BUG: Timedelta.__new__

doc/source/whatsnew/v1.6.0.rst

@@ -172,6 +172,8 @@ Datetimelike
 Timedelta
 ^^^^^^^^^
 - Bug in :func:`to_timedelta` raising error when input has nullable dtype ``Float64`` (:issue:`48796`)
+- Bug in :class:`Timedelta` constructor incorrectly raising instead of returning ``NaT`` when given a ``np.timedelta64("nat")`` (:issue:`48898`)
+- Bug in :class:`Timedelta` constructor failing to raise when passed both a :class:`Timedelta` object and keywords (e.g. days, seconds) (:issue:`48898`)
 -
 
 Timezones

pandas/_libs/tslibs/timedeltas.pyx

@@ -1662,10 +1662,15 @@ class Timedelta(_Timedelta):
 
         # GH 30543 if pd.Timedelta already passed, return it
         # check that only value is passed
-        if isinstance(value, _Timedelta) and unit is None and len(kwargs) == 0:
+        if isinstance(value, _Timedelta):
+            # 'unit' is benign in this case, but e.g. days or seconds
+            #  doesn't make sense here.
+            if len(kwargs):
+                # GH#48898
+                raise ValueError(
+                    "Cannot pass both a Timedelta input and timedelta keyword arguments."
+                )
             return value
-        elif isinstance(value, _Timedelta):
-            value = value.value
         elif isinstance(value, str):
             if unit is not None:
                 raise ValueError("unit must not be specified if the value is a str")
@@ -1679,6 +1684,9 @@ class Timedelta(_Timedelta):
         elif PyDelta_Check(value):
             value = convert_to_timedelta64(value, 'ns')
         elif is_timedelta64_object(value):
+            if get_timedelta64_value(value) == NPY_NAT:
+                # i.e. np.timedelta64("NaT")
+                return NaT
             value = ensure_td64ns(value)
         elif is_tick_object(value):
             value = np.timedelta64(value.nanos, 'ns')

pandas/tests/scalar/timedelta/test_constructors.py

@@ -7,6 +7,7 @@
 from pandas._libs.tslibs import OutOfBoundsTimedelta
 
 from pandas import (
+    NaT,
     Timedelta,
     offsets,
     to_timedelta,
@@ -371,6 +372,14 @@ def test_timedelta_constructor_identity():
     assert result is expected
 
 
+def test_timedelta_pass_td_and_kwargs_raises():
+    # don't silently ignore the kwargs GH#48898
+    td = Timedelta(days=1)
+    msg = "Cannot pass both a Timedelta input and timedelta keyword arguments"
+    with pytest.raises(ValueError, match=msg):
+        Timedelta(td, days=2)
+
+
 @pytest.mark.parametrize(
     "constructor, value, unit, expectation",
     [
@@ -402,3 +411,9 @@ def test_string_without_numbers(value):
     )
     with pytest.raises(ValueError, match=msg):
         Timedelta(value)
+
+
+def test_timedelta_new_npnat():
+    # GH#48898
+    nat = np.timedelta64("NaT", "h")
+    assert Timedelta(nat) is NaT