Add multiple result exception for GAX.

vision/google/cloud/vision/_gax.py

@@ -53,6 +53,8 @@ def annotate(self, image, features):
         images = annotator_client.batch_annotate_images(requests)
         if len(images.responses) == 1:
             return Annotations.from_pb(images.responses[0])
+        elif len(images.responses) > 1:
+            raise ValueError('Multiple image processing is not yet supported.')
 
 
 def _to_gapic_feature(feature):

vision/unit_tests/test__gax.py

@@ -82,6 +82,31 @@ def test_annotate_no_results(self):
 
         gax_api._annotator_client.batch_annotate_images.assert_called()
 
+    def test_annotate_multiple_results(self):
+        from google.cloud.vision.feature import Feature
+        from google.cloud.vision.feature import FeatureTypes
+        from google.cloud.vision.image import Image
+
+        client = mock.Mock(spec_set=[])
+        feature = Feature(FeatureTypes.LABEL_DETECTION, 5)
+        image_content = b'abc 1 2 3'
+        image = Image(client, content=image_content)
+        with mock.patch('google.cloud.vision._gax.image_annotator_client.'
+                        'ImageAnnotatorClient'):
+            gax_api = self._make_one(client)
+
+        mock_response = {
+            'batch_annotate_images.return_value': mock.Mock(responses=[1, 2]),
+        }
+
+        gax_api._annotator_client = mock.Mock(
+            spec_set=['batch_annotate_images'], **mock_response)
+        with mock.patch('google.cloud.vision._gax.Annotations'):
+            with self.assertRaises(ValueError):
+                gax_api.annotate(image, [feature])
+
+        gax_api._annotator_client.batch_annotate_images.assert_called()
+
 
 class Test__to_gapic_feature(unittest.TestCase):
     def _call_fut(self, feature):