TODO fulfillment: use enum for enumerated types.

src/python/src/_adapter/_links_test.py

@@ -80,8 +80,8 @@ class RoundTripTest(unittest.TestCase):
     rear_link.start()
 
     front_to_back_ticket = tickets.FrontToBackPacket(
-        test_operation_id, 0, tickets.Kind.ENTIRE, test_method, interfaces.FULL,
-        None, None, _TIMEOUT)
+        test_operation_id, 0, tickets.Kind.ENTIRE, test_method,
+        interfaces.ServicedSubscription.Kind.FULL, None, None, _TIMEOUT)
     rear_link.accept_front_to_back_ticket(front_to_back_ticket)
 
     with test_fore_link.condition:
@@ -133,8 +133,9 @@ class RoundTripTest(unittest.TestCase):
     rear_link.start()
 
     front_to_back_ticket = tickets.FrontToBackPacket(
-        test_operation_id, 0, tickets.Kind.ENTIRE, test_method, interfaces.FULL,
-        None, test_front_to_back_datum, _TIMEOUT)
+        test_operation_id, 0, tickets.Kind.ENTIRE, test_method,
+        interfaces.ServicedSubscription.Kind.FULL, None,
+        test_front_to_back_datum, _TIMEOUT)
     rear_link.accept_front_to_back_ticket(front_to_back_ticket)
 
     with test_fore_link.condition:
@@ -196,7 +197,7 @@ class RoundTripTest(unittest.TestCase):
 
     commencement_ticket = tickets.FrontToBackPacket(
         test_operation_id, 0, tickets.Kind.COMMENCEMENT, test_method,
-        interfaces.FULL, None, None, _TIMEOUT)
+        interfaces.ServicedSubscription.Kind.FULL, None, None, _TIMEOUT)
     fore_sequence_number = 1
     rear_link.accept_front_to_back_ticket(commencement_ticket)
     for request in scenario.requests():

src/python/src/_adapter/_lonely_rear_link_test.py

@@ -69,7 +69,7 @@ class LonelyRearLinkTest(unittest.TestCase):
 
     front_to_back_ticket = packets.FrontToBackPacket(
         test_operation_id, 0, front_to_back_ticket_kind, test_method,
-        interfaces.FULL, None, None, _TIMEOUT)
+        interfaces.ServicedSubscription.Kind.FULL, None, None, _TIMEOUT)
     rear_link.accept_front_to_back_ticket(front_to_back_ticket)
 
     with fore_link.condition:

src/python/src/_adapter/fore.py

@@ -116,7 +116,8 @@ class ForeLink(ticket_interfaces.ForeLink):
         self._response_serializers[method])
 
     ticket = tickets.FrontToBackPacket(
-        call, 0, tickets.Kind.COMMENCEMENT, method, interfaces.FULL, None, None,
+        call, 0, tickets.Kind.COMMENCEMENT, method,
+        interfaces.ServicedSubscription.Kind.FULL, None, None,
         service_acceptance.deadline - time.time())
     self._rear_link.accept_front_to_back_ticket(ticket)
 

src/python/src/_framework/base/interfaces.py

@@ -29,27 +29,24 @@
 
 """Interfaces defined and used by the base layer of RPC Framework."""
 
-# TODO(nathaniel): Use Python's new enum library for enumerated types rather
-# than constants merely placed close together.
-
 import abc
+import enum
 
 # stream is referenced from specification in this module.
 from _framework.foundation import stream  # pylint: disable=unused-import
 
-# Operation outcomes.
-COMPLETED = 'completed'
-CANCELLED = 'cancelled'
-EXPIRED = 'expired'
-RECEPTION_FAILURE = 'reception failure'
-TRANSMISSION_FAILURE = 'transmission failure'
-SERVICER_FAILURE = 'servicer failure'
-SERVICED_FAILURE = 'serviced failure'
 
-# Subscription categories.
-FULL = 'full'
-TERMINATION_ONLY = 'termination only'
-NONE = 'none'
+@enum.unique
+class Outcome(enum.Enum):
+  """Operation outcomes."""
+
+  COMPLETED = 'completed'
+  CANCELLED = 'cancelled'
+  EXPIRED = 'expired'
+  RECEPTION_FAILURE = 'reception failure'
+  TRANSMISSION_FAILURE = 'transmission failure'
+  SERVICER_FAILURE = 'servicer failure'
+  SERVICED_FAILURE = 'serviced failure'
 
 
 class OperationContext(object):
@@ -70,9 +67,7 @@ class OperationContext(object):
     """Adds a function to be called upon operation termination.
 
     Args:
-      callback: A callable that will be passed one of COMPLETED, CANCELLED,
-        EXPIRED, RECEPTION_FAILURE, TRANSMISSION_FAILURE, SERVICER_FAILURE, or
-        SERVICED_FAILURE.
+      callback: A callable that will be passed an Outcome value.
     """
     raise NotImplementedError()
 
@@ -167,11 +162,20 @@ class ServicedSubscription(object):
   """A sum type representing a serviced's interest in an operation.
 
