code: def assert_key_is_string(key):
      if not isinstance(key, six.string_types):
          raise ValueError(
              "key must be a string. Got: type {}. Given key: {}.".format(
                  type(key), key
              )
          )
  
file_path: tensorflow-upstream/tensorflow/python/feature_column/utils.py
imports: import six
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import ops
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import math_ops
  from tensorflow.python.util import nest
label_desc: Use image node_id 3 for calling a global function with example usage: assert_key_is_string(key) without return types