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

@@ -6949,7 +6949,15 @@ 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):
+            new_index.freq = self.freq
+
+        return new_index
 
     @final
     def infer_objects(self, copy: bool = True) -> Index:

pandas/tests/generic/test_generic.py

@@ -3,6 +3,7 @@
     deepcopy,
 )
 
+from datetime import datetime
 import numpy as np
 import pytest
 
@@ -12,6 +13,7 @@
     DataFrame,
     Index,
     Series,
+    Timestamp,
     date_range,
 )
 import pandas._testing as tm
@@ -483,3 +485,17 @@ 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", ["Y", "M", "D"])
+    def test_drop_method_freq_preservation(self, freq):
+        start = "1970-01-01"
+        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], inplace=False)
+        tm.assert_equal(test_df.index.freq, index.freq)
+
+        # set inplace as true
+        df.drop(index=df.index[0], inplace=True)
+        tm.assert_equal(df.index.freq, index.freq)