   Attributes:
-    category: One of FULL, TERMINATION_ONLY, or NONE.
-    ingestor: A ServicedIngestor. Must be present if category is FULL.
+    kind: A Kind value.
+    ingestor: A ServicedIngestor. Must be present if kind is Kind.FULL. Must
+      be None if kind is Kind.TERMINATION_ONLY or Kind.NONE.
   """
   __metaclass__ = abc.ABCMeta
 
+  @enum.unique
+  class Kind(enum.Enum):
+    """Kinds of subscription."""
+
+    FULL = 'full'
+    TERMINATION_ONLY = 'termination only'
+    NONE = 'none'
+
 
 class End(object):
   """Common type for entry-point objects on both sides of an operation."""
@@ -182,9 +186,8 @@ class End(object):
     """Reports the number of terminated operations broken down by outcome.
 
     Returns:
-      A dictionary from operation outcome constant (COMPLETED, CANCELLED,
-        EXPIRED, and so on) to an integer representing the number of operations
-        that terminated with that outcome.
+      A dictionary from Outcome value to an integer identifying the number
+        of operations that terminated with that outcome.
     """
     raise NotImplementedError()
 

src/python/src/_framework/base/interfaces_test.py

@@ -49,13 +49,13 @@ TRIGGERED_FAILURE = 'triggered failure'
 WAIT_ON_CONDITION = 'wait on condition'
 
 EMPTY_OUTCOME_DICT = {
-    interfaces.COMPLETED: 0,
-    interfaces.CANCELLED: 0,
-    interfaces.EXPIRED: 0,
-    interfaces.RECEPTION_FAILURE: 0,
-    interfaces.TRANSMISSION_FAILURE: 0,
-    interfaces.SERVICER_FAILURE: 0,
-    interfaces.SERVICED_FAILURE: 0,
+    interfaces.Outcome.COMPLETED: 0,
+    interfaces.Outcome.CANCELLED: 0,
+    interfaces.Outcome.EXPIRED: 0,
+    interfaces.Outcome.RECEPTION_FAILURE: 0,
+    interfaces.Outcome.TRANSMISSION_FAILURE: 0,
+    interfaces.Outcome.SERVICER_FAILURE: 0,
+    interfaces.Outcome.SERVICED_FAILURE: 0,
     }
 
 
@@ -169,7 +169,8 @@ class FrontAndBackTest(object):
         SYNCHRONOUS_ECHO, None, True, SMALL_TIMEOUT,
         util.none_serviced_subscription(), 'test trace ID')
     util.wait_for_idle(self.front)
-    self.assertEqual(1, self.front.operation_stats()[interfaces.COMPLETED])
+    self.assertEqual(
+        1, self.front.operation_stats()[interfaces.Outcome.COMPLETED])
 
     # Assuming nothing really pathological (such as pauses on the order of
     # SMALL_TIMEOUT interfering with this test) there are a two different ways
@@ -183,7 +184,7 @@ class FrontAndBackTest(object):
     first_back_possibility = EMPTY_OUTCOME_DICT
     # (2) The packet arrived at the back and the back completed the operation.
     second_back_possibility = dict(EMPTY_OUTCOME_DICT)
-    second_back_possibility[interfaces.COMPLETED] = 1
+    second_back_possibility[interfaces.Outcome.COMPLETED] = 1
     self.assertIn(
         back_operation_stats, (first_back_possibility, second_back_possibility))
     # It's true that if the packet had arrived at the back and the back had
@@ -204,8 +205,10 @@ class FrontAndBackTest(object):
 
     util.wait_for_idle(self.front)
     util.wait_for_idle(self.back)
-    self.assertEqual(1, self.front.operation_stats()[interfaces.COMPLETED])
-    self.assertEqual(1, self.back.operation_stats()[interfaces.COMPLETED])
+    self.assertEqual(
+        1, self.front.operation_stats()[interfaces.Outcome.COMPLETED])
+    self.assertEqual(
+        1, self.back.operation_stats()[interfaces.Outcome.COMPLETED])
     self.assertListEqual([(test_payload, True)], test_consumer.calls)
 
   def testBidirectionalStreamingEcho(self):
@@ -226,8 +229,10 @@ class FrontAndBackTest(object):
 
     util.wait_for_idle(self.front)
     util.wait_for_idle(self.back)
-    self.assertEqual(1, self.front.operation_stats()[interfaces.COMPLETED])
-    self.assertEqual(1, self.back.operation_stats()[interfaces.COMPLETED])
+    self.assertEqual(
+        1, self.front.operation_stats()[interfaces.Outcome.COMPLETED])
+    self.assertEqual(
+        1, self.back.operation_stats()[interfaces.Outcome.COMPLETED])
     self.assertListEqual(test_payloads, test_consumer.values())
 
   def testCancellation(self):
@@ -242,7 +247,8 @@ class FrontAndBackTest(object):
     operation.cancel()
 
     util.wait_for_idle(self.front)
-    self.assertEqual(1, self.front.operation_stats()[interfaces.CANCELLED])
+    self.assertEqual(
+        1, self.front.operation_stats()[interfaces.Outcome.CANCELLED])
     util.wait_for_idle(self.back)
     self.assertListEqual([], test_consumer.calls)
 
