Raise error in Choice on duplicate outlets (#545)

* Raise error in `Choice` on duplicate outlets

* Move validation

* Add tests, improve error messages

storey/flow.py

@@ -23,7 +23,7 @@
 from asyncio import Task
 from collections import defaultdict
 from concurrent.futures import ProcessPoolExecutor, ThreadPoolExecutor
-from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Union
+from typing import Any, Callable, Collection, Dict, Iterable, List, Optional, Set, Union
 
 import aiohttp
 
@@ -363,12 +363,12 @@ def _init(self):
         # TODO: hacky way of supporting mlrun preview, which replaces targets with a DFTarget
         self._passthrough_for_preview = list(self._name_to_outlet) == ["dataframe"]
 
-    def select_outlets(self, event) -> List[str]:
+    def select_outlets(self, event) -> Collection[str]:
         """
         Override this method to route events based on a customer logic. The default implementation will route all
         events to all outlets.
         """
-        return list(self._name_to_outlet.keys())
+        return self._name_to_outlet.keys()
 
     async def _do(self, event):
         if event is _termination_obj:
@@ -381,6 +381,11 @@ async def _do(self, event):
                 outlet = self._name_to_outlet["dataframe"]
                 outlets.append(outlet)
             else:
+                if len(set(outlet_names)) != len(outlet_names):
+                    raise ValueError(
+                        "select_outlets() returned duplicate outlets among the defined outlets: "
+                        + ", ".join(outlet_names)
+                    )
                 for outlet_name in outlet_names:
                     if outlet_name not in self._name_to_outlet:
                         raise ValueError(

tests/test_flow.py

@@ -1728,6 +1728,50 @@ def select_outlets(self, event):
     assert termination_result == expected
 
 
+def test_duplicate_choice():
+    class DuplicateChoice(Choice):
+        def select_outlets(self, event):
+            outlets = ["all_events", "all_events"]
+            return outlets
+
+    source = SyncEmitSource()
+    duplicate_choice = DuplicateChoice(termination_result_fn=lambda x, y: x + y)
+    all_events = Map(lambda x: x, name="all_events")
+
+    source.to(duplicate_choice).to(all_events)
+
+    controller = source.run()
+    controller.emit(0)
+    controller.terminate()
+    with pytest.raises(
+        ValueError,
+        match=r"select_outlets\(\) returned duplicate outlets among the defined outlets: all_events, all_events",
+    ):
+        controller.await_termination()
+
+
+def test_nonexistent_choice():
+    class NonexistentChoice(Choice):
+        def select_outlets(self, event):
+            outlets = ["wrong"]
+            return outlets
+
+    source = SyncEmitSource()
+    nonexistent_choice = NonexistentChoice(termination_result_fn=lambda x, y: x + y)
+    all_events = Map(lambda x: x, name="all_events")
+
+    source.to(nonexistent_choice).to(all_events)
+
+    controller = source.run()
+    controller.emit(0)
+    controller.terminate()
+    with pytest.raises(
+        ValueError,
+        match=r"select_outlets\(\) returned outlet name 'wrong', which is not one of the defined outlets: all_events",
+    ):
+        controller.await_termination()
+
+
 def test_metadata():
     def mapf(x):
         x.key = x.key + 1