BUG: DataFrame.drop unexpectedly drops frequency

doc/source/whatsnew/v3.0.0.rst

@@ -388,6 +388,7 @@ Categorical
 Datetimelike
 ^^^^^^^^^^^^
 - Bug in :class:`Timestamp` constructor failing to raise when ``tz=None`` is explicitly specified in conjunction with timezone-aware ``tzinfo`` or data (:issue:`48688`)
+- Bug in :func:`Datafreme.drop` returning ``Freq=None`` when the dataframe has a ``DatetimeIndex`` (:issue:`58743`)
 - Bug in :func:`date_range` where the last valid timestamp would sometimes not be produced (:issue:`56134`)
 - Bug in :func:`date_range` where using a negative frequency value would not include all points between the start and end values (:issue:`56382`)
 - Bug in :func:`tseries.api.guess_datetime_format` would fail to infer time format when "%Y" == "%H%M" (:issue:`57452`)

pandas/core/indexes/base.py

@@ -39,6 +39,7 @@
     IncompatibleFrequency,
     OutOfBoundsDatetime,
     Timestamp,
+    to_offset,
     tz_compare,
 )
 from pandas._typing import (
@@ -6949,7 +6950,15 @@ def drop(
             if errors != "ignore":
                 raise KeyError(f"{labels[mask].tolist()} not found in axis")
             indexer = indexer[~mask]
-        return self.delete(indexer)
+        new_index = self.delete(indexer)
+
+        # check if we need to set the freq attribute
+        from pandas import DatetimeIndex
+
+        if isinstance(self, DatetimeIndex) and isinstance(new_index, DatetimeIndex):
+            new_index.freq = to_offset(new_index.inferred_freq)
+
+        return new_index
 
     @final
     def infer_objects(self, copy: bool = True) -> Index:

pandas/tests/generic/test_generic.py

@@ -483,3 +483,19 @@ def test_flags_identity(self, frame_or_series):
         assert obj.flags is obj.flags
         obj2 = obj.copy()
         assert obj2.flags is not obj.flags
+
+    @pytest.mark.parametrize("freq", ["YE", "ME", "D"])
+    def test_drop_method_freq_preservation(self, freq):
+        start = "1970-01-01"
+        periods = 10
+
+        index = date_range(start=start, periods=10, freq=freq)
+        df = DataFrame((np.ones(len(index))), index=index)
+
+        # set inplace as false
+        test_df = df.drop(index=df.index[[0, periods - 1]], inplace=False)
+        tm.assert_equal(test_df.index.freq, index.freq)
+
+        # set inplace as true
+        df.drop(index=df.index[[0, periods - 1]], inplace=True)
+        tm.assert_equal(df.index.freq, index.freq)

pandas/tests/indexes/datetimes/methods/test_delete.py

@@ -132,9 +132,9 @@ def test_delete_slice2(self, tz, unit):
         assert result.freq == expected.freq
         assert result.tz == expected.tz
 
-        # reset freq to None
+        # reset freq
         result = ts.drop(ts.index[[1, 3, 5, 7, 9]]).index
-        expected = dti[::2]._with_freq(None)
+        expected = dti[::2]._with_freq("infer")
         tm.assert_index_equal(result, expected)
         assert result.name == expected.name
         assert result.freq == expected.freq