Fix the input of executor

python/paddle/fluid/executor.py

@@ -320,8 +320,9 @@ class Executor(object):
         # append fetch_operators
         if not has_fetch_operators(global_block, fetch_list, fetch_var_name):
             for i, var in enumerate(fetch_list):
-                assert isinstance(var, Variable) or isinstance(var, str), (
-                    "Wrong type for fetch_list[%s]: %s" % (i, type(var)))
+                assert isinstance(var, Variable) or isinstance(
+                    var, six.string_types), (
+                        "Wrong type for fetch_list[%s]: %s" % (i, type(var)))
                 global_block.append_op(
                     type='fetch',
                     inputs={'X': [var]},