Add 'Session.labels' property.

Read-only, set via ctor.

spanner/google/cloud/spanner_v1/session.py

@@ -44,13 +44,19 @@ class Session(object):
 
     :type database: :class:`~google.cloud.spanner_v1.database.Database`
     :param database: The database to which the session is bound.
+
+    :type labels: dict (str -> str)
+    :param labels: (Optional) User-assigned labels for the session.
     """
 
     _session_id = None
     _transaction = None
 
-    def __init__(self, database):
+    def __init__(self, database, labels=None):
         self._database = database
+        if labels is None:
+            labels = {}
+        self._labels = labels
 
     def __lt__(self, other):
         return self._session_id < other._session_id
@@ -60,6 +66,15 @@ def session_id(self):
         """Read-only ID, set by the back-end during :meth:`create`."""
         return self._session_id
 
+    @property
+    def labels(self):
+        """User-assigned labels for the session.
+
+        :rtype: dict (str -> str)
+        :returns: the labels dict (empty if no labels were assigned.
+        """
+        return self._labels
+
     @property
     def name(self):
         """Session name used in requests.

spanner/tests/unit/test_session.py

@@ -44,11 +44,20 @@ def _getTargetClass(self):
     def _make_one(self, *args, **kwargs):
         return self._getTargetClass()(*args, **kwargs)
 
-    def test_constructor(self):
+    def test_constructor_wo_labels(self):
         database = _Database(self.DATABASE_NAME)
         session = self._make_one(database)
         self.assertIs(session.session_id, None)
         self.assertIs(session._database, database)
+        self.assertEqual(session.labels, {})
+
+    def test_constructor_w_labels(self):
+        database = _Database(self.DATABASE_NAME)
+        labels = {'foo': 'bar'}
+        session = self._make_one(database, labels=labels)
+        self.assertIs(session.session_id, None)
+        self.assertIs(session._database, database)
+        self.assertEqual(session.labels, labels)
 
     def test___lt___(self):
         database = _Database(self.DATABASE_NAME)