Check exclude from indexes type in Entity ctor.

Fixes #819.

gcloud/datastore/entity.py

@@ -15,6 +15,9 @@
 """Class for representing a single entity in the Cloud Datastore."""
 
 
+from gcloud._helpers import _ensure_tuple_or_list
+
+
 class Entity(dict):
     """Entities are akin to rows in a relational database
 
@@ -76,7 +79,8 @@ class Entity(dict):
     def __init__(self, key=None, exclude_from_indexes=()):
         super(Entity, self).__init__()
         self.key = key
-        self._exclude_from_indexes = set(exclude_from_indexes)
+        self._exclude_from_indexes = set(_ensure_tuple_or_list(
+            'exclude_from_indexes', exclude_from_indexes))
 
     def __eq__(self, other):
         """Compare two entities for equality.

gcloud/datastore/test_entity.py

@@ -52,6 +52,12 @@ def test_ctor_explicit(self):
         self.assertEqual(sorted(entity.exclude_from_indexes),
                          sorted(_EXCLUDE_FROM_INDEXES))
 
+    def test_ctor_bad_exclude_from_indexes(self):
+        BAD_EXCLUDE_FROM_INDEXES = object()
+        key = _Key()
+        self.assertRaises(TypeError, self._makeOne, key=key,
+                          exclude_from_indexes=BAD_EXCLUDE_FROM_INDEXES)
+
     def test___eq_____ne___w_non_entity(self):
         from gcloud.datastore.key import Key
         key = Key(_KIND, _ID, dataset_id=_DATASET_ID)