BUG: Timedelta drops decimals if precision is greater than nanoseconds

doc/source/whatsnew/v1.2.0.rst

@@ -339,6 +339,7 @@ Datetimelike
 - Bug in :class:`DatetimeIndex.shift` incorrectly raising when shifting empty indexes (:issue:`14811`)
 - :class:`Timestamp` and :class:`DatetimeIndex` comparisons between timezone-aware and timezone-naive objects now follow the standard library ``datetime`` behavior, returning ``True``/``False`` for ``!=``/``==`` and raising for inequality comparisons (:issue:`28507`)
 - Bug in :meth:`DatetimeIndex.equals` and :meth:`TimedeltaIndex.equals` incorrectly considering ``int64`` indexes as equal (:issue:`36744`)
+- Bug in :class:`Timedelta` incorrectly deleted all decimals when input had a higher precision than nanoseconds (:issue:`36738`)
 
 Timedelta
 ^^^^^^^^^

pandas/_libs/tslibs/timedeltas.pyx

@@ -404,9 +404,11 @@ cdef inline int64_t parse_timedelta_string(str ts) except? -1:
             m = 10**(3 -len(frac)) * 1000 * 1000
         elif len(frac) > 3 and len(frac) <= 6:
             m = 10**(6 -len(frac)) * 1000
-        else:
+        elif len(frac) > 6 and len(frac) <= 9:
             m = 10**(9 -len(frac))
-
+        else:
+            m = 1
+            frac = frac[:9]
         r = <int64_t>int(''.join(frac)) * m
         result += timedelta_as_neg(r, neg)
 
@@ -1132,6 +1134,9 @@ class Timedelta(_Timedelta):
     Notes
     -----
     The ``.value`` attribute is always in ns.
+
+    If the precision is higher than nanoseconds, the precision of the duration is
+    truncated to nanoseconds.
     """
 
     def __new__(cls, object value=_no_input, unit=None, **kwargs):

pandas/core/tools/timedeltas.py

@@ -64,6 +64,12 @@ def to_timedelta(arg, unit=None, errors="raise"):
     to_datetime : Convert argument to datetime.
     convert_dtypes : Convert dtypes.
 
+    Notes
+    -----
+
+    If the precision is higher than nanoseconds, the precision of the duration is
+    truncated to nanoseconds.
+
     Examples
     --------
     Parsing a single string to a Timedelta:

pandas/tests/tools/test_to_timedelta.py

@@ -179,3 +179,10 @@ def test_to_timedelta_nullable_int64_dtype(self):
         result = to_timedelta(Series([1, None], dtype="Int64"), unit="days")
 
         tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize("func", ["Timedelta", "to_timedelta"])
+    def test_to_timedelta_precision_over_nanos(self, func):
+        # GH: 36738
+        expected = pd.Timedelta("8:53:08.718")
+        result = getattr(pd, func)("8:53:08.71800000001")
+        assert result == expected