From 5066013417573475dd0fcbd1b49a3a1145b3a470 Mon Sep 17 00:00:00 2001
From: Jared Deckard <jared@shademaps.com>
Date: Fri, 20 Aug 2021 09:35:26 -0500
Subject: [PATCH] Use microsecond integers to fix timedelta precision errors

---
 src/marshmallow/fields.py   | 4 +++-
 src/marshmallow/utils.py    | 8 ++++++++
 tests/test_serialization.py | 9 +++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/marshmallow/fields.py b/src/marshmallow/fields.py
index 88c1bc719..ea5d08cff 100644
--- a/src/marshmallow/fields.py
+++ b/src/marshmallow/fields.py
@@ -1471,7 +1471,9 @@ def _serialize(self, value, attr, obj, **kwargs):
         if value is None:
             return None
         base_unit = dt.timedelta(**{self.precision: 1})
-        return int(value.total_seconds() / base_unit.total_seconds())
+        delta = utils.timedelta_to_microseconds(value)
+        unit = utils.timedelta_to_microseconds(base_unit)
+        return delta // unit
 
     def _deserialize(self, value, attr, data, **kwargs):
         try:
diff --git a/src/marshmallow/utils.py b/src/marshmallow/utils.py
index cb5e5002d..74ee67392 100644
--- a/src/marshmallow/utils.py
+++ b/src/marshmallow/utils.py
@@ -323,3 +323,11 @@ def resolve_field_instance(cls_or_instance):
         if not isinstance(cls_or_instance, FieldABC):
             raise FieldInstanceResolutionError
         return cls_or_instance
+
+
+def timedelta_to_microseconds(value: dt.timedelta) -> int:
+    """Compute the total microseconds of a timedelta
+
+    https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/Lib/datetime.py#L665-L667  # noqa: B950
+    """
+    return (value.days * (24 * 3600) + value.seconds) * 1000000 + value.microseconds
diff --git a/tests/test_serialization.py b/tests/test_serialization.py
index ca6fd0b9a..d191dff37 100644
--- a/tests/test_serialization.py
+++ b/tests/test_serialization.py
@@ -712,6 +712,15 @@ def test_timedelta_field(self, user):
         user.d7 = None
         assert field.serialize("d7", user) is None
 
+        # https://github.com/marshmallow-code/marshmallow/issues/1856
+        user.d8 = dt.timedelta(milliseconds=345)
+        field = fields.TimeDelta(fields.TimeDelta.MILLISECONDS)
+        assert field.serialize("d8", user) == 345
+
+        user.d9 = dt.timedelta(milliseconds=1999)
+        field = fields.TimeDelta(fields.TimeDelta.SECONDS)
+        assert field.serialize("d9", user) == 1
+
     def test_datetime_list_field(self):
         obj = DateTimeList([dt.datetime.utcnow(), dt.datetime.now()])
         field = fields.List(fields.DateTime)