@@ -260,7 +266,7 @@ class FrontAndBackTest(object):
     # The back started processing based on the first packet and then stopped
     # upon receiving the cancellation packet.
     second_back_possibility = dict(EMPTY_OUTCOME_DICT)
-    second_back_possibility[interfaces.CANCELLED] = 1
+    second_back_possibility[interfaces.Outcome.CANCELLED] = 1
     self.assertIn(
         back_operation_stats, (first_back_possibility, second_back_possibility))
 
@@ -292,8 +298,10 @@ class FrontAndBackTest(object):
     duration = termination_time_cell[0] - start_time
     self.assertLessEqual(timeout, duration)
     self.assertLess(duration, timeout + allowance)
-    self.assertEqual(interfaces.EXPIRED, outcome_cell[0])
+    self.assertEqual(interfaces.Outcome.EXPIRED, outcome_cell[0])
     util.wait_for_idle(self.front)
-    self.assertEqual(1, self.front.operation_stats()[interfaces.EXPIRED])
+    self.assertEqual(
+        1, self.front.operation_stats()[interfaces.Outcome.EXPIRED])
     util.wait_for_idle(self.back)
-    self.assertLessEqual(1, self.back.operation_stats()[interfaces.EXPIRED])
+    self.assertLessEqual(
+        1, self.back.operation_stats()[interfaces.Outcome.EXPIRED])

src/python/src/_framework/base/packets/_ends.py

@@ -51,13 +51,13 @@ from _framework.foundation import callable_util
 _IDLE_ACTION_EXCEPTION_LOG_MESSAGE = 'Exception calling idle action!'
 
 _OPERATION_OUTCOMES = (
-    base_interfaces.COMPLETED,
-    base_interfaces.CANCELLED,
-    base_interfaces.EXPIRED,
-    base_interfaces.RECEPTION_FAILURE,
-    base_interfaces.TRANSMISSION_FAILURE,
-    base_interfaces.SERVICER_FAILURE,
-    base_interfaces.SERVICED_FAILURE,
+    base_interfaces.Outcome.COMPLETED,
+    base_interfaces.Outcome.CANCELLED,
+    base_interfaces.Outcome.EXPIRED,
+    base_interfaces.Outcome.RECEPTION_FAILURE,
+    base_interfaces.Outcome.TRANSMISSION_FAILURE,
+    base_interfaces.Outcome.SERVICER_FAILURE,
+    base_interfaces.Outcome.SERVICED_FAILURE,
     )
 
 
@@ -193,10 +193,10 @@ def _front_operate(
   lock = threading.Lock()
   with lock:
     termination_manager = _termination.front_termination_manager(
-        work_pool, utility_pool, termination_action, subscription.category)
+        work_pool, utility_pool, termination_action, subscription.kind)
     transmission_manager = _transmission.front_transmission_manager(
         lock, transmission_pool, callback, operation_id, name,
-        subscription.category, trace_id, timeout, termination_manager)
+        subscription.kind, trace_id, timeout, termination_manager)
     operation_context = _context.OperationContext(
         lock, operation_id, packets.Kind.SERVICED_FAILURE,
         termination_manager, transmission_manager)
@@ -225,9 +225,10 @@ def _front_operate(
 
     transmission_manager.inmit(payload, complete)
 
-    returned_reception_manager = (
-        None if subscription.category == base_interfaces.NONE
-        else reception_manager)
+    if subscription.kind is base_interfaces.ServicedSubscription.Kind.NONE:
+      returned_reception_manager = None
+    else:
+      returned_reception_manager = reception_manager
 
     return _FrontManagement(
         returned_reception_manager, emission_manager, operation_context,

src/python/src/_framework/base/packets/_ingestion.py

@@ -111,7 +111,7 @@ class _FrontConsumerCreator(_ConsumerCreator):
 
   def create_consumer(self, requirement):
     """See _ConsumerCreator.create_consumer for specification."""
-    if self._subscription.category == interfaces.FULL:
+    if self._subscription.kind is interfaces.ServicedSubscription.Kind.FULL:
       try:
         return _ConsumerCreation(
             self._subscription.ingestor.consumer(self._operation_context),

src/python/src/_framework/base/packets/_interfaces.py

@@ -58,10 +58,7 @@ class TerminationManager(object):
     immediately.
 
     Args:
-      callback: A callable that will be passed one of base_interfaces.COMPLETED,
-        base_interfaces.CANCELLED, base_interfaces.EXPIRED,
-        base_interfaces.RECEPTION_FAILURE, base_interfaces.TRANSMISSION_FAILURE,
-        base_interfaces.SERVICER_FAILURE, or base_interfaces.SERVICED_FAILURE.
+      callback: A callable that will be passed a base_interfaces.Outcome value.
     """
     raise NotImplementedError()
 

src/python/src/_framework/base/packets/_termination.py

@@ -29,6 +29,8 @@
 
 """State and behavior for operation termination."""
 
+import enum
+
 from _framework.base import interfaces
 from _framework.base.packets import _constants
 from _framework.base.packets import _interfaces
@@ -37,26 +39,32 @@ from _framework.foundation import callable_util
 
 _CALLBACK_EXCEPTION_LOG_MESSAGE = 'Exception calling termination callback!'
 
