fix: Handle optional (None) values for custom serializers (#84)

Fixes #82

CHANGELOG.rst

@@ -16,6 +16,12 @@ Unreleased
 Changed
 ~~~~~~~
 
+[0.11.1] - 2022-07-28
+---------------------
+Fixed
+~~~~~
+* Handle optional (None) values for custom serializers
+
 [0.11.0] - 2022-07-21
 ---------------------
 Added

openedx_events/__init__.py

@@ -5,4 +5,4 @@
 more information about the project.
 """
 
-__version__ = "0.11.0"
+__version__ = "0.11.1"

openedx_events/event_bus/avro/serializer.py

@@ -25,6 +25,21 @@ def _get_non_attrs_serializer(serializers=None):
     all_serializers = {**DEFAULT_SERIALIZERS, **param_serializers}
 
     def _serialize_non_attrs_values(inst, field, value):  # pylint: disable=unused-argument
+        if value is None:
+            # All default=None fields are implicit union types of NoneType
+            # and something else. (See ADR 7.) Note that if there isn't a
+            # default at all, field.default will be attrs.NOTHING, not None.
+            if field.default is None:
+                return None
+            else:
+                # Bail out early with an informative message rather
+                # than ending up with an inscrutable error from inside
+                # a custom serializer.
+                #
+                # If we ever make a custom serializer that can handle
+                # None as an input, we can remove this check.
+                raise Exception("None cannot be handled by custom serializers (and default=None was not set)")
+
         for extended_class, serializer in all_serializers.items():
             if field:
                 if issubclass(field.type, extended_class):

openedx_events/event_bus/avro/tests/test_serializer.py

@@ -2,11 +2,14 @@
 
 from datetime import datetime
 
+import pytest
 from django.test import TestCase
 from opaque_keys.edx.keys import CourseKey
 
 from openedx_events.event_bus.avro.serializer import AvroSignalSerializer
 from openedx_events.event_bus.avro.tests.test_utilities import (
+    CustomAttrsWithDefaults,
+    CustomAttrsWithoutDefaults,
     EventData,
     NestedAttrsWithDefaults,
     NestedNonAttrs,
@@ -91,7 +94,7 @@ def test_serialization_with_custom_serializer_on_nested_fields(self):
             "field_0": "a.val:a.nother.val"
         }})
 
-    def test_serialization_of_optional_fields(self):
+    def test_serialization_of_optional_simple_fields(self):
         SIGNAL = create_simple_signal({
             "data": SimpleAttrsWithDefaults
         })
@@ -105,6 +108,23 @@ def test_serialization_of_optional_fields(self):
                                                   'string_field': None,
                                                   'attrs_field': None}})
 
+    def test_serialization_of_optional_custom_fields(self):
+        SIGNAL = create_simple_signal({"data": CustomAttrsWithDefaults})
+        serializer = AvroSignalSerializer(SIGNAL)
+        event_data = {"data": CustomAttrsWithDefaults(coursekey_field=None, datetime_field=None)}
+        data_dict = serializer.to_dict(event_data)
+        self.assertDictEqual(data_dict, {"data": {'coursekey_field': None,
+                                                  'datetime_field': None}})
+
+    def test_serialization_of_none_mandatory_custom_fields(self):
+        """Check that None isn't accepted if field not optional."""
+        SIGNAL = create_simple_signal({"data": CustomAttrsWithoutDefaults})
+        serializer = AvroSignalSerializer(SIGNAL)
+        event_data = {"data": CustomAttrsWithoutDefaults(coursekey_field=None, datetime_field=None)}
+        with pytest.raises(Exception) as excinfo:
+            serializer.to_dict(event_data)
+        assert excinfo.value.args[0] == "None cannot be handled by custom serializers (and default=None was not set)"
+
     def test_serialization_of_nested_optional_fields(self):
         SIGNAL = create_simple_signal({
             "data": NestedAttrsWithDefaults

openedx_events/event_bus/avro/tests/test_utilities.py

@@ -3,9 +3,11 @@
 """
 import io
 import re
+from datetime import datetime
 
 import attr
 import fastavro
+from opaque_keys.edx.keys import CourseKey
 
 from openedx_events.event_bus.avro.custom_serializers import BaseCustomTypeAvroSerializer
 from openedx_events.event_bus.avro.deserializer import AvroSignalDeserializer
@@ -108,6 +110,20 @@ class SimpleAttrsWithDefaults:
     attrs_field = attr.ib(type=SimpleAttrs, default=None)
 
 
+@attr.s(frozen=True)
+class CustomAttrsWithDefaults:
+    """Test attrs with nullable values"""
+    coursekey_field = attr.ib(type=CourseKey, default=None)
+    datetime_field = attr.ib(type=datetime, default=None)
+
+
+@attr.s(frozen=True)
+class CustomAttrsWithoutDefaults:
+    """Test attrs without nullable values"""
+    coursekey_field = attr.ib(type=CourseKey)
+    datetime_field = attr.ib(type=datetime)
+
+
 @attr.s(frozen=True)
 class NestedAttrsWithDefaults:
     """Test attrs with nullable values"""