code: def test_append_row(self):
      r_set = row_set.empty()
      r_set.append("row1")
      expected = 'row_keys: "row1"\n'
      self.assertEqual(expected, repr(r_set))
  
imports: from tensorflow_io.python.ops import core_ops
  import tensorflow_io.python.ops.bigtable.bigtable_row_range
  import tensorflow_io.python.ops.bigtable.bigtable_row_set
  from tensorflow import test
label_desc: Use image node_id 2 for calling the TestRowSet obj's underlying member method code with example usage: obj.test_append_row() without return types