-# TODO(nathaniel): enum module.
-_EMISSION = 'emission'
-_TRANSMISSION = 'transmission'
-_INGESTION = 'ingestion'
-
-_FRONT_NOT_LISTENING_REQUIREMENTS = (_TRANSMISSION,)
-_BACK_NOT_LISTENING_REQUIREMENTS = (_EMISSION, _INGESTION,)
-_LISTENING_REQUIREMENTS = (_TRANSMISSION, _INGESTION,)
-
 _KINDS_TO_OUTCOMES = {
-    packets.Kind.COMPLETION: interfaces.COMPLETED,
-    packets.Kind.CANCELLATION: interfaces.CANCELLED,
-    packets.Kind.EXPIRATION: interfaces.EXPIRED,
-    packets.Kind.RECEPTION_FAILURE: interfaces.RECEPTION_FAILURE,
-    packets.Kind.TRANSMISSION_FAILURE: interfaces.TRANSMISSION_FAILURE,
-    packets.Kind.SERVICER_FAILURE: interfaces.SERVICER_FAILURE,
-    packets.Kind.SERVICED_FAILURE: interfaces.SERVICED_FAILURE,
+    packets.Kind.COMPLETION: interfaces.Outcome.COMPLETED,
+    packets.Kind.CANCELLATION: interfaces.Outcome.CANCELLED,
+    packets.Kind.EXPIRATION: interfaces.Outcome.EXPIRED,
+    packets.Kind.RECEPTION_FAILURE: interfaces.Outcome.RECEPTION_FAILURE,
+    packets.Kind.TRANSMISSION_FAILURE: interfaces.Outcome.TRANSMISSION_FAILURE,
+    packets.Kind.SERVICER_FAILURE: interfaces.Outcome.SERVICER_FAILURE,
+    packets.Kind.SERVICED_FAILURE: interfaces.Outcome.SERVICED_FAILURE,
     }
 
 
+@enum.unique
+class _Requirement(enum.Enum):
+  """Symbols indicating events required for termination."""
+
+  EMISSION = 'emission'
+  TRANSMISSION = 'transmission'
+  INGESTION = 'ingestion'
+
+_FRONT_NOT_LISTENING_REQUIREMENTS = (_Requirement.TRANSMISSION,)
+_BACK_NOT_LISTENING_REQUIREMENTS = (
+    _Requirement.EMISSION, _Requirement.INGESTION,)
+_LISTENING_REQUIREMENTS = (
+    _Requirement.TRANSMISSION, _Requirement.INGESTION,)
+
+
 class _TerminationManager(_interfaces.TerminationManager):
   """An implementation of _interfaces.TerminationManager."""
 
@@ -68,9 +76,8 @@ class _TerminationManager(_interfaces.TerminationManager):
       work_pool: A thread pool in which customer work will be done.
       utility_pool: A thread pool in which work utility work will be done.
       action: An action to call on operation termination.
-      requirements: A combination of _EMISSION, _TRANSMISSION, and _INGESTION
-        identifying what must finish for the operation to be considered
-        completed.
+      requirements: A combination of _Requirement values identifying what
+        must finish for the operation to be considered completed.
       local_failure: A packets.Kind specifying what constitutes local failure of
         customer work.
     """
@@ -137,21 +144,21 @@ class _TerminationManager(_interfaces.TerminationManager):
   def emission_complete(self):
     """See superclass method for specification."""
     if self._outstanding_requirements is not None:
-      self._outstanding_requirements.discard(_EMISSION)
+      self._outstanding_requirements.discard(_Requirement.EMISSION)
       if not self._outstanding_requirements:
         self._terminate(packets.Kind.COMPLETION)
 
   def transmission_complete(self):
     """See superclass method for specification."""
     if self._outstanding_requirements is not None:
-      self._outstanding_requirements.discard(_TRANSMISSION)
+      self._outstanding_requirements.discard(_Requirement.TRANSMISSION)
       if not self._outstanding_requirements:
         self._terminate(packets.Kind.COMPLETION)
 
   def ingestion_complete(self):
     """See superclass method for specification."""
     if self._outstanding_requirements is not None:
-      self._outstanding_requirements.discard(_INGESTION)
+      self._outstanding_requirements.discard(_Requirement.INGESTION)
       if not self._outstanding_requirements:
         self._terminate(packets.Kind.COMPLETION)
 
@@ -163,39 +170,46 @@ class _TerminationManager(_interfaces.TerminationManager):
       self._terminate(kind)
 
 
-def front_termination_manager(work_pool, utility_pool, action, subscription):
+def front_termination_manager(
+    work_pool, utility_pool, action, subscription_kind):
   """Creates a TerminationManager appropriate for front-side use.
 
   Args:
     work_pool: A thread pool in which customer work will be done.
     utility_pool: A thread pool in which work utility work will be done.
     action: An action to call on operation termination.
-    subscription: One of interfaces.FULL, interfaces.termination_only, or
-      interfaces.NONE.
+    subscription_kind: An interfaces.ServicedSubscription.Kind value.
 
