code: def _is_variable_op(op):
      """Returns true if 'op' refers to a Variable node."""
      return op in _VARIABLE_OPS
  
file_path: tensorflow/tensorflow/python/framework/graph_util_impl.py
imports: import copy
  import re
  from tensorflow.core.framework import graph_pb2
  from tensorflow.core.framework import node_def_pb2
  from tensorflow.python.framework import _proto_comparators
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import ops
  from tensorflow.python.util import deprecation
  from tensorflow.python.util.tf_export import tf_export
label_desc: Use image node_id 1 for calling a global function with example usage: _is_variable_op(op) and returns: unknown