check invalid string type for dest_nodes in extract_sub_graph (#13057)

* BUG: check str type

* TST: add unit test

* CLN: remove list check

* CLN: use warning

* CLN: 2 indent

* CLN: raise TypeError if not list

* CLN: check string only

tensorflow/python/framework/graph_util_impl.py

@@ -21,6 +21,7 @@ from __future__ import division
 from __future__ import print_function
 import copy
 import re
+import six
 
 from tensorflow.core.framework import attr_value_pb2
 from tensorflow.core.framework import graph_pb2
@@ -123,6 +124,9 @@ def extract_sub_graph(graph_def, dest_nodes):
   if not isinstance(graph_def, graph_pb2.GraphDef):
     raise TypeError("graph_def must be a graph_pb2.GraphDef proto.")
 
+  if isinstance(dest_nodes, six.string_types):
+    raise TypeError("dest_nodes must be a list.")
+
   edges = {}  # Keyed by the dest node name.
   name_to_node_map = {}  # Keyed by node name.
 

tensorflow/python/framework/graph_util_test.py

@@ -188,6 +188,13 @@ class DeviceFunctionsTest(test.TestCase):
     self.assertEqual("n3", sub_graph.node[2].name)
     self.assertEqual("n5", sub_graph.node[3].name)
 
+  def testExtractSubGraphWithInvalidDestNodes(self):
+    graph_def = graph_pb2.GraphDef()
+    n1 = graph_def.node.add()
+    n1.name = "n1"
+    with self.assertRaisesRegexp(TypeError, "must be a list"):
+      graph_util.extract_sub_graph(graph_def, "n1")
+
   def testConvertVariablesToConstsWithFunctions(self):
     @function.Defun(dtypes.float32)
     def plus_one(x):