BUG: DataFrame.resample changing index type to MultiIndex when dataframe is empty and upsampling

doc/source/whatsnew/v2.1.4.rst

@@ -23,6 +23,7 @@ Bug fixes
 - Bug in :class:`Series` constructor raising DeprecationWarning when ``index`` is a list of :class:`Series` (:issue:`55228`)
 - Bug in :class:`Series` when trying to cast date-like string inputs to :class:`ArrowDtype` of ``pyarrow.timestamp`` (:issue:`56266`)
 - Bug in :class:`Timestamp` construction with ``ts_input="now"`` or ``ts_input="today"`` giving a different unit from :meth:`Timestamp.now` or :meth:`Timestamp.today` (:issue:`55879`)
+- 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:`Index.__getitem__` returning wrong result for Arrow dtypes and negative stepsize (:issue:`55832`)
 - Fixed bug in :func:`read_csv` not respecting object dtype when ``infer_string`` option is set (:issue:`56047`)
 - Fixed bug in :func:`to_numeric` converting to extension dtype for ``string[pyarrow_numpy]`` dtype (:issue:`56179`)

pandas/core/resample.py

@@ -494,22 +494,12 @@ 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)
 
@@ -1675,6 +1665,17 @@ def func(x):
             return x.apply(f, *args, **kwargs)
 
         result = _apply(self._groupby, func, include_groups=self.include_groups)
+
+        # 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

@@ -289,6 +289,33 @@ def test_resample_size_empty_dataframe(freq, empty_frame_dti):
     tm.assert_series_equal(result, expected)
 
 
+@all_ts
+@pytest.mark.parametrize("freq", ["ME", "D", "h"])
+@pytest.mark.parametrize(
+    "method", ["ffill", "bfill", "nearest", "asfreq", "interpolate"]
+)
+def test_resample_upsample_empty_dataframe(freq, method, empty_frame_dti):
+    # GH#55572
+    if freq == "ME" and isinstance(empty_frame_dti.index, TimedeltaIndex):
+        msg = (
+            "Resampling on a TimedeltaIndex requires fixed-duration `freq`, "
+            "e.g. '24h' or '3D', not <MonthEnd>"
+        )
+        with pytest.raises(ValueError, match=msg):
+            empty_frame_dti.resample(freq)
+        return
+    elif freq == "ME" and isinstance(empty_frame_dti.index, PeriodIndex):
+        # index is PeriodIndex, so convert to corresponding Period freq
+        freq = "M"
+    rs = empty_frame_dti.resample(freq)
+    result = getattr(rs, method)()
+
+    index = _asfreq_compat(empty_frame_dti.index, freq)
+    expected = DataFrame([], index=index)
+
+    tm.assert_frame_equal(result, expected)
+
+
 @pytest.mark.parametrize(
     "index",
     [