BUG: DataFrame.apply not adding a frequency if freq=None (#22150)

doc/source/whatsnew/v0.24.0.txt

@@ -635,7 +635,7 @@ Datetimelike
 - Bug in :meth:`DataFrame.eq` comparison against ``NaT`` incorrectly returning ``True`` or ``NaN`` (:issue:`15697`, :issue:`22163`)
 - Bug in :class:`DatetimeIndex` subtraction that incorrectly failed to raise ``OverflowError`` (:issue:`22492`, :issue:`22508`)
 - Bug in :class:`DatetimeIndex` incorrectly allowing indexing with ``Timedelta`` object (:issue:`20464`)
--
+- Bug in :class:`DatetimeIndex` where frequency was being set if original frequency was ``None`` (:issue:`22150`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/indexes/datetimes.py

@@ -860,8 +860,6 @@ def union_many(self, others):
                 if isinstance(this, DatetimeIndex):
                     this._tz = timezones.tz_standardize(tz)
 
-        if this.freq is None:
-            this.freq = to_offset(this.inferred_freq)
         return this
 
     def join(self, other, how='left', level=None, return_indexers=False,

pandas/tests/frame/test_apply.py

@@ -11,6 +11,8 @@
 
 import warnings
 import numpy as np
+from hypothesis import given
+from hypothesis.strategies import composite, dates, integers, sampled_from
 
 from pandas import (notna, DataFrame, Series, MultiIndex, date_range,
                     Timestamp, compat)
@@ -1155,3 +1157,24 @@ def test_agg_cython_table_raises(self, df, func, expected, axis):
         # GH21224
         with pytest.raises(expected):
             df.agg(func, axis=axis)
+
+    @composite
+    def indices(draw, max_length=5):
+        date = draw(
+            dates(
+                min_value=Timestamp.min.ceil("D").to_pydatetime().date(),
+                max_value=Timestamp.max.floor("D").to_pydatetime().date(),
+            ).map(Timestamp)
+        )
+        periods = draw(integers(0, max_length))
+        freq = draw(sampled_from(list("BDHTS")))
+        dr = date_range(date, periods=periods, freq=freq)
+        return pd.DatetimeIndex(list(dr))
+
+    @given(index=indices(5), num_columns=integers(0, 5))
+    def test_frequency_is_original(self, index, num_columns):
+        # GH22150
+        original = index.copy()
+        df = DataFrame(True, index=index, columns=range(num_columns))
+        df.apply(lambda x: x)
+        assert index.freq == original.freq