BUG: Series.sum on overflowing timedelta64 raises OutOfBoundsTimedelta (GH-43178)

doc/source/whatsnew/v3.1.0.rst

@@ -269,6 +269,7 @@ Timedelta
 - Bug in :class:`DateOffset` where ``DateOffset(1)`` and ``DateOffset(days=1)`` returned different results near daylight saving time transitions (:issue:`61862`)
 - Bug in :class:`Timedelta` constructor where keyword arguments (e.g. ``days=365000``) that exceeded nanosecond int64 bounds raised ``OutOfBoundsTimedelta`` instead of falling back to a coarser resolution (:issue:`46587`)
 - Bug in :func:`to_timedelta` where passing ``np.str_`` objects would fail in Cython string parsing (:issue:`48974`)
+- Bug in :meth:`Series.sum` on an overflowing ``timedelta64`` series raising a plain ``ValueError`` instead of :class:`OutOfBoundsTimedelta` (:issue:`43178`)
 - Bug in ``Series.dt.seconds`` and ``Series.dt.microseconds`` with :class:`ArrowDtype` durations returning the ``Series.dt.components`` field values (e.g. 0-59 for seconds, or 0 for ``microseconds`` on a ``"ms"`` unit) instead of the totals within each day and second respectively, inconsistent with NumPy-backed timedeltas (:issue:`63470`)
 
 Timezones

pandas/core/nanops.py

@@ -20,6 +20,7 @@
     lib,
 )
 import pandas._libs.algos as libalgos
+from pandas._libs.tslibs import OutOfBoundsTimedelta
 from pandas.compat._optional import import_optional_dependency
 
 from pandas.core.dtypes.common import (
@@ -375,8 +376,8 @@ def _wrap_results(result, dtype: np.dtype, fill_value=None):
                 result = np.timedelta64("NaT", unit)  # type: ignore[call-overload]
 
             elif np.fabs(result) > lib.i8max:
-                # raise if we have a timedelta64[ns] which is too large
-                raise ValueError("overflow in timedelta operation")
+                # GH#43178: raise if the result is too large for the dtype's unit
+                raise OutOfBoundsTimedelta("overflow in timedelta operation")
             else:
                 # return a timedelta64 with the original unit
                 result = np.int64(result).astype(dtype, copy=False)

pandas/tests/series/test_reductions.py

@@ -114,12 +114,13 @@ def test_td64_summation_overflow():
     assert np.allclose(result._value / 1000, expected._value / 1000)
 
     # sum
+    # GH#43178: OutOfBoundsTimedelta (a ValueError subclass) is raised
     msg = "overflow in timedelta operation"
-    with pytest.raises(ValueError, match=msg):
+    with pytest.raises(pd.errors.OutOfBoundsTimedelta, match=msg):
         (ser - ser.min()).sum()
 
     s1 = ser[0:10000]
-    with pytest.raises(ValueError, match=msg):
+    with pytest.raises(pd.errors.OutOfBoundsTimedelta, match=msg):
         (s1 - s1.min()).sum()
     s2 = ser[0:1000]
     (s2 - s2.min()).sum()