API (string): return str dtype for .dt methods, DatetimeIndex methods (#59526)

* API (string): return str dtype for .dt methods, DatetimeIndex methods

* mypy fixup

Author: jbrockmendel

pandas/core/arrays/datetimelike.py

@@ -20,6 +20,8 @@
 
 import numpy as np
 
+from pandas._config import using_string_dtype
+
 from pandas._libs import (
     algos,
     lib,
@@ -1789,6 +1791,10 @@ def strftime(self, date_format: str) -> npt.NDArray[np.object_]:
               dtype='object')
         """
         result = self._format_native_types(date_format=date_format, na_rep=np.nan)
+        if using_string_dtype():
+            from pandas import StringDtype
+
+            return pd_array(result, dtype=StringDtype(na_value=np.nan))  # type: ignore[return-value]
         return result.astype(object, copy=False)
 
 

pandas/core/arrays/datetimes.py

@@ -14,6 +14,8 @@
 
 import numpy as np
 
+from pandas._config import using_string_dtype
+
 from pandas._libs import (
     lib,
     tslib,
@@ -1306,6 +1308,13 @@ def month_name(self, locale=None) -> npt.NDArray[np.object_]:
             values, "month_name", locale=locale, reso=self._creso
         )
         result = self._maybe_mask_results(result, fill_value=None)
+        if using_string_dtype():
+            from pandas import (
+                StringDtype,
+                array as pd_array,
+            )
+
+            return pd_array(result, dtype=StringDtype(na_value=np.nan))  # type: ignore[return-value]
         return result
 
     def day_name(self, locale=None) -> npt.NDArray[np.object_]:
@@ -1363,6 +1372,14 @@ def day_name(self, locale=None) -> npt.NDArray[np.object_]:
             values, "day_name", locale=locale, reso=self._creso
         )
         result = self._maybe_mask_results(result, fill_value=None)
+        if using_string_dtype():
+            # TODO: no tests that check for dtype of result as of 2024-08-15
+            from pandas import (
+                StringDtype,
+                array as pd_array,
+            )
+
+            return pd_array(result, dtype=StringDtype(na_value=np.nan))  # type: ignore[return-value]
         return result
 
     @property

pandas/core/indexes/datetimes.py

@@ -276,7 +276,7 @@ def _engine_type(self) -> type[libindex.DatetimeEngine]:
     @doc(DatetimeArray.strftime)
     def strftime(self, date_format) -> Index:
         arr = self._data.strftime(date_format)
-        return Index(arr, name=self.name, dtype=object)
+        return Index(arr, name=self.name, dtype=arr.dtype)
 
     @doc(DatetimeArray.tz_convert)
     def tz_convert(self, tz) -> Self:

pandas/core/indexes/extension.py

@@ -71,7 +71,7 @@ def fget(self):
                         return type(self)._simple_new(result, name=self.name)
                     elif isinstance(result, ABCDataFrame):
                         return result.set_index(self)
-                    return Index(result, name=self.name)
+                    return Index(result, name=self.name, dtype=result.dtype)
                 return result
 
             def fset(self, value) -> None:
@@ -98,7 +98,7 @@ def method(self, *args, **kwargs):  # type: ignore[misc]
                     return type(self)._simple_new(result, name=self.name)
                 elif isinstance(result, ABCDataFrame):
                     return result.set_index(self)
-                return Index(result, name=self.name)
+                return Index(result, name=self.name, dtype=result.dtype)
             return result
 
         # error: "property" has no attribute "__name__"

pandas/tests/arrays/test_datetimelike.py

@@ -889,20 +889,24 @@ def test_concat_same_type_different_freq(self, unit):
 
         tm.assert_datetime_array_equal(result, expected)
 
