Backport PR #49080 on branch 1.5.x (REGR: midx.values resetting freq of underyling index) (#49094)

Backport PR #49080: REGR: midx.values resetting freq of underyling index

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

Author: meeseeksmachine

doc/source/whatsnew/v1.5.1.rst

@@ -76,6 +76,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.loc` raising ``FutureWarning`` when setting an empty :class:`DataFrame` (:issue:`48480`)
 - Fixed regression in :meth:`DataFrame.describe` raising ``TypeError`` when result contains ``NA`` (:issue:`48778`)
 - Fixed regression in :meth:`DataFrame.plot` ignoring invalid ``colormap`` for ``kind="scatter"`` (:issue:`48726`)
+- Fixed regression in :meth:`MultiIndex.values`` resetting ``freq`` attribute of underlying :class:`Index` object (:issue:`49054`)
 - Fixed performance regression in :func:`factorize` when ``na_sentinel`` is not ``None`` and ``sort=False`` (:issue:`48620`)
 - Fixed regression causing an ``AttributeError`` during warning emitted if the provided table name in :meth:`DataFrame.to_sql` and the table name actually used in the database do not match (:issue:`48733`)
 - Fixed regression in :func:`to_datetime` when ``arg`` was a date string with nanosecond and ``format`` contained ``%f`` would raise a ``ValueError`` (:issue:`48767`)

pandas/core/indexes/multi.py

@@ -723,21 +723,23 @@ def _values(self) -> np.ndarray:
                 vals = cast("CategoricalIndex", vals)
                 vals = vals._data._internal_get_values()
 
-            if isinstance(vals, ABCDatetimeIndex):
+            is_dti = isinstance(vals, ABCDatetimeIndex)
+
+            if is_dti:
                 # TODO: this can be removed after Timestamp.freq is removed
                 # The astype(object) below does not remove the freq from
                 # the underlying Timestamps so we remove it here to match
                 # the behavior of self._get_level_values
-                vals = vals.copy()
-                vals.freq = None
+                vals = algos.take_nd(vals, codes, fill_value=index._na_value)
 
             if isinstance(vals.dtype, ExtensionDtype) or isinstance(
                 vals, (ABCDatetimeIndex, ABCTimedeltaIndex)
             ):
                 vals = vals.astype(object)
 
             vals = np.array(vals, copy=False)
-            vals = algos.take_nd(vals, codes, fill_value=index._na_value)
+            if not is_dti:
+                vals = algos.take_nd(vals, codes, fill_value=index._na_value)
             values.append(vals)
 
         arr = lib.fast_zip(values)

pandas/tests/indexes/multi/test_get_level_values.py

@@ -112,3 +112,14 @@ def test_get_level_values_when_periods():
         [idx._get_level_values(level) for level in range(idx.nlevels)]
     )
     assert all(x.is_monotonic_increasing for x in idx2.levels)
+
+
+def test_values_loses_freq_of_underlying_index():
+    # GH#49054
+    idx = pd.DatetimeIndex(date_range("20200101", periods=3, freq="BM"))
+    expected = idx.copy(deep=True)
+    idx2 = Index([1, 2, 3])
+    midx = MultiIndex(levels=[idx, idx2], codes=[[0, 1, 2], [0, 1, 2]])
+    midx.values
+    assert idx.freq is not None
+    tm.assert_index_equal(idx, expected)