   Returns:
     A TerminationManager appropriate for front-side use.
   """
+  if subscription_kind is interfaces.ServicedSubscription.Kind.NONE:
+    requirements = _FRONT_NOT_LISTENING_REQUIREMENTS
+  else:
+    requirements = _LISTENING_REQUIREMENTS
+
   return _TerminationManager(
-      work_pool, utility_pool, action,
-      _FRONT_NOT_LISTENING_REQUIREMENTS if subscription == interfaces.NONE else
-      _LISTENING_REQUIREMENTS, packets.Kind.SERVICED_FAILURE)
+      work_pool, utility_pool, action, requirements,
+      packets.Kind.SERVICED_FAILURE)
 
 
-def back_termination_manager(work_pool, utility_pool, action, subscription):
+def back_termination_manager(work_pool, utility_pool, action, subscription_kind):
   """Creates a TerminationManager appropriate for back-side use.
 
   Args:
     work_pool: A thread pool in which customer work will be done.
     utility_pool: A thread pool in which work utility work will be done.
     action: An action to call on operation termination.
-    subscription: One of interfaces.FULL, interfaces.termination_only, or
-      interfaces.NONE.
+    subscription_kind: An interfaces.ServicedSubscription.Kind value.
 
   Returns:
     A TerminationManager appropriate for back-side use.
   """
+  if subscription_kind is interfaces.ServicedSubscription.Kind.NONE:
+    requirements = _BACK_NOT_LISTENING_REQUIREMENTS
+  else:
+    requirements = _LISTENING_REQUIREMENTS
+
   return _TerminationManager(
-      work_pool, utility_pool, action,
-      _BACK_NOT_LISTENING_REQUIREMENTS if subscription == interfaces.NONE else
-      _LISTENING_REQUIREMENTS, packets.Kind.SERVICER_FAILURE)
+      work_pool, utility_pool, action, requirements,
+      packets.Kind.SERVICER_FAILURE)

src/python/src/_framework/base/packets/_transmission.py

@@ -91,20 +91,19 @@ class _Packetizer(object):
 class _FrontPacketizer(_Packetizer):
   """Front-side packet-creating behavior."""
 
-  def __init__(self, name, subscription, trace_id, timeout):
+  def __init__(self, name, subscription_kind, trace_id, timeout):
     """Constructor.
 
     Args:
       name: The name of the operation.
-      subscription: One of interfaces.FULL, interfaces.TERMINATION_ONLY, or
-        interfaces.NONE describing the interest the front has in packets sent
-        from the back.
+      subscription_kind: An interfaces.ServicedSubscription.Kind value
+        describing the interest the front has in packets sent from the back.
       trace_id: A uuid.UUID identifying a set of related operations to which
         this operation belongs.
       timeout: A length of time in seconds to allow for the entire operation.
     """
     self._name = name
-    self._subscription = subscription
+    self._subscription_kind = subscription_kind
     self._trace_id = trace_id
     self._timeout = timeout
 
@@ -114,13 +113,13 @@ class _FrontPacketizer(_Packetizer):
       return packets.FrontToBackPacket(
           operation_id, sequence_number,
           packets.Kind.COMPLETION if complete else packets.Kind.CONTINUATION,
-          self._name, self._subscription, self._trace_id, payload,
+          self._name, self._subscription_kind, self._trace_id, payload,
           self._timeout)
     else:
       return packets.FrontToBackPacket(
           operation_id, 0,
           packets.Kind.ENTIRE if complete else packets.Kind.COMMENCEMENT,
-          self._name, self._subscription, self._trace_id, payload,
+          self._name, self._subscription_kind, self._trace_id, payload,
           self._timeout)
 
   def packetize_abortion(self, operation_id, sequence_number, kind):
@@ -335,8 +334,8 @@ class _TransmittingTransmissionManager(TransmissionManager):
 
 
 def front_transmission_manager(
-    lock, pool, callback, operation_id, name, subscription, trace_id, timeout,
-    termination_manager):
+    lock, pool, callback, operation_id, name, subscription_kind, trace_id,
+    timeout, termination_manager):
   """Creates a TransmissionManager appropriate for front-side use.
 
   Args:
@@ -347,9 +346,8 @@ def front_transmission_manager(
       of the operation.
     operation_id: The operation's ID.
     name: The name of the operation.
-    subscription: One of interfaces.FULL, interfaces.TERMINATION_ONLY, or
-      interfaces.NONE describing the interest the front has in packets sent
-      from the back.
+    subscription_kind: An interfaces.ServicedSubscription.Kind value
+      describing the interest the front has in packets sent from the back.
     trace_id: A uuid.UUID identifying a set of related operations to which
       this operation belongs.
     timeout: A length of time in seconds to allow for the entire operation.
@@ -361,12 +359,13 @@ def front_transmission_manager(
   """
   return _TransmittingTransmissionManager(
       lock, pool, callback, operation_id, _FrontPacketizer(
-          name, subscription, trace_id, timeout),
+          name, subscription_kind, trace_id, timeout),
       termination_manager)
 
 
 def back_transmission_manager(
-    lock, pool, callback, operation_id, termination_manager, subscription):
+    lock, pool, callback, operation_id, termination_manager,
+    subscription_kind):
   """Creates a TransmissionManager appropriate for back-side use.
 
   Args:
@@ -378,14 +377,13 @@ def back_transmission_manager(
     operation_id: The operation's ID.
     termination_manager: The _interfaces.TerminationManager associated with
       this operation.
-    subscription: One of interfaces.FULL, interfaces.TERMINATION_ONLY, or
-      interfaces.NONE describing the interest the front has in packets sent from
-      the back.
+    subscription_kind: An interfaces.ServicedSubscription.Kind value
+      describing the interest the front has in packets sent from the back.
 
