code: def test_generate_scaffold(self):
      from deepchem.splits.splitters import _generate_scaffold
  
      valid_smiles = r"s1cc(nc1\[N]=C(\N)N)C"
      scaffold = _generate_scaffold(valid_smiles)
      self.assertTrue(scaffold == "c1cscn1")
  
      # Invalid because valence for atom 5 N is greater than permitted (4)
      invalid_smiles = r"s1cc(nc1\[NH]=C(\N)N)C"
      scaffold = _generate_scaffold(invalid_smiles)
      self.assertIsNone(scaffold)
  
imports: import unittest
  import deepchem
  from deepchem.splits.splitters import ScaffoldSplitter
label_desc: Use image node_id 2 for calling the TestScaffoldSplitter obj's underlying member method code with example usage: obj.test_generate_scaffold() without return types