Ensuring query constructor args are correct types.

Fixes #887.

gcloud/datastore/query.py

@@ -90,9 +90,32 @@ def __init__(self,
         self._namespace = namespace
         self._ancestor = ancestor
         self._filters = list(filters)
-        self._projection = list(projection)
-        self._order = list(order)
-        self._group_by = list(group_by)
+        self._projection = self._ensure_tuple_or_list('projection', projection)
+        self._order = self._ensure_tuple_or_list('order', order)
+        self._group_by = self._ensure_tuple_or_list('group_by', group_by)
+
+    @staticmethod
+    def _ensure_tuple_or_list(arg_name, tuple_or_list):
+        """Ensures an input is a tuple or list.
+
+        This effectively reduces the iterable types allowed to a very short
+        whitelist: list and tuple.
+
+        :type arg_name: string
+        :param arg_name: Name of argument to use in error message.
+
+        :type tuple_or_list: sequence of string
+        :param tuple_or_list: Sequence to be verified.
+
+        :rtype: list of string
+        :returns: The ``tuple_or_list`` passed in cast to a ``list``.
+        :raises: class:`TypeError` if the ``tuple_or_list`` is not a tuple or
+                 list.
+        """
+        if not isinstance(tuple_or_list, (tuple, list)):
+            raise TypeError('Expected %s to be a tuple or list. '
+                            'Received %r' % (arg_name, tuple_or_list))
+        return list(tuple_or_list)
 
     @property
     def dataset_id(self):

gcloud/datastore/test_query.py

@@ -79,6 +79,27 @@ def test_ctor_explicit(self):
         self.assertEqual(query.order, ORDER)
         self.assertEqual(query.group_by, GROUP_BY)
 
+    def test_ctor_bad_projection(self):
+        _DATASET = 'DATASET'
+        _KIND = 'KIND'
+        BAD_PROJECTION = object()
+        self.assertRaises(TypeError, self._makeOne, _KIND, _DATASET,
+                          projection=BAD_PROJECTION)
+
+    def test_ctor_bad_order(self):
+        _DATASET = 'DATASET'
+        _KIND = 'KIND'
+        BAD_ORDER = object()
+        self.assertRaises(TypeError, self._makeOne, _KIND, _DATASET,
+                          order=BAD_ORDER)
+
+    def test_ctor_bad_group_by(self):
+        _DATASET = 'DATASET'
+        _KIND = 'KIND'
+        BAD_GROUP_BY = object()
+        self.assertRaises(TypeError, self._makeOne, _KIND, _DATASET,
+                          group_by=BAD_GROUP_BY)
+
     def test_namespace_setter_w_non_string(self):
         _DATASET = 'DATASET'
         query = self._makeOne(dataset_id=_DATASET)