BUG: disallow resample with non-Tick on TimedeltaIndex

doc/source/whatsnew/v2.1.0.rst

@@ -200,6 +200,7 @@ Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^
 - Bug in :meth:`DataFrameGroupBy.idxmin`, :meth:`SeriesGroupBy.idxmin`, :meth:`DataFrameGroupBy.idxmax`, :meth:`SeriesGroupBy.idxmax` return wrong dtype when used on empty DataFrameGroupBy or SeriesGroupBy (:issue:`51423`)
 - Bug in weighted rolling aggregations when specifying ``min_periods=0`` (:issue:`51449`)
+- Bug in :meth:`DataFrame.resample` and :meth:`Series.resample` in incorrectly allowing non-fixed ``freq`` when resampling on a :class:`TimedeltaIndex` (:issue:`51896`)
 -
 
 Reshaping
@@ -226,6 +227,7 @@ Styler
 
 Other
 ^^^^^
+-
 
 .. ***DO NOT USE THIS SECTION***
 

pandas/core/arrays/datetimelike.py

@@ -1851,6 +1851,8 @@ def __init__(
             values = values.copy()
         if freq:
             freq = to_offset(freq)
+            if values.dtype.kind == "m" and not isinstance(freq, Tick):
+                raise TypeError("TimedeltaArray/Index freq must be a Tick")
 
         NDArrayBacked.__init__(self, values=values, dtype=dtype)
         self._freq = freq
@@ -1874,6 +1876,8 @@ def freq(self, value) -> None:
         if value is not None:
             value = to_offset(value)
             self._validate_frequency(self, value)
+            if self.dtype.kind == "m" and not isinstance(value, Tick):
+                raise TypeError("TimedeltaArray/Index freq must be a Tick")
 
             if self.ndim > 1:
                 raise ValueError("Cannot set freq with ndim > 1")
@@ -2067,9 +2071,9 @@ def _with_freq(self, freq):
             # Always valid
             pass
         elif len(self) == 0 and isinstance(freq, BaseOffset):
-            # Always valid.  In the TimedeltaArray case, we assume this
-            #  is a Tick offset.
-            pass
+            # Always valid.  In the TimedeltaArray case, we require a Tick offset
+            if self.dtype.kind == "m" and not isinstance(freq, Tick):
+                raise TypeError("TimedeltaArray/Index freq must be a Tick")
         else:
             # As an internal method, we can ensure this assertion always holds
             assert freq == "infer"

pandas/core/arrays/timedeltas.py

@@ -202,6 +202,7 @@ def _simple_new(  # type: ignore[override]
         assert not tslibs.is_unitless(dtype)
         assert isinstance(values, np.ndarray), type(values)
         assert dtype == values.dtype
+        assert freq is None or isinstance(freq, Tick)
 
         result = super()._simple_new(values=values, dtype=dtype)
         result._freq = freq

pandas/core/resample.py

@@ -1826,6 +1826,13 @@ def _get_time_delta_bins(self, ax: TimedeltaIndex):
                 f"an instance of {type(ax).__name__}"
             )
 
+        if not isinstance(self.freq, Tick):
+            # GH#51896
+            raise ValueError(
+                "Resampling on a TimedeltaIndex requires fixed-duration `freq`, "
+                f"e.g. '24H' or '3D', not {self.freq}"
+            )
+
         if not len(ax):
             binner = labels = TimedeltaIndex(data=[], freq=self.freq, name=ax.name)
             return binner, [], labels

pandas/tests/indexes/timedeltas/test_freq_attr.py

@@ -6,6 +6,7 @@
     DateOffset,
     Day,
     Hour,
+    MonthEnd,
 )
 
 
@@ -25,6 +26,16 @@ def test_freq_setter(self, values, freq):
         idx._data.freq = None
         assert idx.freq is None
 
