Implement day_name and month_name to match pandas (#15479)

This PR implements the `month_name` and `day_name` datetime methods, matching the equivalent [month_name](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.dt.month_name.html) and [day_name](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.dt.day_name.html) methods from pandas.

Currently this is implemented just for English locale, though it could be expanded to include additional languages in the future. 

Closes #12407

Authors:
  - Brian Tepera (https://github.com/btepera)
  - GALI PREM SAGAR (https://github.com/galipremsagar)
  - Matthew Roeschke (https://github.com/mroeschke)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #15479

python/cudf/cudf/core/column/datetime.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import calendar
 import datetime
 import functools
 import locale
@@ -339,6 +340,34 @@ def element_indexing(self, index: int):
     def get_dt_field(self, field: str) -> ColumnBase:
         return libcudf.datetime.extract_datetime_component(self, field)
 
+    def _get_field_names(
+        self,
+        field: Literal["month", "weekday"],
+        labels: list[str],
+        locale: str | None = None,
+    ) -> ColumnBase:
+        if locale is not None:
+            raise NotImplementedError(
+                "Setting a locale is currently not supported. "
+                "Results will be returned in your current locale."
+            )
+        col_labels = as_column(labels)
+        indices = self.get_dt_field(field)
+        has_nulls = indices.has_nulls()
+        if has_nulls:
+            indices = indices.fillna(len(col_labels))
+        return col_labels.take(indices, nullify=True, check_bounds=has_nulls)
+
+    def get_day_names(self, locale: str | None = None) -> ColumnBase:
+        return self._get_field_names(
+            "weekday", list(calendar.day_name), locale=locale
+        )
+
+    def get_month_names(self, locale: str | None = None) -> ColumnBase:
+        return self._get_field_names(
+            "month", list(calendar.month_name), locale=locale
+        )
+
     def ceil(self, freq: str) -> ColumnBase:
         return libcudf.datetime.ceil_datetime(self, freq)
 

python/cudf/cudf/core/index.py

@@ -2120,6 +2120,45 @@ def quarter(self):
         res = extract_quarter(self._values)
         return Index(res, dtype="int8")
 
+    @_cudf_nvtx_annotate
+    def day_name(self, locale: str | None = None) -> Index:
+        """
+        Return the day names. Currently supports English locale only.
+
+        Examples
+        --------
+        >>> import cudf
+        >>> datetime_index = cudf.date_range("2016-12-31", "2017-01-08", freq="D")
+        >>> datetime_index
+        DatetimeIndex(['2016-12-31', '2017-01-01', '2017-01-02', '2017-01-03',
+                       '2017-01-04', '2017-01-05', '2017-01-06', '2017-01-07'],
+                      dtype='datetime64[ns]', freq='D')
+        >>> datetime_index.day_name()
+        Index(['Saturday', 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday',
+               'Friday', 'Saturday'], dtype='object')
+        """
+        day_names = self._column.get_day_names(locale)
+        return Index._from_data({self.name: day_names})
+
+    @_cudf_nvtx_annotate
+    def month_name(self, locale: str | None = None) -> Index:
+        """
+        Return the month names. Currently supports English locale only.
+
+        Examples
+        --------
+        >>> import cudf
+        >>> datetime_index = cudf.date_range("2017-12-30", periods=6, freq='W')
+        >>> datetime_index
+        DatetimeIndex(['2017-12-30', '2018-01-06', '2018-01-13', '2018-01-20',
+                    '2018-01-27', '2018-02-03'],
+                      dtype='datetime64[ns]', freq='7D')
+        >>> datetime_index.month_name()
+        Index(['December', 'January', 'January', 'January', 'January', 'February'], dtype='object')
+        """
+        month_names = self._column.get_month_names(locale)
+        return Index._from_data({self.name: month_names})
+
     @_cudf_nvtx_annotate
     def isocalendar(self):
         """

python/cudf/cudf/core/series.py

@@ -4201,6 +4201,78 @@ def quarter(self):
             name=self.series.name,
         )
 
