Lazily fetch keys on request; fixes #33

Note that this changes ResultCursor.keys from a property to a method

Author: Nigel Small

neo4j/v1/session.py

@@ -129,7 +129,7 @@ def __init__(self, connection, statement, parameters):
         super(ResultCursor, self).__init__()
         self.statement = statement
         self.parameters = parameters
-        self.keys = None
+        self._keys = None
         self._connection = connection
         self._current = None
         self._next = deque()
@@ -156,7 +156,7 @@ def next(self):
         """
         if self._next:
             values = self._next.popleft()
-            self._current = Record(self.keys, tuple(map(hydrated, values)))
+            self._current = Record(self.keys(), tuple(map(hydrated, values)))
             self._position += 1
             return True
         elif self._consumed:
@@ -199,6 +199,14 @@ def __getitem__(self, item):
             raise TypeError("No current record")
         return current[item]
 
+    def keys(self):
+        """ Return the keys for the records.
+        """
+        # Fetch messages until we have the header or a failure
+        while self._keys is None and not self._consumed:
+            self._connection.fetch_next()
+        return self._keys
+
     def get(self, item, default=None):
         current = self._current
         if current is None:
@@ -224,7 +232,7 @@ def _consume(self):
 
     def _on_header(self, metadata):
         # Called on receipt of the result header.
-        self.keys = metadata["fields"]
+        self._keys = metadata["fields"]
 
     def _on_record(self, values):
         # Called on receipt of each result record.

test/test_session.py

@@ -261,6 +261,19 @@ def test_can_obtain_notification_info(self):
             assert position.line == 1
             assert position.column == 1
 
+    def test_keys_are_available_before_and_after_stream(self):
+        with GraphDatabase.driver("bolt://localhost").session() as session:
+            cursor = session.run("UNWIND range(1, 10) AS n RETURN n")
+            assert list(cursor.keys()) == ["n"]
+            _ = list(cursor.stream())
+            assert list(cursor.keys()) == ["n"]
+
+    def test_keys_with_an_error(self):
+        with GraphDatabase.driver("bolt://localhost").session() as session:
+            cursor = session.run("X")
+            with self.assertRaises(CypherError):
+                _ = list(cursor.keys())
+
 
 class ResetTestCase(TestCase):