code: def test_len_(self):
      ds = DataSet({"x": [[1, 2, 3, 4]] * 40, "y": [[5, 6]] * 40})
      assert len(ds) == 40
  
      ds = DataSet()
      assert len(ds) == 0
  
imports: import os
  import pytest
  import numpy
  from fastNLP.core.dataset import DataSet, FieldArray, Instance, ApplyResultException
  from fastNLP import logger
label_desc: Use image node_id 8 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_len_() without return types