improve accuracy of to_pytimedelta

doc/source/whatsnew/v3.0.0.rst

@@ -313,7 +313,7 @@ Datetimelike
 
 Timedelta
 ^^^^^^^^^
--
+- Accuracy improvement in :meth:`Timedelta.to_pytimedelta` to round microseconds consistently for large nanosecond based Timedelta (:issue:`57841`)
 -
 
 Timezones

pandas/_libs/tslibs/timedeltas.pyx

@@ -1376,7 +1376,10 @@ cdef class _Timedelta(timedelta):
         datetime.timedelta(days=3)
         """
         if self._creso == NPY_FR_ns:
-            return timedelta(microseconds=int(self._value) / 1000)
+            us, remainder = divmod(self._value, 1000)
+            if remainder >= 500:
+                us += 1
+            return timedelta(microseconds=us)
 
         # TODO(@WillAyd): is this the right way to use components?
         self._ensure_components()

pandas/tests/scalar/timedelta/test_timedelta.py

@@ -665,3 +665,10 @@ def test_timedelta_attribute_precision():
     result += td.nanoseconds
     expected = td._value
     assert result == expected
+
+
+def test_to_pytimedelta_large_values():
+    td = Timedelta(1152921504609987375, unit="ns")
+    result = td.to_pytimedelta()
+    expected = timedelta(days=13343, seconds=86304, microseconds=609987)
+    assert result == expected