   Returns:
     A TransmissionManager appropriate for back-side use.
   """
-  if subscription == interfaces.NONE:
+  if subscription_kind is interfaces.ServicedSubscription.Kind.NONE:
     return _EmptyTransmissionManager()
   else:
     return _TransmittingTransmissionManager(

src/python/src/_framework/base/packets/packets.py

@@ -71,10 +71,9 @@ class FrontToBackPacket(
       Kind.RECEPTION_FAILURE, or Kind.TRANSMISSION_FAILURE.
     name: The name of an operation. Must be present if kind is Kind.COMMENCEMENT
       or Kind.ENTIRE. Must be None for any other kind.
-    subscription: One of interfaces.FULL, interfaces.TERMINATION_ONLY, or
-      interfaces.NONE describing the interest the front has in packets sent from
-      the back. Must be present if kind is Kind.COMMENCEMENT or Kind.ENTIRE.
-      Must be None for any other kind.
+    subscription: An interfaces.ServicedSubscription.Kind value describing the
+      interest the front has in packets sent from the back. Must be present if
+      kind is Kind.COMMENCEMENT or Kind.ENTIRE. Must be None for any other kind.
     trace_id: A uuid.UUID identifying a set of related operations to which this
       operation belongs. May be None.
     payload: A customer payload object. Must be present if kind is

src/python/src/_framework/base/util.py

@@ -36,13 +36,14 @@ from _framework.base import interfaces
 
 
 class _ServicedSubscription(
-    collections.namedtuple('_ServicedSubscription', ['category', 'ingestor']),
+    collections.namedtuple('_ServicedSubscription', ['kind', 'ingestor']),
     interfaces.ServicedSubscription):
   """See interfaces.ServicedSubscription for specification."""
 
-_NONE_SUBSCRIPTION = _ServicedSubscription(interfaces.NONE, None)
+_NONE_SUBSCRIPTION = _ServicedSubscription(
+    interfaces.ServicedSubscription.Kind.NONE, None)
 _TERMINATION_ONLY_SUBSCRIPTION = _ServicedSubscription(
-    interfaces.TERMINATION_ONLY, None)
+    interfaces.ServicedSubscription.Kind.TERMINATION_ONLY, None)
 
 
 def none_serviced_subscription():
@@ -72,12 +73,14 @@ def full_serviced_subscription(ingestor):
   """Creates a "full" interfaces.ServicedSubscription object.
 
   Args:
-    ingestor: A ServicedIngestor.
+    ingestor: An interfaces.ServicedIngestor.
 
   Returns:
-    A ServicedSubscription object indicating a full subscription.
+    An interfaces.ServicedSubscription object indicating a full
+      subscription.
   """
-  return _ServicedSubscription(interfaces.FULL, ingestor)
+  return _ServicedSubscription(
+      interfaces.ServicedSubscription.Kind.FULL, ingestor)
 
 
 def wait_for_idle(end):

src/python/src/_framework/face/_calls.py

@@ -94,7 +94,7 @@ class _OperationCancellableIterator(interfaces.CancellableIterator):
 
   def cancel(self):
     self._operation.cancel()
-    self._rendezvous.set_outcome(base_interfaces.CANCELLED)
+    self._rendezvous.set_outcome(base_interfaces.Outcome.CANCELLED)
 
 
 class _OperationFuture(future.Future):
@@ -150,15 +150,12 @@ class _OperationFuture(future.Future):
     """Indicates to this object that the operation has terminated.
 
     Args:
-      operation_outcome: One of base_interfaces.COMPLETED,
-        base_interfaces.CANCELLED, base_interfaces.EXPIRED,
-        base_interfaces.RECEPTION_FAILURE, base_interfaces.TRANSMISSION_FAILURE,
-        base_interfaces.SERVICED_FAILURE, or base_interfaces.SERVICER_FAILURE
-        indicating the categorical outcome of the operation.
+      operation_outcome: A base_interfaces.Outcome value indicating the
+        outcome of the operation.
     """
     with self._condition:
       if (self._outcome is None and
-          operation_outcome != base_interfaces.COMPLETED):
+          operation_outcome is not base_interfaces.Outcome.COMPLETED):
         self._outcome = future.raised(
             _control.abortion_outcome_to_exception(operation_outcome))
         self._condition.notify_all()

src/python/src/_framework/face/_control.py

@@ -40,13 +40,17 @@ from _framework.foundation import stream
 INTERNAL_ERROR_LOG_MESSAGE = ':-( RPC Framework (Face) Internal Error! :-('
 
