api_compat.py
+++ b/xarray/compat/array_api_compat.py
@@ -38,10 +38,15 @@ def _future_array_api_result_type(*arrays_and_dtypes, xp):
 
 
 def result_type(*arrays_and_dtypes, xp) -> np.dtype:
-    if xp is np or any(
-        isinstance(getattr(t, "dtype", t), np.dtype) for t in arrays_and_dtypes
-    ):
-        return xp.result_type(*arrays_and_dtypes)
+    is_np_dtype = [
+        hasattr(t, "dtype") and isinstance(t.dtype, np.dtype) for t in arrays_and_dtypes
+    ]
+    if xp is np or any(is_np_dtype):
+        all_valid_arrays_and_dtypes = [
+            t if is_np_dtype[i] or t.__class__.__module__ == "builtins" else np.object_
+            for i, t in enumerate(arrays_and_dtypes)
+        ]
+        return xp.result_type(*all_valid_arrays_and_dtypes)
     else:
         return _future_array_api_result_type(*arrays_and_dtypes, xp=xp)
 
diff --git a/xarray/tests/test_dtypes.py b/xarray/tests/test_dtypes.py
index 0ccda1d8074..9b76e7d431a 100644
--- a/xarray/tests/test_dtypes.py
+++ b/xarray/tests/test_dtypes.py
@@ -32,6 +32,10 @@ class DummyArrayAPINamespace:
         ([np.dtype("<U2"), str], np.dtype("U")),
         ([np.dtype("S3"), np.bytes_], np.dtype("S")),
         ([np.dtype("S10"), bytes], np.dtype("S")),
+        ([type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
+        ([np.float32, type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
+        ([np.str_, type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
+        ([np.bytes_, type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
     ],
 )
 def test_result_type(args, expected) -> None:

From ef3df17dbf4c7e209a820cc3d9a54918a513d2a8 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 20 May 2025 14:57:50 +0000
Subject: [PATCH 2/3] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 xarray/tests/test_dtypes.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xarray/tests/test_dtypes.py b/xarray/tests/test_dtypes.py
index 9b76e7d431a..3b9c60517f9 100644
--- a/xarray/tests/test_dtypes.py
+++ b/xarray/tests/test_dtypes.py
@@ -32,10 +32,10 @@ class DummyArrayAPINamespace:
         ([np.dtype("<U2"), str], np.dtype("U")),
         ([np.dtype("S3"), np.bytes_], np.dtype("S")),
         ([np.dtype("S10"), bytes], np.dtype("S")),
-        ([type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
-        ([np.float32, type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
-        ([np.str_, type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
-        ([np.bytes_, type('Foo', (object,), {'foo' : 'bar'})()], np.object_),
+        ([type("Foo", (object,), {"foo": "bar"})()], np.object_),
+        ([np.float32, type("Foo", (object,), {"foo": "bar"})()], np.object_),
+        ([np.str_, type("Foo", (object,), {"foo": "bar"})()], np.object_),
+        ([np.bytes_, type("Foo", (object,), {"foo": "bar"})()], np.object_),
     ],
 )
 def test_result_type(args, expected) -> None:

From 1756bc4ba9bb4a05f85b66de3ca07b2a3d7c129b Mon Sep 17 00:00:00 2001
From: Bryn Pickering <17178478+brynpickering@users.noreply.github.com>
Date: Fri, 23 May 2025 14:34:39 +0100
Subject: [PATCH 3/3] Add comment on setting obj dtype

---
 xarray/compat/array_api_compat.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/xarray/compat/array_api_compat.py b/xarray/compat/array_api_compat.py
index 1b6c440cc7a..166a23ba2f8 100644
--- a/xarray/compat/array_api_compat.py
+++ b/xarray/compat/array_api_compat.py
@@ -42,6 +42,9 @@ def result_type(*arrays_and_dtypes, xp) -> np.dtype:
         hasattr(t, "dtype") and isinstance(t.dtype, np.dtype) for t in arrays_and_dtypes
     ]
     if xp is np or any(is_np_dtype):
+        # Numpy can only apply type promotion rules on non-builtin & numpy-compatible Python objects.
+        # So we convert any incompatible objects (e.g. user-defined class instances) stored in the array to numpy Object dtype.
+        # Then, numpy's type promotion will fall back to Object dtype rather than raising an exception.
         all_valid_arrays_and_dtypes = [
             t if is_np_dtype[i] or t.__class__.__module__ == "builtins" else np.object_
             for i, t in enumerate(arrays_and_dtypes)