+    @_cudf_nvtx_annotate
+    def day_name(self, locale=None):
+        """
+        Return the day names. Currently supports English locale only.
+
+        Examples
+        --------
+        >>> import cudf
+        >>> datetime_series = cudf.Series(cudf.date_range('2016-12-31',
+        ...     '2017-01-08', freq='D'))
+        >>> datetime_series
+        0   2016-12-31
+        1   2017-01-01
+        2   2017-01-02
+        3   2017-01-03
+        4   2017-01-04
+        5   2017-01-05
+        6   2017-01-06
+        7   2017-01-07
+        8   2017-01-08
+        dtype: datetime64[ns]
+        >>> datetime_series.dt.day_name()
+        0     Saturday
+        1       Sunday
+        2       Monday
+        3      Tuesday
+        4    Wednesday
+        5     Thursday
+        6       Friday
+        7     Saturday
+        dtype: object
+        """
+        day_names = self.series._column.get_day_names(locale)
+        return Series._from_data(
+            ColumnAccessor({None: day_names}),
+            index=self.series.index,
+            name=self.series.name,
+        )
+
+    @_cudf_nvtx_annotate
+    def month_name(self, locale: str | None = None) -> Series:
+        """
+        Return the month names. Currently supports English locale only.
+
+        Examples
+        --------
+        >>> import cudf
+        >>> datetime_series = cudf.Series(cudf.date_range("2017-12-30", periods=6, freq='W'))
+        >>> datetime_series
+        0   2017-12-30
+        1   2018-01-06
+        2   2018-01-13
+        3   2018-01-20
+        4   2018-01-27
+        5   2018-02-03
+        dtype: datetime64[ns]
+        >>> datetime_series.dt.month_name()
+        0    December
+        1     January
+        2     January
+        3     January
+        4     January
+        5    February
+        dtype: object
+        """
+        month_names = self.series._column.get_month_names(locale)
+        return Series._from_data(
+            ColumnAccessor({None: month_names}),
+            index=self.series.index,
+            name=self.series.name,
+        )
+
     @_cudf_nvtx_annotate
     def isocalendar(self):
         """

python/cudf/cudf/tests/test_datetime.py

@@ -2419,3 +2419,42 @@ def test_date_range_tz():
     result = pd.date_range("2020-01-01", "2020-01-02", periods=2, tz="UTC")
     expected = cudf.date_range("2020-01-01", "2020-01-02", periods=2, tz="UTC")
     assert_eq(result, expected)
+
+
+@pytest.mark.parametrize("meth", ["day_name", "month_name"])
+@pytest.mark.parametrize("klass", [pd.Series, pd.DatetimeIndex])
+def test_day_month_name(meth, klass):
+    data = [
+        "2020-05-31 08:00:00",
+        None,
+        "1999-12-31 18:40:00",
+        "2000-12-31 04:00:00",
+        None,
+        "1900-02-28 07:00:00",
+        "1800-03-14 07:30:00",
+        "2100-03-14 07:30:00",
+        "1970-01-01 00:00:00",
+        "1969-12-31 12:59:00",
+    ]
+
+    p_obj = klass(data, dtype="datetime64[s]")
+    g_obj = cudf.from_pandas(p_obj)
+
+    if klass is pd.Series:
+        p_obj = p_obj.dt
+        g_obj = g_obj.dt
+
+    expect = getattr(p_obj, meth)()
+    got = getattr(g_obj, meth)()
+
+    assert_eq(expect, got)
+
+
+@pytest.mark.parametrize("meth", ["day_name", "month_name"])
+@pytest.mark.parametrize("klass", [cudf.Series, cudf.DatetimeIndex])
+def test_day_month_name_locale_not_implemented(meth, klass):
+    obj = klass(cudf.date_range("2020-01-01", periods=7))
+    if klass is cudf.Series:
+        obj = obj.dt
+    with pytest.raises(NotImplementedError):
+        getattr(obj, meth)(locale="pt_BR.utf8")