Improve error msg for empty data (#1178)

python/langsmith/evaluation/_arunner.py

@@ -520,9 +520,18 @@ async def aget_evaluation_results(self) -> AsyncIterator[EvaluationResults]:
                 yield result
 
     async def astart(self) -> _AsyncExperimentManager:
-        first_example = await aitertools.py_anext(await self.aget_examples())
+        try:
+            first_example = await aitertools.py_anext(await self.aget_examples())
+        except StopAsyncIteration:
+            raise ValueError(
+                "No examples found in the dataset. "
+                "Please ensure the data provided to aevaluate is not empty."
+            )
         if not first_example:
-            raise ValueError("No examples found in the dataset.")
+            raise ValueError(
+                "No examples found in the dataset."
+                "Please ensure the data provided to aevaluate is not empty."
+            )
         project = self._get_project(first_example)
         self._print_experiment_start(project, first_example)
         self._metadata["num_repetitions"] = self._num_repetitions

python/tests/evaluation/test_evaluation.py

@@ -456,3 +456,31 @@ def test_pytest_skip():
 @test
 async def test_async_pytest_skip():
     pytest.skip("Skip this test")
+
+
+async def test_aevaluate_good_error():
+    client = Client()
+    ds_name = "__Empty Dataset Do Not Modify"
+    if not client.has_dataset(dataset_name=ds_name):
+        client.create_dataset(dataset_name=ds_name)
+
+    async def predict(inputs: dict):
+        return {}
+
+    match_val = "No examples found in the dataset."
+    with pytest.raises(ValueError, match=match_val):
+        await aevaluate(
+            predict,
+            data=ds_name,
+        )
+
+    with pytest.raises(ValueError, match=match_val):
+        await aevaluate(
+            predict,
+            data=[],
+        )
+    with pytest.raises(ValueError, match=match_val):
+        await aevaluate(
+            predict,
+            data=(_ for _ in range(0)),
+        )