Fix #135: enforce contract for Key.parent().

gcloud/datastore/key.py

@@ -268,7 +268,9 @@ def parent(self):#pragma NO COVER
     :returns: a new `Key` instance, whose path consists of all but the last
     element of self's path.  If self has only one path element, return None.
     """
-    raise NotImplementedError
+    if len(self._path) <= 1:
+        return None
+    return self.path(self.path()[:-1])
 
   def __repr__(self): #pragma NO COVER
     return '<Key%s>' % self.path()

gcloud/datastore/test_key.py

@@ -339,6 +339,14 @@ def test_id_or_name_w_name_only(self):
         key = self._makeOne(path=_PATH)
         self.assertEqual(key.id_or_name(), _NAME)
 
-    def _ugh(self):
-        protokey = key.to_protobuf()
-        self.assertEqual(protokey.partition_id.dataset_id, _DATASET)
+    def test_parent_default(self):
+        key = self._makeOne()
+        self.assertEqual(key.parent(), None)
+
+    def test_parent_explicit_top_level(self):
+        key = self._getTargetClass().from_path('abc', 'def')
+        self.assertEqual(key.parent(), None)
+
+    def test_parent_explicit_nested(self):
+        key = self._getTargetClass().from_path('abc', 'def', 'ghi', 123)
+        self.assertEqual(key.parent().path(), [{'kind': 'abc', 'name': 'def'}])