BUG: Timedelta(td64, unit=foo) silent overflow

doc/source/whatsnew/v1.5.0.rst

@@ -479,6 +479,8 @@ Datetimelike
 Timedelta
 ^^^^^^^^^
 - Bug in :func:`astype_nansafe` astype("timedelta64[ns]") fails when np.nan is included (:issue:`45798`)
+- Bug in constructing a :class:`Timedelta` with a ``np.timedelta64`` object and a ``unit`` sometimes silently overflowing and returning incorrect results instead of raising ``OutOfBoundsTimedelta`` (:issue:`46827`)
+-
 
 Time Zones
 ^^^^^^^^^^

pandas/_libs/tslibs/dtypes.pyx

@@ -277,6 +277,16 @@ cdef str npy_unit_to_abbrev(NPY_DATETIMEUNIT unit):
         return "M"
     elif unit == NPY_DATETIMEUNIT.NPY_FR_Y:
         return "Y"
+
+    # Checks for not-really-supported units go at the end, as we don't expect
+    #  to see these often
+    elif unit == NPY_DATETIMEUNIT.NPY_FR_ps:
+        return "ps"
+    elif unit == NPY_DATETIMEUNIT.NPY_FR_fs:
+        return "fs"
+    elif unit == NPY_DATETIMEUNIT.NPY_FR_as:
+        return "as"
+
     else:
         raise NotImplementedError(unit)
 

pandas/_libs/tslibs/timedeltas.pyx

@@ -1487,8 +1487,6 @@ class Timedelta(_Timedelta):
         elif PyDelta_Check(value):
             value = convert_to_timedelta64(value, 'ns')
         elif is_timedelta64_object(value):
-            if unit is not None:
-                value = value.astype(f'timedelta64[{unit}]')
             value = ensure_td64ns(value)
         elif is_tick_object(value):
             value = np.timedelta64(value.nanos, 'ns')

pandas/tests/scalar/timedelta/test_constructors.py

@@ -13,6 +13,21 @@
 )
 
 
+def test_construct_from_td64_with_unit():
+    # ignore the unit, as it may cause silently overflows leading to incorrect
+    #  results, and in non-overflow cases is irrelevant GH#46827
+    obj = np.timedelta64(123456789, "h")
+
+    with pytest.raises(OutOfBoundsTimedelta, match="123456789 hours"):
+        Timedelta(obj, unit="ps")
+
+    with pytest.raises(OutOfBoundsTimedelta, match="123456789 hours"):
+        Timedelta(obj, unit="ns")
+
+    with pytest.raises(OutOfBoundsTimedelta, match="123456789 hours"):
+        Timedelta(obj)
+
+
 def test_construction():
     expected = np.timedelta64(10, "D").astype("m8[ns]").view("i8")
     assert Timedelta(10, unit="d").value == expected