Merge branch 'SSA-6320-workflow-service' of...

Merge branch 'SSA-6320-workflow-service' of https://open-bitbucket.nrao.edu/scm/ssa/data into SSA-6320-workflow-service

shared/channels/src/channels/amqp_helpers.py

 from abc import ABC, abstractmethod
-from typing import Callable, Any, Optional, Union, TypeVar, Protocol
+from typing import Callable, Any, Optional, Union, TypeVar, Protocol, Generic
 
 import pika
 from marshmallow import Schema
@@ -8,10 +8,10 @@ from pycapo import CapoConfig
 from workspaces.json import WorkflowEventSchema
 from workspaces.schema import WorkflowEvent
 
-T = TypeVar('T')
+T = TypeVar('T', contravariant=True)
 
 
-class ChannelDefinition(ABC, Protocol[T]):
+class ChannelDefinition(Protocol[T]):
     @abstractmethod
     def routing_key_for(self, message: T) -> str:
         pass
@@ -49,10 +49,13 @@ class WorkflowEventChannel(ChannelDefinition[WorkflowEvent]):
         return self.EXCHANGE
 
 
-class Channel(Protocol[T]):
-    CONNECTION = None
-    CHANNEL = None
-    CONFIG = None
+ChannelDef = TypeVar('ChannelDef', bound=ChannelDefinition, covariant=True)
+
+
+class Channel(Generic[ChannelDef]):
+    CONNECTION : pika.BlockingConnection = None
+    CHANNEL : BlockingChannel = None
+    CONFIG : CapoConfig = None
 
     def __init__(self, definition: ChannelDefinition[T]):
         self.definition = definition
@@ -144,4 +147,4 @@ class Channel(Protocol[T]):
 
 
 # Predefined event channels for ease of use
-workflow_events = Channel(WorkflowEventChannel())
+workflow_events : Channel[WorkflowEvent] = Channel(WorkflowEventChannel())