 _OPERATION_OUTCOME_TO_RPC_ABORTION = {
-    base_interfaces.CANCELLED: interfaces.CANCELLED,
-    base_interfaces.EXPIRED: interfaces.EXPIRED,
-    base_interfaces.RECEPTION_FAILURE: interfaces.NETWORK_FAILURE,
-    base_interfaces.TRANSMISSION_FAILURE: interfaces.NETWORK_FAILURE,
-    base_interfaces.SERVICED_FAILURE: interfaces.SERVICED_FAILURE,
-    base_interfaces.SERVICER_FAILURE: interfaces.SERVICER_FAILURE,
-    }
+    base_interfaces.Outcome.CANCELLED: interfaces.Abortion.CANCELLED,
+    base_interfaces.Outcome.EXPIRED: interfaces.Abortion.EXPIRED,
+    base_interfaces.Outcome.RECEPTION_FAILURE:
+        interfaces.Abortion.NETWORK_FAILURE,
+    base_interfaces.Outcome.TRANSMISSION_FAILURE:
+        interfaces.Abortion.NETWORK_FAILURE,
+    base_interfaces.Outcome.SERVICED_FAILURE:
+        interfaces.Abortion.SERVICED_FAILURE,
+    base_interfaces.Outcome.SERVICER_FAILURE:
+        interfaces.Abortion.SERVICER_FAILURE,
+}
 
 
 def _as_operation_termination_callback(rpc_abortion_callback):
@@ -59,13 +63,13 @@ def _as_operation_termination_callback(rpc_abortion_callback):
 
 
 def _abortion_outcome_to_exception(abortion_outcome):
-  if abortion_outcome == base_interfaces.CANCELLED:
+  if abortion_outcome == base_interfaces.Outcome.CANCELLED:
     return exceptions.CancellationError()
-  elif abortion_outcome == base_interfaces.EXPIRED:
+  elif abortion_outcome == base_interfaces.Outcome.EXPIRED:
     return exceptions.ExpirationError()
-  elif abortion_outcome == base_interfaces.SERVICER_FAILURE:
+  elif abortion_outcome == base_interfaces.Outcome.SERVICER_FAILURE:
     return exceptions.ServicerError()
-  elif abortion_outcome == base_interfaces.SERVICED_FAILURE:
+  elif abortion_outcome == base_interfaces.Outcome.SERVICED_FAILURE:
     return exceptions.ServicedError()
   else:
     return exceptions.NetworkError()
@@ -133,7 +137,7 @@ class Rendezvous(stream.Consumer):
 
   def set_outcome(self, outcome):
     with self._condition:
-      if outcome != base_interfaces.COMPLETED:
+      if outcome is not base_interfaces.Outcome.COMPLETED:
         self._abortion = outcome
         self._condition.notify()
 

src/python/src/_framework/face/interfaces.py

@@ -30,6 +30,7 @@
 """Interfaces for the face layer of RPC Framework."""
 
 import abc
+import enum
 
 # exceptions, abandonment, and future are referenced from specification in this
 # module.
@@ -58,14 +59,15 @@ class CancellableIterator(object):
     raise NotImplementedError()
 
 
-# Constants that categorize RPC abortion.
-# TODO(nathaniel): Learn and use Python's enum library for this de facto
-# enumerated type
-CANCELLED = 'abortion: cancelled'
-EXPIRED = 'abortion: expired'
-NETWORK_FAILURE = 'abortion: network failure'
-SERVICED_FAILURE = 'abortion: serviced failure'
-SERVICER_FAILURE = 'abortion: servicer failure'
+@enum.unique
+class Abortion(enum.Enum):
+  """Categories of RPC abortion."""
+
+  CANCELLED = 'cancelled'
+  EXPIRED = 'expired'
+  NETWORK_FAILURE = 'network failure'
+  SERVICED_FAILURE = 'serviced failure'
+  SERVICER_FAILURE = 'servicer failure'
 
 
 class RpcContext(object):
@@ -93,9 +95,8 @@ class RpcContext(object):
     """Registers a callback to be called if the RPC is aborted.
 
     Args:
-      abortion_callback: A callable to be called and passed one of CANCELLED,
-        EXPIRED, NETWORK_FAILURE, SERVICED_FAILURE, or SERVICER_FAILURE in the
-        event of RPC abortion.
+      abortion_callback: A callable to be called and passed an Abortion value
+        in the event of RPC abortion.
     """
     raise NotImplementedError()
 
@@ -474,9 +475,8 @@ class Stub(object):
       request: The request value for the RPC.
       response_callback: A callback to be called to accept the response value
         of the RPC.
-      abortion_callback: A callback to be called to accept one of CANCELLED,
-        EXPIRED, NETWORK_FAILURE, or SERVICER_FAILURE in the event of RPC
-        abortion.
+      abortion_callback: A callback to be called and passed an Abortion value
+        in the event of RPC abortion.
       timeout: A duration of time in seconds to allow for the RPC.
 
     Returns:
@@ -494,9 +494,8 @@ class Stub(object):
       request: The request value for the RPC.
       response_consumer: A stream.Consumer to be called to accept the response
         values of the RPC.
-      abortion_callback: A callback to be called to accept one of CANCELLED,
-        EXPIRED, NETWORK_FAILURE, or SERVICER_FAILURE in the event of RPC
-        abortion.
+      abortion_callback: A callback to be called and passed an Abortion value
+        in the event of RPC abortion.
       timeout: A duration of time in seconds to allow for the RPC.
 
     Returns:
@@ -513,9 +512,8 @@ class Stub(object):
       name: The RPC method name.
       response_callback: A callback to be called to accept the response value
         of the RPC.
-      abortion_callback: A callback to be called to accept one of CANCELLED,
-        EXPIRED, NETWORK_FAILURE, or SERVICER_FAILURE in the event of RPC
-        abortion.
+      abortion_callback: A callback to be called and passed an Abortion value
+        in the event of RPC abortion.
       timeout: A duration of time in seconds to allow for the RPC.
 
