fix kafka unmarshaller args typing and defaults (#240)

* fix kafka unmarshaller args typing and defaults

Signed-off-by: Christoph Hösler <[email protected]>

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---------

Signed-off-by: Christoph Hösler <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

cloudevents/kafka/conversion.py

@@ -21,9 +21,14 @@
 from cloudevents.kafka.exceptions import KeyMapperError
 from cloudevents.sdk import types
 
-DEFAULT_MARSHALLER: types.MarshallerType = json.dumps
-DEFAULT_UNMARSHALLER: types.MarshallerType = json.loads
-DEFAULT_EMBEDDED_DATA_MARSHALLER: types.MarshallerType = lambda x: x
+JSON_MARSHALLER: types.MarshallerType = json.dumps
+JSON_UNMARSHALLER: types.UnmarshallerType = json.loads
+IDENTITY_MARSHALLER = IDENTITY_UNMARSHALLER = lambda x: x
+
+DEFAULT_MARSHALLER: types.MarshallerType = JSON_MARSHALLER
+DEFAULT_UNMARSHALLER: types.UnmarshallerType = JSON_UNMARSHALLER
+DEFAULT_EMBEDDED_DATA_MARSHALLER: types.MarshallerType = IDENTITY_MARSHALLER
+DEFAULT_EMBEDDED_DATA_UNMARSHALLER: types.UnmarshallerType = IDENTITY_UNMARSHALLER
 
 
 class KafkaMessage(typing.NamedTuple):
@@ -109,7 +114,7 @@ def to_binary(
 def from_binary(
     message: KafkaMessage,
     event_type: typing.Optional[typing.Type[AnyCloudEvent]] = None,
-    data_unmarshaller: typing.Optional[types.MarshallerType] = None,
+    data_unmarshaller: typing.Optional[types.UnmarshallerType] = None,
 ) -> AnyCloudEvent:
     """
     Returns a CloudEvent from a KafkaMessage in binary format.
@@ -208,7 +213,7 @@ def to_structured(
 def from_structured(
     message: KafkaMessage,
     event_type: typing.Optional[typing.Type[AnyCloudEvent]] = None,
-    data_unmarshaller: typing.Optional[types.MarshallerType] = None,
+    data_unmarshaller: typing.Optional[types.UnmarshallerType] = None,
     envelope_unmarshaller: typing.Optional[types.UnmarshallerType] = None,
 ) -> AnyCloudEvent:
     """
@@ -222,7 +227,7 @@ def from_structured(
     :returns: CloudEvent
     """
 
-    data_unmarshaller = data_unmarshaller or DEFAULT_EMBEDDED_DATA_MARSHALLER
+    data_unmarshaller = data_unmarshaller or DEFAULT_EMBEDDED_DATA_UNMARSHALLER
     envelope_unmarshaller = envelope_unmarshaller or DEFAULT_UNMARSHALLER
     try:
         structure = envelope_unmarshaller(message.value)