Disallow cumprod for timedelta (#50246)

Author: phofl

doc/source/whatsnew/v2.0.0.rst

@@ -485,6 +485,7 @@ Other API changes
   or :attr:`~DataFrame.iloc` (thus, ``df.loc[:, :]`` or ``df.iloc[:, :]``) now returns a
   new DataFrame (shallow copy) instead of the original DataFrame, consistent with other
   methods to get a full slice (for example ``df.loc[:]`` or ``df[:]``) (:issue:`49469`)
+- Disallow computing ``cumprod`` for :class:`Timedelta` object; previously this returned incorrect values (:issue:`50246`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/arrays/timedeltas.py

@@ -417,12 +417,13 @@ def _accumulate(self, name: str, *, skipna: bool = True, **kwargs):
 
         data = self._ndarray.copy()
 
-        if name in {"cumsum", "cumprod"}:
-            # TODO: cumprod should not work here GH#48111
-            func = np.cumsum if name == "cumsum" else np.cumprod
+        if name == "cumsum":
+            func = np.cumsum
             result = cast(np.ndarray, nanops.na_accum_func(data, func, skipna=skipna))
 
             return type(self)._simple_new(result, freq=None, dtype=self.dtype)
+        elif name == "cumprod":
+            raise TypeError("cumprod not supported for Timedelta.")
 
         else:
             return super()._accumulate(name, skipna=skipna, **kwargs)

pandas/tests/series/test_cumulative.py

@@ -129,3 +129,9 @@ def test_cummethods_bool_in_object_dtype(self, method, expected):
         ser = pd.Series([False, True, np.nan, False])
         result = getattr(ser, method)()
         tm.assert_series_equal(result, expected)
+
+    def test_cumprod_timedelta(self):
+        # GH#48111
+        ser = pd.Series([pd.Timedelta(days=1), pd.Timedelta(days=3)])
+        with pytest.raises(TypeError, match="cumprod not supported for Timedelta"):
+            ser.cumprod()