From 1a0e56feb5b9c85c5b48d0da9233ef688e41c388 Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Sun, 6 Oct 2024 15:20:34 +0200
Subject: [PATCH 1/6] check that the length of fixed-width numpy strings is
 reset

---
 xarray/tests/test_dtypes.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xarray/tests/test_dtypes.py b/xarray/tests/test_dtypes.py
index 498ba2ce59f..dbc38deca6f 100644
--- a/xarray/tests/test_dtypes.py
+++ b/xarray/tests/test_dtypes.py
@@ -28,6 +28,7 @@ class DummyArrayAPINamespace:
         ([np.str_, np.int64], np.object_),
         ([np.str_, np.str_], np.str_),
         ([np.bytes_, np.str_], np.object_),
+        ([np.dtype("<U2"), np.str_], np.dtype("U")),
     ],
 )
 def test_result_type(args, expected) -> None:

From ed9e1b8f8d1a56994fa3e362474fe787ac36ba5b Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Sun, 6 Oct 2024 15:21:04 +0200
Subject: [PATCH 2/6] drop the length from numpy's fixed-width string dtypes

---
 xarray/core/dtypes.py | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/xarray/core/dtypes.py b/xarray/core/dtypes.py
index 7464c1e8a89..6315a1fda0b 100644
--- a/xarray/core/dtypes.py
+++ b/xarray/core/dtypes.py
@@ -215,9 +215,14 @@ def isdtype(dtype, kind: str | tuple[str, ...], xp=None) -> bool:
         return xp.isdtype(dtype, kind)
 
 
-def preprocess_scalar_types(t):
+def preprocess_types(t):
     if isinstance(t, str | bytes):
         return type(t)
+    elif isinstance(
+        dtype := getattr(t, "dtype", t), np.dtypes.StrDType | np.dtypes.BytesDType
+    ):
+        # drop the length from numpy's fixed-width string dtypes, it is better to recalculate that
+        return dtype.type
     else:
         return t
 
@@ -249,7 +254,7 @@ def result_type(
         xp = get_array_namespace(arrays_and_dtypes)
 
     types = {
-        array_api_compat.result_type(preprocess_scalar_types(t), xp=xp)
+        array_api_compat.result_type(preprocess_types(t), xp=xp)
         for t in arrays_and_dtypes
     }
     if any(isinstance(t, np.dtype) for t in types):
@@ -262,5 +267,5 @@ def result_type(
                 return np.dtype(object)
 
     return array_api_compat.result_type(
-        *map(preprocess_scalar_types, arrays_and_dtypes), xp=xp
+        *map(preprocess_types, arrays_and_dtypes), xp=xp
     )

From 4d8dcb05ce8bd03de99f18b9108a1f8e906a9851 Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Thu, 10 Oct 2024 17:00:48 +0200
Subject: [PATCH 3/6] compatibility with `numpy<2`

---
 xarray/core/dtypes.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xarray/core/dtypes.py b/xarray/core/dtypes.py
index 6315a1fda0b..fb430b1699b 100644
--- a/xarray/core/dtypes.py
+++ b/xarray/core/dtypes.py
@@ -218,8 +218,9 @@ def isdtype(dtype, kind: str | tuple[str, ...], xp=None) -> bool:
 def preprocess_types(t):
     if isinstance(t, str | bytes):
         return type(t)
-    elif isinstance(
-        dtype := getattr(t, "dtype", t), np.dtypes.StrDType | np.dtypes.BytesDType
+    elif isinstance(dtype := getattr(t, "dtype", t), np.dtype) and dtype.type in (
+        np.str_,
+        np.bytes_,
     ):
         # drop the length from numpy's fixed-width string dtypes, it is better to recalculate that
         return dtype.type

From 0faec8489f4493c99c85de119e7d7172879eedb4 Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Thu, 10 Oct 2024 18:44:00 +0200
Subject: [PATCH 4/6] use `issubdtype` instead

---
 xarray/core/dtypes.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/xarray/core/dtypes.py b/xarray/core/dtypes.py
index fb430b1699b..af21830ecf7 100644
--- a/xarray/core/dtypes.py
+++ b/xarray/core/dtypes.py
@@ -218,9 +218,8 @@ def isdtype(dtype, kind: str | tuple[str, ...], xp=None) -> bool:
 def preprocess_types(t):
     if isinstance(t, str | bytes):
         return type(t)
-    elif isinstance(dtype := getattr(t, "dtype", t), np.dtype) and dtype.type in (
-        np.str_,
-        np.bytes_,
+    elif isinstance(dtype := getattr(t, "dtype", t), np.dtype) and (
+        np.issubdtype(dtype, np.str_) or np.issubdtype(dtype, np.bytes_)
     ):
         # drop the length from numpy's fixed-width string dtypes, it is better to recalculate that
         return dtype.type

From a6dffe06c08c248a5a5408d3b158fc530afaeaf5 Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Thu, 10 Oct 2024 20:02:51 +0200
Subject: [PATCH 5/6] some more test cases

---
 xarray/tests/test_dtypes.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/xarray/tests/test_dtypes.py b/xarray/tests/test_dtypes.py
index dbc38deca6f..0ccda1d8074 100644
--- a/xarray/tests/test_dtypes.py
+++ b/xarray/tests/test_dtypes.py
@@ -29,6 +29,9 @@ class DummyArrayAPINamespace:
         ([np.str_, np.str_], np.str_),
         ([np.bytes_, np.str_], np.object_),
         ([np.dtype("<U2"), np.str_], np.dtype("U")),
+        ([np.dtype("<U2"), str], np.dtype("U")),
+        ([np.dtype("S3"), np.bytes_], np.dtype("S")),
+        ([np.dtype("S10"), bytes], np.dtype("S")),
     ],
 )
 def test_result_type(args, expected) -> None:

From d163934f324b824eabdcce5db405b0fbc8964f03 Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Thu, 10 Oct 2024 20:05:59 +0200
Subject: [PATCH 6/6] more details in the comment

---
 xarray/core/dtypes.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/xarray/core/dtypes.py b/xarray/core/dtypes.py
index af21830ecf7..b81432c19e5 100644
--- a/xarray/core/dtypes.py
+++ b/xarray/core/dtypes.py
@@ -221,7 +221,10 @@ def preprocess_types(t):
     elif isinstance(dtype := getattr(t, "dtype", t), np.dtype) and (
         np.issubdtype(dtype, np.str_) or np.issubdtype(dtype, np.bytes_)
     ):
-        # drop the length from numpy's fixed-width string dtypes, it is better to recalculate that
+        # drop the length from numpy's fixed-width string dtypes, it is better to
+        # recalculate
+        # TODO(keewis): remove once the minimum version of `numpy.result_type` does this
+        # for us
         return dtype.type
     else:
         return t