BUG: DataFrame.resample is changing the index type to MultiIndex when the dataframe is empty (#61174)

* BUG: DataFrame.resample is changing the index type to MultiIndex when the dataframe is empty

* reformat

---------

Co-authored-by: rmorotti <[email protected]>

Author: morotti

doc/source/whatsnew/v3.0.0.rst

@@ -772,6 +772,7 @@ Groupby/resample/rolling
 - Bug in :meth:`.DataFrameGroupBy.quantile` when ``interpolation="nearest"`` is inconsistent with :meth:`DataFrame.quantile` (:issue:`47942`)
 - Bug in :meth:`.Resampler.interpolate` on a :class:`DataFrame` with non-uniform sampling and/or indices not aligning with the resulting resampled index would result in wrong interpolation (:issue:`21351`)
 - Bug in :meth:`DataFrame.ewm` and :meth:`Series.ewm` when passed ``times`` and aggregation functions other than mean (:issue:`51695`)
+- Bug in :meth:`DataFrame.resample` changing index type to :class:`MultiIndex` when the dataframe is empty and using an upsample method (:issue:`55572`)
 - Bug in :meth:`DataFrameGroupBy.agg` that raises ``AttributeError`` when there is dictionary input and duplicated columns, instead of returning a DataFrame with the aggregation of all duplicate columns. (:issue:`55041`)
 - Bug in :meth:`DataFrameGroupBy.apply` and :meth:`SeriesGroupBy.apply` for empty data frame with ``group_keys=False`` still creating output index using group keys. (:issue:`60471`)
 - Bug in :meth:`DataFrameGroupBy.apply` that was returning a completely empty DataFrame when all return values of ``func`` were ``None`` instead of returning an empty DataFrame with the original columns and dtypes. (:issue:`57775`)

pandas/core/resample.py

@@ -507,22 +507,12 @@ def _wrap_result(self, result):
         """
         Potentially wrap any results.
         """
-        # GH 47705
-        obj = self.obj
-        if (
-            isinstance(result, ABCDataFrame)
-            and len(result) == 0
-            and not isinstance(result.index, PeriodIndex)
-        ):
-            result = result.set_index(
-                _asfreq_compat(obj.index[:0], freq=self.freq), append=True
-            )
-
         if isinstance(result, ABCSeries) and self._selection is not None:
             result.name = self._selection
 
         if isinstance(result, ABCSeries) and result.empty:
             # When index is all NaT, result is empty but index is not
+            obj = self.obj
             result.index = _asfreq_compat(obj.index[:0], freq=self.freq)
             result.name = getattr(obj, "name", None)
 
@@ -1756,6 +1746,17 @@ def func(x):
             return x.apply(f, *args, **kwargs)
 
         result = self._groupby.apply(func)
+
+        # GH 47705
+        if (
+            isinstance(result, ABCDataFrame)
+            and len(result) == 0
+            and not isinstance(result.index, PeriodIndex)
+        ):
+            result = result.set_index(
+                _asfreq_compat(self.obj.index[:0], freq=self.freq), append=True
+            )
+
         return self._wrap_result(result)
 
     _upsample = _apply

pandas/tests/resample/test_base.py

@@ -438,6 +438,24 @@ def test_resample_size_empty_dataframe(freq, index):
     tm.assert_series_equal(result, expected)
 
 
+@pytest.mark.parametrize("index", [DatetimeIndex([]), TimedeltaIndex([])])
+@pytest.mark.parametrize("freq", ["D", "h"])
+@pytest.mark.parametrize(
+    "method", ["ffill", "bfill", "nearest", "asfreq", "interpolate", "mean"]
+)
+def test_resample_apply_empty_dataframe(index, freq, method):
+    # GH#55572
+    empty_frame_dti = DataFrame(index=index)
+
+    rs = empty_frame_dti.resample(freq)
+    result = rs.apply(getattr(rs, method))
+
+    expected_index = _asfreq_compat(empty_frame_dti.index, freq)
+    expected = DataFrame([], index=expected_index)
+
+    tm.assert_frame_equal(result, expected)
+
+
 @pytest.mark.parametrize(
     "index",
     [