Check exclude from indexes type in Entity ctor.

Fixes googleapis#819.

gcloud/datastore/entity.py

@@ -70,11 +70,21 @@ class Entity(dict):
     :type exclude_from_indexes: tuple of string
     :param exclude_from_indexes: Names of fields whose values are not to be
                                  indexed for this entity.
+
+    :raises: class:`TypeError` if ``exclude_from_indexes`` is not a tuple or
+             list.
     """
 
     def __init__(self, key=None, exclude_from_indexes=()):
         super(Entity, self).__init__()
         self.key = key
+
+        # NOTE: This is the same whitelist used for sequence inputs
+        #       in query.py.
+        if not isinstance(exclude_from_indexes, (tuple, list)):
+            raise TypeError('Expected exclude_from_indexes to be a tuple or '
+                            'list. Received %r' % (exclude_from_indexes,))
+
         self._exclude_from_indexes = set(exclude_from_indexes)
 
     def __eq__(self, other):

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)