code: def test_default_username(self):
      if "USER" in os.environ:
          username = os.environ["USER"]
          u = utils.get_default_username()
          self.assertEqual(u, username)
  
          del os.environ["USER"]
          u = utils.get_default_username()
          self.assertEqual(u, "default")
          os.environ["USER"] = username
      else:
          u = utils.get_default_username()
          self.assertEqual(u, "default")
          os.environ["USER"] = "test"
          u = utils.get_default_username()
          self.assertEqual("test", u)
          del os.environ["USER"]
  
      u = utils.get_default_username(username="new")
      self.assertEqual("new", u)
  
file_path: auptimizer/tests/test_utils.py
imports: import unittest
  import os
  import sys
  from six import StringIO
  from aup import utils
  import tempfile
label_desc: Use image node_id 3 for calling the UtilTestCase obj's underlying member method code with example usage: obj.test_default_username() without return types