+    def test_with_freq_empty_requires_tick(self):
+        idx = TimedeltaIndex([])
+
+        off = MonthEnd(1)
+        msg = "TimedeltaArray/Index freq must be a Tick"
+        with pytest.raises(TypeError, match=msg):
+            idx._with_freq(off)
+        with pytest.raises(TypeError, match=msg):
+            idx._data._with_freq(off)
+
     def test_freq_setter_errors(self):
         # GH#20678
         idx = TimedeltaIndex(["0 days", "2 days", "4 days"])

pandas/tests/resample/test_base.py

@@ -8,6 +8,7 @@
     NaT,
     PeriodIndex,
     Series,
+    TimedeltaIndex,
 )
 import pandas._testing as tm
 from pandas.core.groupby.groupby import DataError
@@ -110,7 +111,17 @@ def test_resample_empty_series(freq, empty_series_dti, resample_method, request)
         )
 
     ser = empty_series_dti
-    result = getattr(ser.resample(freq), resample_method)()
+    if freq == "M" and isinstance(ser.index, TimedeltaIndex):
+        msg = (
+            "Resampling on a TimedeltaIndex requires fixed-duration `freq`, "
+            "e.g. '24H' or '3D', not <MonthEnd>"
+        )
+        with pytest.raises(ValueError, match=msg):
+            ser.resample(freq)
+        return
+
+    rs = ser.resample(freq)
+    result = getattr(rs, resample_method)()
 
     expected = ser.copy()
     expected.index = _asfreq_compat(ser.index, freq)
@@ -150,11 +161,23 @@ def test_resample_nat_index_series(request, freq, series, resample_method):
 @pytest.mark.parametrize("resample_method", ["count", "size"])
 def test_resample_count_empty_series(freq, empty_series_dti, resample_method):
     # GH28427
-    result = getattr(empty_series_dti.resample(freq), resample_method)()
+    ser = empty_series_dti
+    if freq == "M" and isinstance(ser.index, TimedeltaIndex):
+        msg = (
+            "Resampling on a TimedeltaIndex requires fixed-duration `freq`, "
+            "e.g. '24H' or '3D', not <MonthEnd>"
+        )
+        with pytest.raises(ValueError, match=msg):
+            ser.resample(freq)
+        return
+
+    rs = ser.resample(freq)
+
+    result = getattr(rs, resample_method)()
 
-    index = _asfreq_compat(empty_series_dti.index, freq)
+    index = _asfreq_compat(ser.index, freq)
 
-    expected = Series([], dtype="int64", index=index, name=empty_series_dti.name)
+    expected = Series([], dtype="int64", index=index, name=ser.name)
 
     tm.assert_series_equal(result, expected)
 
@@ -165,7 +188,17 @@ def test_resample_empty_dataframe(empty_frame_dti, freq, resample_method):
     # GH13212
     df = empty_frame_dti
     # count retains dimensions too
-    result = getattr(df.resample(freq, group_keys=False), resample_method)()
+    if freq == "M" and isinstance(df.index, TimedeltaIndex):
+        msg = (
+            "Resampling on a TimedeltaIndex requires fixed-duration `freq`, "
+            "e.g. '24H' or '3D', not <MonthEnd>"
+        )
+        with pytest.raises(ValueError, match=msg):
+            df.resample(freq, group_keys=False)
+        return
+
+    rs = df.resample(freq, group_keys=False)
+    result = getattr(rs, resample_method)()
     if resample_method != "size":
         expected = df.copy()
     else:
@@ -188,6 +221,15 @@ def test_resample_count_empty_dataframe(freq, empty_frame_dti):
 
     empty_frame_dti["a"] = []
 
+    if freq == "M" 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
+
     result = empty_frame_dti.resample(freq).count()
 
     index = _asfreq_compat(empty_frame_dti.index, freq)
@@ -204,6 +246,15 @@ def test_resample_size_empty_dataframe(freq, empty_frame_dti):
 
     empty_frame_dti["a"] = []
 
+    if freq == "M" 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
+
     result = empty_frame_dti.resample(freq).size()
 
     index = _asfreq_compat(empty_frame_dti.index, freq)
@@ -233,6 +284,16 @@ def test_resample_empty_dtypes(index, dtype, resample_method):
 def test_apply_to_empty_series(empty_series_dti, freq):
     # GH 14313
     ser = empty_series_dti
+
+    if freq == "M" and isinstance(empty_series_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_series_dti.resample(freq)
+        return
+
     result = ser.resample(freq, group_keys=False).apply(lambda x: 1)
     expected = ser.resample(freq).apply(np.sum)