-    def test_strftime(self, arr1d):
+    def test_strftime(self, arr1d, using_infer_string):
         arr = arr1d
 
         result = arr.strftime("%Y %b")
         expected = np.array([ts.strftime("%Y %b") for ts in arr], dtype=object)
-        tm.assert_numpy_array_equal(result, expected)
+        if using_infer_string:
+            expected = pd.array(expected, dtype=pd.StringDtype(na_value=np.nan))
+        tm.assert_equal(result, expected)
 
-    def test_strftime_nat(self):
+    def test_strftime_nat(self, using_infer_string):
         # GH 29578
         arr = DatetimeIndex(["2019-01-01", NaT])._data
 
         result = arr.strftime("%Y-%m-%d")
         expected = np.array(["2019-01-01", np.nan], dtype=object)
-        tm.assert_numpy_array_equal(result, expected)
+        if using_infer_string:
+            expected = pd.array(expected, dtype=pd.StringDtype(na_value=np.nan))
+        tm.assert_equal(result, expected)
 
 
 class TestTimedeltaArray(SharedTests):
@@ -1159,20 +1163,24 @@ def test_array_interface(self, arr1d):
         expected = np.asarray(arr).astype("S20")
         tm.assert_numpy_array_equal(result, expected)
 
-    def test_strftime(self, arr1d):
+    def test_strftime(self, arr1d, using_infer_string):
         arr = arr1d
 
         result = arr.strftime("%Y")
         expected = np.array([per.strftime("%Y") for per in arr], dtype=object)
-        tm.assert_numpy_array_equal(result, expected)
+        if using_infer_string:
+            expected = pd.array(expected, dtype=pd.StringDtype(na_value=np.nan))
+        tm.assert_equal(result, expected)
 
-    def test_strftime_nat(self):
+    def test_strftime_nat(self, using_infer_string):
         # GH 29578
         arr = PeriodArray(PeriodIndex(["2019-01-01", NaT], dtype="period[D]"))
 
         result = arr.strftime("%Y-%m-%d")
         expected = np.array(["2019-01-01", np.nan], dtype=object)
-        tm.assert_numpy_array_equal(result, expected)
+        if using_infer_string:
+            expected = pd.array(expected, dtype=pd.StringDtype(na_value=np.nan))
+        tm.assert_equal(result, expected)
 
 
 @pytest.mark.parametrize(

pandas/tests/series/accessors/test_dt_accessor.py

@@ -29,6 +29,7 @@
     Period,
     PeriodIndex,
     Series,
+    StringDtype,
     TimedeltaIndex,
     date_range,
     period_range,
@@ -528,7 +529,6 @@ def test_dt_accessor_datetime_name_accessors(self, time_locale):
         ser = pd.concat([ser, Series([pd.NaT])])
         assert np.isnan(ser.dt.month_name(locale=time_locale).iloc[-1])
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     def test_strftime(self):
         # GH 10086
         ser = Series(date_range("20130101", periods=5))
@@ -599,10 +599,9 @@ def test_strftime_period_days(self, using_infer_string):
             dtype="=U10",
         )
         if using_infer_string:
-            expected = expected.astype("str")
+            expected = expected.astype(StringDtype(na_value=np.nan))
         tm.assert_index_equal(result, expected)
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     def test_strftime_dt64_microsecond_resolution(self):
         ser = Series([datetime(2013, 1, 1, 2, 32, 59), datetime(2013, 1, 2, 14, 32, 1)])
         result = ser.dt.strftime("%Y-%m-%d %H:%M:%S")
@@ -635,7 +634,6 @@ def test_strftime_period_minutes(self):
         )
         tm.assert_series_equal(result, expected)
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)", strict=False)
     @pytest.mark.parametrize(
         "data",
         [
@@ -658,7 +656,7 @@ def test_strftime_all_nat(self, data):
         ser = Series(data)
         with tm.assert_produces_warning(None):
             result = ser.dt.strftime("%Y-%m-%d")
-        expected = Series([np.nan], dtype=object)
+        expected = Series([np.nan], dtype="str")
         tm.assert_series_equal(result, expected)
 
     def test_valid_dt_with_missing_values(self):