From ec0cb89275baac5f1cc67bdf25d11b4ab7c7b52d Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Mon, 7 Jul 2025 14:49:16 +0200
Subject: [PATCH] TST: update expected dtype for sum of decimals with pyarrow
 21+

---
 pandas/compat/__init__.py            | 2 ++
 pandas/compat/pyarrow.py             | 2 ++
 pandas/tests/extension/test_arrow.py | 6 +++++-
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/pandas/compat/__init__.py b/pandas/compat/__init__.py
index 8ed19f97958b9..d5dbcb74d29e4 100644
--- a/pandas/compat/__init__.py
+++ b/pandas/compat/__init__.py
@@ -35,6 +35,7 @@
     pa_version_under18p0,
     pa_version_under19p0,
     pa_version_under20p0,
+    pa_version_under21p0,
 )
 
 if TYPE_CHECKING:
@@ -168,4 +169,5 @@ def is_ci_environment() -> bool:
     "pa_version_under18p0",
     "pa_version_under19p0",
     "pa_version_under20p0",
+    "pa_version_under21p0",
 ]
diff --git a/pandas/compat/pyarrow.py b/pandas/compat/pyarrow.py
index 569d702592982..1e1989b276eb6 100644
--- a/pandas/compat/pyarrow.py
+++ b/pandas/compat/pyarrow.py
@@ -18,6 +18,7 @@
     pa_version_under18p0 = _palv < Version("18.0.0")
     pa_version_under19p0 = _palv < Version("19.0.0")
     pa_version_under20p0 = _palv < Version("20.0.0")
+    pa_version_under21p0 = _palv < Version("21.0.0")
     HAS_PYARROW = _palv >= Version("12.0.1")
 except ImportError:
     pa_version_under12p1 = True
@@ -30,4 +31,5 @@
     pa_version_under18p0 = True
     pa_version_under19p0 = True
     pa_version_under20p0 = True
+    pa_version_under21p0 = True
     HAS_PYARROW = False
diff --git a/pandas/tests/extension/test_arrow.py b/pandas/tests/extension/test_arrow.py
index 1bec5f7303355..8db837b176fe9 100644
--- a/pandas/tests/extension/test_arrow.py
+++ b/pandas/tests/extension/test_arrow.py
@@ -43,6 +43,7 @@
     pa_version_under14p0,
     pa_version_under19p0,
     pa_version_under20p0,
+    pa_version_under21p0,
 )
 
 from pandas.core.dtypes.dtypes import (
@@ -542,7 +543,10 @@ def _get_expected_reduction_dtype(self, arr, op_name: str, skipna: bool):
             else:
                 cmp_dtype = arr.dtype
         elif arr.dtype.name == "decimal128(7, 3)[pyarrow]":
-            if op_name not in ["median", "var", "std", "sem", "skew"]:
+            if op_name == "sum" and not pa_version_under21p0:
+                # https://github.com/apache/arrow/pull/44184
+                cmp_dtype = ArrowDtype(pa.decimal128(38, 3))
+            elif op_name not in ["median", "var", "std", "sem", "skew"]:
                 cmp_dtype = arr.dtype
             else:
                 cmp_dtype = "float64[pyarrow]"