BUG: ArrowDtype.numpy_dtype returning ns units for non-nano timestamp and duration types (#51800)

* BUG: ArrowDtype.numpy_dtype returning ns units for non-nano timestamp and duration types

* fix comment

* whatsnew

* reference arrow issue tracker

Author: lukemanley

doc/source/whatsnew/v2.1.0.rst

@@ -145,7 +145,7 @@ Numeric
 
 Conversion
 ^^^^^^^^^^
--
+- Bug in :meth:`ArrowDtype.numpy_dtype` returning nanosecond units for non-nanosecond ``pyarrow.timestamp`` and ``pyarrow.duration`` types (:issue:`51800`)
 -
 
 Strings

pandas/core/arrays/arrow/dtype.py

@@ -148,6 +148,18 @@ def name(self) -> str:  # type: ignore[override]
     @cache_readonly
     def numpy_dtype(self) -> np.dtype:
         """Return an instance of the related numpy dtype"""
+        if pa.types.is_timestamp(self.pyarrow_dtype):
+            # pa.timestamp(unit).to_pandas_dtype() returns ns units
+            # regardless of the pyarrow timestamp units.
+            # This can be removed if/when pyarrow addresses it:
+            # https://github.com/apache/arrow/issues/34462
+            return np.dtype(f"datetime64[{self.pyarrow_dtype.unit}]")
+        if pa.types.is_duration(self.pyarrow_dtype):
+            # pa.duration(unit).to_pandas_dtype() returns ns units
+            # regardless of the pyarrow duration units
+            # This can be removed if/when pyarrow addresses it:
+            # https://github.com/apache/arrow/issues/34462
+            return np.dtype(f"timedelta64[{self.pyarrow_dtype.unit}]")
         if pa.types.is_string(self.pyarrow_dtype):
             # pa.string().to_pandas_dtype() = object which we don't want
             return np.dtype(str)

pandas/tests/extension/test_arrow.py

@@ -640,11 +640,7 @@ def test_get_common_dtype(self, dtype, request):
         if (
             pa.types.is_date(pa_dtype)
             or pa.types.is_time(pa_dtype)
-            or (
-                pa.types.is_timestamp(pa_dtype)
-                and (pa_dtype.unit != "ns" or pa_dtype.tz is not None)
-            )
-            or (pa.types.is_duration(pa_dtype) and pa_dtype.unit != "ns")
+            or (pa.types.is_timestamp(pa_dtype) and pa_dtype.tz is not None)
             or pa.types.is_binary(pa_dtype)
             or pa.types.is_decimal(pa_dtype)
         ):