class_docstr: Unit tests for Visualizer.
class_docstr_tok: ["Unit","tests","for","Visualizer","."]
code: def test_tracer(self):
      p = (
          pipe.input("a")
          .map("a", "b", lambda x: x + 1)
          .map("a", "b", lambda x: x + 2)
          .output("a", "b")
      )
      v = p.debug(3, tracer=True)
      v.tracer.show()
      t = v.tracer[0]
      self.assertEqual(len(t["lambda"]), 2)
  
      self.assertEqual(t["_input"].next_node, ["lambda-0"])
      self.assertEqual(t["lambda-0"].next_node, ["lambda-1"])
      self.assertEqual(t["lambda-0"].previous_node, ["_input"])
      self.assertEqual(t["lambda-1"].next_node, ["_output"])
      self.assertEqual(t["lambda-1"].previous_node, ["lambda-0"])
  
file_path: towhee/tests/unittests/tools/test_visualizer.py
imports: import unittest
  from towhee import pipe
  from towhee.tools import visualizer
  from towhee.tools.visualizer import Visualizer
  from towhee.runtime.data_queue import DataQueue
label_desc: Use image node_id 8 for calling the TestVisualizer obj's underlying member method code with example usage: obj.test_tracer() without return types