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

Author: David Jun

doc/source/whatsnew/v3.0.0.rst

@@ -75,7 +75,6 @@ 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`)
@@ -655,6 +654,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
 - Bug in :meth:`DatetimeIndex.union` and :meth:`DatetimeIndex.intersection` when ``unit`` was non-nanosecond (:issue:`59036`)
 - Bug in :meth:`Series.dt.microsecond` producing incorrect results for pyarrow backed :class:`Series`. (:issue:`59154`)
+- Bug in :meth:`_validate_inferred_freq` where DateTimeIndex incorrectly raised a ``ValueError`` when assigning a logically equivalent frequency (:issue:`61086`)
 - Bug in :meth:`to_datetime` not respecting dayfirst if an uncommon date string was passed. (:issue:`58859`)
 - Bug in :meth:`to_datetime` on float array with missing values throwing ``FloatingPointError`` (:issue:`58419`)
 - Bug in :meth:`to_datetime` on float32 df with year, month, day etc. columns leads to precision issues and incorrect result. (:issue:`60506`)

pandas/core/indexes/datetimes.py

@@ -421,34 +421,6 @@ 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

@@ -1205,11 +1205,9 @@ def test_dti_constructor_object_dtype_dayfirst_yearfirst_with_tz(self):
         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"
+    def test_validate_inferred_freq_equivalence(self):
+        idx = date_range("2020-02-01", freq="QS-FEB", periods=4)
+
+        new_idx = DatetimeIndex(idx, freq="QS-MAY")
+
+        assert isinstance(new_idx, DatetimeIndex)