code: def test_str(self):
      net = """net {
  }
  node asia{
      states = ("yes"  "no");
      weight = None;
  }
  node bronc{
      states = ("yes"  "no");
      weight = None;
  }
  node dysp{
      states = ("yes"  "no");
      weight = None;
  }
  node either{
      states = ("yes"  "no");
      weight = None;
  }
  node lung{
      states = ("yes"  "no");
      weight = None;
  }
  node smoke{
      states = ("yes"  "no");
      weight = None;
  }
  node tub{
      states = ("yes"  "no");
      weight = None;
  }
  node xray{
      states = ("yes"  "no");
      weight = None;
  }
  potential (asia |){
   data = (0.01 0.99);
  }
  potential (bronc | smoke){
   data = ((0.6 0.4)
   (0.3 0.7));
  }
  potential (dysp | bronc either){
   data = (((0.9 0.1)
    (0.8 0.2))
  
   ((0.7 0.3)
    (0.1 0.9)));
  }
  potential (either | lung tub){
   data = (((1.0 0.0)
    (1.0 0.0))
  
   ((1.0 0.0)
    (0.0 1.0)));
  }
  potential (lung | smoke){
   data = ((0.1  0.9 )
   (0.01 0.99));
  }
  potential (smoke |){
   data = (0.5 0.5);
  }
  potential (tub | asia){
   data = ((0.05 0.95)
   (0.01 0.99));
  }
  potential (xray | either){
   data = ((0.98 0.02)
   (0.05 0.95));
  }
  """
      self.assertEqual(str(self.writer), net)
  
imports: import unittest
  import numpy
  from pgmpy import config
  from pgmpy.readwrite import NETReader, NETWriter
  from pgmpy.utils import compat_fns, get_example_model
label_desc: Use image node_id 7 for calling the TestNETWriter obj's underlying member method code with example usage: obj.test_str() without return types