Return dtype with appropriate resolution in DatetimeTZDtype.base (#52706)

* Return dtype with appropriate resolution in DatetimeTZDtype.base

* Whatsnew entry

* Ignore type error

---------

Co-authored-by: Ashwin Srinath <[email protected]>

Author: shwina

doc/source/whatsnew/v2.1.0.rst

@@ -300,6 +300,7 @@ Timedelta
 Timezones
 ^^^^^^^^^
 - Bug in :func:`infer_freq` that raises ``TypeError`` for ``Series`` of timezone-aware timestamps (:issue:`52456`)
+- Bug in :meth:`DatetimeTZDtype.base` that always returns a NumPy dtype with nanosecond resolution (:issue:`52705`)
 -
 
 Numeric

pandas/core/dtypes/dtypes.py

@@ -676,7 +676,6 @@ class DatetimeTZDtype(PandasExtensionDtype):
     type: type[Timestamp] = Timestamp
     kind: str_type = "M"
     num = 101
-    base = np.dtype("M8[ns]")  # TODO: depend on reso?
     _metadata = ("unit", "tz")
     _match = re.compile(r"(datetime64|M8)\[(?P<unit>.+), (?P<tz>.+)\]")
     _cache_dtypes: dict[str_type, PandasExtensionDtype] = {}
@@ -685,6 +684,10 @@ class DatetimeTZDtype(PandasExtensionDtype):
     def na_value(self) -> NaTType:
         return NaT
 
+    @cache_readonly
+    def base(self) -> DtypeObj:  # type: ignore[override]
+        return np.dtype(f"M8[{self.unit}]")
+
     # error: Signature of "str" incompatible with supertype "PandasExtensionDtype"
     @cache_readonly
     def str(self) -> str:  # type: ignore[override]

pandas/tests/dtypes/test_dtypes.py

@@ -276,6 +276,7 @@ def test_construction_non_nanosecond(self):
         assert res._creso == NpyDatetimeUnit.NPY_FR_ms.value
         assert res.str == "|M8[ms]"
         assert str(res) == "datetime64[ms, US/Eastern]"
+        assert res.base == np.dtype("M8[ms]")
 
     def test_day_not_supported(self):
         msg = "DatetimeTZDtype only supports s, ms, us, ns units"