code: def testSingleString(self, dtype, unit, rank):
      test_string = {
          "BYTE": [b"abcdefghijklmnopqrstuvwxyz"],
          "UTF8_CHAR": [
              "\U0001d229\U0001d227n\U0001d229\U0001d227n\U0001d229\U0001d227n".encode(
                  "utf-8"
              )
          ],
      }[unit]
      position = np.array([1, 2, 3], dtype)
      length = np.array([1, 2, 1], dtype)
      expected_value = {
          "BYTE": [b"b", b"cd", b"d"],
          "UTF8_CHAR": [
              x.decode("utf-8")
              for x in [
                  b"\xf0\x9d\x88\xa7",
                  b"n\xf0\x9d\x88\xa9",
                  b"\xf0\x9d\x88\xa9",
              ]
          ],
      }[unit]
  
      test_string_tensor = np.array(test_string)
      expected_string_tensor = np.array(expected_value)
      if rank == 2:
          test_string_tensor = np.expand_dims(
              test_string_tensor, axis=0
          )
          expected_string_tensor = np.expand_dims(
              expected_string_tensor, axis=0
          )
  
      substr_op = string_ops.substr(
          test_string_tensor, position, length, unit=unit
      )
      with self.cached_session():
          substr = self.evaluate(substr_op)
          self.assertAllEqual(substr, expected_string_tensor)
          self.assertEqual(substr.ndim, rank)
  
imports: from absl.testing import parameterized
  import numpy
  from tensorflow.python.framework import errors_impl
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops import string_ops
  from tensorflow.python.platform import test
label_desc: Use image node_id 16 for calling the SubstrOpTest obj's underlying member method code with example usage: obj.testSingleString(dtype, unit, rank) without return types