TST: update expected dtype for sum of decimals with pyarrow 21+ (#61799)

Author: jorisvandenbossche

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",
 ]

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

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]"