Resample fix dst transition

doc/source/whatsnew/v1.2.0.rst

@@ -307,6 +307,7 @@ Groupby/resample/rolling
 
 - Bug in :meth:`DataFrameGroupBy.count` and :meth:`SeriesGroupBy.sum` returning ``NaN`` for missing categories when grouped on multiple ``Categoricals``. Now returning ``0`` (:issue:`35028`)
 - Bug in :meth:`DataFrameGroupBy.apply` that would some times throw an erroneous ``ValueError`` if the grouping axis had duplicate entries (:issue:`16646`)
+- Bug in :meth:`DataFrame.resample(...)` that would throw a ``ValueError`` when resampling from "D" to "24H" over a transition into daylight savings time (DST) (:issue:`35219`)
 - Bug when combining methods :meth:`DataFrame.groupby` with :meth:`DataFrame.resample` and :meth:`DataFrame.interpolate` raising an ``TypeError`` (:issue:`35325`)
 - Bug in :meth:`DataFrameGroupBy.apply` where a non-nuisance grouping column would be dropped from the output columns if another groupby method was called before ``.apply()`` (:issue:`34656`)
 - Bug in :meth:`DataFrameGroupby.apply` would drop a :class:`CategoricalIndex` when grouped on. (:issue:`35792`)

pandas/core/resample.py

@@ -1087,7 +1087,11 @@ def _upsample(self, method, limit=None, fill_value=None):
         res_index = self._adjust_binner_for_upsample(binner)
 
         # if we have the same frequency as our axis, then we are equal sampling
-        if limit is None and to_offset(ax.inferred_freq) == self.freq:
+        if (
+            limit is None
+            and to_offset(ax.inferred_freq) == self.freq
+            and len(obj) == len(res_index)
+        ):
             result = obj.copy()
             result.index = res_index
         else:

pandas/tests/resample/test_datetime_index.py

@@ -1740,3 +1740,50 @@ def test_resample_apply_product():
         columns=["A", "B"],
     )
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "first,last,freq_in,freq_out,exp_last",
+    [
+        (
+            "2020-03-28",
+            "2020-03-31",
+            "D",
+            "24H",
+            "2020-03-30 01:00",
+        ),  # includes transition into DST
+        (
+            "2020-03-28",
+            "2020-10-27",
+            "D",
+            "24H",
+            "2020-10-27 00:00",
+        ),  # includes transition into and out of DST
+        (
+            "2020-10-25",
+            "2020-10-27",
+            "D",
+            "24H",
+            "2020-10-26 23:00",
+        ),  # includes transition out of DST
+        (
+            "2020-03-28",
+            "2020-03-31",
+            "24H",
+            "D",
+            "2020-03-30 00:00",
+        ),  # same as above, but from 24H to D
+        ("2020-03-28", "2020-10-27", "24H", "D", "2020-10-27 00:00"),
+        ("2020-10-25", "2020-10-27", "24H", "D", "2020-10-26 00:00"),
+    ],
+)
+def test_resample_calendar_day_with_dst(
+    first: str, last: str, freq_in: str, freq_out: str, exp_last: str
+):
+    # GH 35219
+    ts = pd.Series(1.0, pd.date_range(first, last, freq=freq_in, tz="Europe/Amsterdam"))
+    result = ts.resample(freq_out).pad()
+    expected = pd.Series(
+        1.0, pd.date_range(first, exp_last, freq=freq_out, tz="Europe/Amsterdam")
+    )
+    tm.assert_series_equal(result, expected)