Raise error when no entites requested

link/service/ensure.py

@@ -0,0 +1,16 @@
+"""Contains preconditions that are applied to the handlers."""
+from link.domain.commands import BatchCommand
+
+
+class NoEntitiesRequested(Exception):
+    """This exception is raised when a batch command that requests no entities is encountered."""
+
+    def __init__(self, command: BatchCommand) -> None:
+        """Initialize the exception."""
+        self.command = command
+
+
+def requests_entities(command: BatchCommand) -> None:
+    """Raise an exception if the given command requests no entities."""
+    if not command.requested:
+        raise NoEntitiesRequested(command)

link/service/handlers.py

@@ -6,6 +6,7 @@
 from link.domain import commands, events
 from link.domain.state import Processes
 
+from . import ensure
 from .messagebus import MessageBus
 from .progress import ProgessDisplay
 from .uow import UnitOfWork
@@ -31,6 +32,7 @@ def delete_entity(command: commands.DeleteEntity, *, uow: UnitOfWork, message_bu
 
 def pull(command: commands.PullEntities, *, message_bus: MessageBus) -> None:
     """Pull entities across the link."""
+    ensure.requests_entities(command)
     message_bus.handle(events.BatchProcessingStarted(Processes.PULL, command.requested))
     for identifier in command.requested:
         message_bus.handle(commands.PullEntity(identifier))
@@ -39,6 +41,7 @@ def pull(command: commands.PullEntities, *, message_bus: MessageBus) -> None:
 
 def delete(command: commands.DeleteEntities, *, message_bus: MessageBus) -> None:
     """Delete shared entities."""
+    ensure.requests_entities(command)
     message_bus.handle(events.BatchProcessingStarted(Processes.DELETE, command.requested))
     for identifier in command.requested:
         message_bus.handle(commands.DeleteEntity(identifier))

tests/integration/test_services.py

@@ -7,6 +7,7 @@
 
 from link.domain import commands, events
 from link.domain.state import Components, Processes, State, states
+from link.service.ensure import NoEntitiesRequested
 from link.service.handlers import delete, delete_entity, pull, pull_entity
 from link.service.messagebus import CommandHandlers, EventHandlers, MessageBus
 from link.service.uow import UnitOfWork
@@ -171,3 +172,17 @@ def test_pulled_entity_ends_in_correct_state(state: EntityConfig, expected: type
     pull_service(commands.PullEntities(frozenset(create_identifiers("1"))))
     with uow:
         assert uow.entities.create_entity(create_identifier("1")).state is expected
+
+
+@pytest.mark.parametrize(
+    ("create_service", "command_cls"),
+    [(create_pull_service, commands.PullEntities), (create_delete_service, commands.DeleteEntities)],
+)
+def test_requesting_no_entities_raises_error(
+    create_service: Callable[[UnitOfWork], Callable[[commands.BatchCommand], None]],
+    command_cls: type[commands.BatchCommand],
+) -> None:
+    uow = create_uow(**STATES[0])
+    service = create_service(uow)
+    with pytest.raises(NoEntitiesRequested):
+        service(command_cls(frozenset()))