Resolving #61086 : BUG: Collision between equivalent frequencies 'QS-FEB' and 'QS-NOV'

Author: David Jun

doc/source/whatsnew/v3.0.0.rst

@@ -75,6 +75,7 @@ Other enhancements
 - :meth:`pandas.concat` will raise a ``ValueError`` when ``ignore_index=True`` and ``keys`` is not ``None`` (:issue:`59274`)
 - :py:class:`frozenset` elements in pandas objects are now natively printed (:issue:`60690`)
 - Add ``"delete_rows"`` option to ``if_exists`` argument in :meth:`DataFrame.to_sql` deleting all records of the table before inserting data (:issue:`37210`).
+- Added :func:`DateTimeIndex.set_freq` and :class:`DateTimeIndex` now supports assigning equivalent quarerly start frequency (:issue:`61086`)
 - Added half-year offset classes :class:`HalfYearBegin`, :class:`HalfYearEnd`, :class:`BHalfYearBegin` and :class:`BHalfYearEnd` (:issue:`60928`)
 - Errors occurring during SQL I/O will now throw a generic :class:`.DatabaseError` instead of the raw Exception type from the underlying driver manager library (:issue:`60748`)
 - Implemented :meth:`Series.str.isascii` and :meth:`Series.str.isascii` (:issue:`59091`)

pandas/core/arrays/datetimelike.py

@@ -2519,8 +2519,13 @@ def _validate_inferred_freq(
     -------
     freq : DateOffset or None
     """
+    offset1 = to_offset(freq)
+    offset2 = to_offset(inferred_freq)
+
+    freq_equal = type(offset1) == type(offset2)
+
     if inferred_freq is not None:
-        if freq is not None and freq != inferred_freq:
+        if freq is not None and not freq_equal:
             raise ValueError(
                 f"Inferred frequency {inferred_freq} from passed "
                 "values does not conform to passed frequency "

pandas/core/indexes/datetimes.py

@@ -421,6 +421,34 @@ def _can_range_setop(self, other) -> bool:
 
     # --------------------------------------------------------------------
 
+    def _set_freq(self, freq, *, inplace: bool = False):
+        """
+        Set a new frequency for this DatetimeIndex.
+
+        Parameters
+        ----------
+        freq : str, Timedelta, datetime.timedelta, or DateOffset, default 'S'
+            Frequency strings can have multiples, e.g. '5h'. See
+            :ref:`here <timeseries.offset_aliases>` for a list of
+            frequency aliases.
+        inplace : bool, default False
+            If True, modifies object in place. Otherwise, returns a new DateTimeIndex
+
+        Returns
+        -------
+        DatetimeIndex
+            Fixed frequency DatetimeIndex
+        """
+
+        if inplace:
+            self._freq = to_offset(freq)
+        else:
+            new_index = self.copy()
+            new_index.freq = to_offset(freq)
+            return new_index
+
+    # --------------------------------------------------------------------
+
     def _get_time_micros(self) -> npt.NDArray[np.int64]:
         """
         Return the number of microseconds since midnight.

pandas/tests/indexes/datetimes/test_constructors.py

@@ -1204,3 +1204,12 @@ def test_dti_constructor_object_dtype_dayfirst_yearfirst_with_tz(self):
         result2 = DatetimeIndex([val], tz="US/Pacific", yearfirst=True)
         expected2 = DatetimeIndex([yfirst]).as_unit("s")
         tm.assert_index_equal(result2, expected2)
+
+    def test_datetimeindex_equivalent_freq(self):
+        idx2 = date_range("2020-02-01", freq="QS-FEB", periods=4)
+        new_idx2 = idx2._set_freq("QS-MAY")
+        tm.assert_index_equal(new_idx2, idx2)
+        assert new_idx2.freq == "QS-MAY"
+
+        idx2._set_freq("QS-MAY", inplace=True)
+        assert idx2.freq == "QS-MAY"