Pack datetime subclasses as timestamps unless strict_types is set

msgpack/_packer.pyx

@@ -1,7 +1,7 @@
 from cpython cimport *
 from cpython.bytearray cimport PyByteArray_Check, PyByteArray_CheckExact
 from cpython.datetime cimport (
-    PyDateTime_CheckExact, PyDelta_CheckExact,
+    PyDateTime_Check, PyDateTime_CheckExact, PyDelta_Check,
     datetime_tzinfo, timedelta_days, timedelta_seconds, timedelta_microseconds,
 )
 from cpython.frozendict cimport PyAnyDict_Check, PyAnyDict_CheckExact
@@ -241,16 +241,20 @@ cdef class Packer:
                 msgpack_pack_raw_body(&self.pk, <char*>view.buf, L)
             finally:
                 PyBuffer_Release(&view);
-        elif self.datetime and PyDateTime_CheckExact(o) and datetime_tzinfo(o) is not None:
+        elif (
+            self.datetime
+            and (PyDateTime_CheckExact(o) if strict else PyDateTime_Check(o))
+            and datetime_tzinfo(o) is not None
+        ):
             delta = o - epoch
-            if not PyDelta_CheckExact(delta):
-                raise ValueError("failed to calculate delta")
+            if not PyDelta_Check(delta):
+                raise TypeError("failed to calculate delta")
             llval = timedelta_days(delta) * <long long>(24*60*60) + timedelta_seconds(delta)
             ulval = timedelta_microseconds(delta) * 1000
             msgpack_pack_timestamp(&self.pk, llval, ulval)
         elif will_default:
             return -2
-        elif self.datetime and PyDateTime_CheckExact(o):
+        elif self.datetime and (PyDateTime_CheckExact(o) if strict else PyDateTime_Check(o)):
             # this should be later than will_default
             PyErr_Format(ValueError, b"can not serialize '%.200s' object where tzinfo=None", Py_TYPE(o).tp_name)
         else:

test/test_timestamp.py

@@ -225,3 +225,70 @@ def test_too_large_timestamp():
     # When timestamp64 is too large, conversion to datetime fails due to int64 -> int32 conversion.
     # https://github.com/msgpack/msgpack-python/issues/696
     print(msgpack.unpackb(b"\xd7\xff" + b"\x00" * 8, timestamp=3))
+
+
+class DatetimeSubclass(datetime.datetime):
+    pass
+
+
+class TimedeltaSubclass(datetime.timedelta):
+    pass
+
+
+class DatetimeSubclassTimedeltaSubclass(datetime.datetime):
+    # Subtraction returns a timedelta subclass instead of datetime.timedelta.
+    def __sub__(self, other):
+        delta = super().__sub__(other)
+        if type(delta) is datetime.timedelta:
+            return TimedeltaSubclass(delta.days, delta.seconds, delta.microseconds)
+        return delta
+
+
+class DatetimeSubclassNotTimedelta(datetime.datetime):
+    # Subtraction does not return a timedelta.
+    def __sub__(self, other):
+        return 0
+
+
+@pytest.mark.parametrize("cls", [DatetimeSubclass, DatetimeSubclassTimedeltaSubclass])
+def test_pack_datetime_subclass(cls):
+    # Without strict_types, an aware datetime subclass packs like datetime.
+    utc = datetime.timezone.utc
+    dt = datetime.datetime(2024, 4, 16, 8, 43, 9, 420317, tzinfo=utc)
+    subdt = cls(2024, 4, 16, 8, 43, 9, 420317, tzinfo=utc)
+    called = []
+    packed = msgpack.packb(subdt, datetime=True, default=called.append)
+    assert packed == msgpack.packb(dt, datetime=True)
+    assert called == []
+    assert msgpack.unpackb(packed, timestamp=3) == dt
+
+
+def test_pack_naive_datetime_subclass():
+    dt = DatetimeSubclass(1970, 1, 1, 0, 0, 42, 14)
+    with pytest.raises(ValueError, match="where tzinfo=None"):
+        msgpack.packb(dt, datetime=True)
+
+
+@pytest.mark.parametrize(
+    "options",
+    [{"datetime": True, "strict_types": True}, {"datetime": False}],
+)
+def test_pack_datetime_subclass_unconverted(options):
+    # strict_types=True and datetime=False leave a subclass to default.
+    utc = datetime.timezone.utc
+    subdt = DatetimeSubclass(2024, 4, 16, 8, 43, 9, 420317, tzinfo=utc)
+    with pytest.raises(TypeError):
+        msgpack.packb(subdt, **options)
+    called = []
+    assert msgpack.packb(subdt, default=called.append, **options) == msgpack.packb(None)
+    assert called == [subdt]
+
+
+@pytest.mark.skipif(
+    msgpack.Packer.__module__ == "msgpack.fallback",
+    reason="subtraction result check is implemented in C extension only",
+)
+def test_pack_datetime_subclass_subtraction_not_timedelta():
+    subdt = DatetimeSubclassNotTimedelta(2024, 4, 16, tzinfo=datetime.timezone.utc)
+    with pytest.raises(TypeError):
+        msgpack.packb(subdt, datetime=True)