     Returns:
@@ -533,9 +531,8 @@ class Stub(object):
       name: The RPC method name.
       response_consumer: A stream.Consumer to be called to accept the response
         values of the RPC.
-      abortion_callback: A callback to be called to accept one of CANCELLED,
-        EXPIRED, NETWORK_FAILURE, or SERVICER_FAILURE in the event of RPC
-        abortion.
+      abortion_callback: A callback to be called and passed an Abortion value
+        in the event of RPC abortion.
       timeout: A duration of time in seconds to allow for the RPC.
 
     Returns:

src/python/src/_framework/face/testing/event_invocation_synchronous_event_service_test_case.py

@@ -176,7 +176,7 @@ class EventInvocationSynchronousEventServiceTestCase(
               name, request, callback.complete, callback.abort, _TIMEOUT)
           callback.block_until_terminated()
 
-        self.assertEqual(interfaces.EXPIRED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.EXPIRED, callback.abortion())
 
   def testExpiredUnaryRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -190,7 +190,7 @@ class EventInvocationSynchronousEventServiceTestCase(
               name, request, callback, callback.abort, _TIMEOUT)
           callback.block_until_terminated()
 
-        self.assertEqual(interfaces.EXPIRED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.EXPIRED, callback.abortion())
 
   def testExpiredStreamRequestUnaryResponse(self):
     for name, test_messages_sequence in (
@@ -202,7 +202,7 @@ class EventInvocationSynchronousEventServiceTestCase(
             name, callback.complete, callback.abort, _TIMEOUT)
         callback.block_until_terminated()
 
-        self.assertEqual(interfaces.EXPIRED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.EXPIRED, callback.abortion())
 
   def testExpiredStreamRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -217,7 +217,7 @@ class EventInvocationSynchronousEventServiceTestCase(
           request_consumer.consume(request)
         callback.block_until_terminated()
 
-        self.assertEqual(interfaces.EXPIRED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.EXPIRED, callback.abortion())
 
   def testFailedUnaryRequestUnaryResponse(self):
     for name, test_messages_sequence in (
@@ -231,7 +231,7 @@ class EventInvocationSynchronousEventServiceTestCase(
               name, request, callback.complete, callback.abort, _TIMEOUT)
           callback.block_until_terminated()
 
-        self.assertEqual(interfaces.SERVICER_FAILURE, callback.abortion())
+        self.assertEqual(interfaces.Abortion.SERVICER_FAILURE, callback.abortion())
 
   def testFailedUnaryRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -245,7 +245,7 @@ class EventInvocationSynchronousEventServiceTestCase(
               name, request, callback, callback.abort, _TIMEOUT)
           callback.block_until_terminated()
 
-        self.assertEqual(interfaces.SERVICER_FAILURE, callback.abortion())
+        self.assertEqual(interfaces.Abortion.SERVICER_FAILURE, callback.abortion())
 
   def testFailedStreamRequestUnaryResponse(self):
     for name, test_messages_sequence in (
@@ -262,7 +262,7 @@ class EventInvocationSynchronousEventServiceTestCase(
           request_consumer.terminate()
           callback.block_until_terminated()
 
-        self.assertEqual(interfaces.SERVICER_FAILURE, callback.abortion())
+        self.assertEqual(interfaces.Abortion.SERVICER_FAILURE, callback.abortion())
 
   def testFailedStreamRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -279,7 +279,7 @@ class EventInvocationSynchronousEventServiceTestCase(
           request_consumer.terminate()
           callback.block_until_terminated()
 
-        self.assertEqual(interfaces.SERVICER_FAILURE, callback.abortion())
+        self.assertEqual(interfaces.Abortion.SERVICER_FAILURE, callback.abortion())
 
   def testParallelInvocations(self):
     for name, test_messages_sequence in (
@@ -321,7 +321,7 @@ class EventInvocationSynchronousEventServiceTestCase(
           call.cancel()
           callback.block_until_terminated()
 
-        self.assertEqual(interfaces.CANCELLED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.CANCELLED, callback.abortion())
 
   def testCancelledUnaryRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -335,7 +335,7 @@ class EventInvocationSynchronousEventServiceTestCase(
         call.cancel()
         callback.block_until_terminated()
 
-        self.assertEqual(interfaces.CANCELLED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.CANCELLED, callback.abortion())
 
   def testCancelledStreamRequestUnaryResponse(self):
     for name, test_messages_sequence in (
@@ -351,7 +351,7 @@ class EventInvocationSynchronousEventServiceTestCase(
         call.cancel()
         callback.block_until_terminated()
 
-        self.assertEqual(interfaces.CANCELLED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.CANCELLED, callback.abortion())
 
   def testCancelledStreamRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -364,4 +364,4 @@ class EventInvocationSynchronousEventServiceTestCase(
         call.cancel()
         callback.block_until_terminated()
 
-        self.assertEqual(interfaces.CANCELLED, callback.abortion())
+        self.assertEqual(interfaces.Abortion.CANCELLED, callback.abortion())