old_contents: import unittest
  
  from . import base
  
  
  class PngTestCase(base.ShellParserTestCase, unittest.TestCase):
      extension = 'png'
  
new_contents: import unittest
  
  from . import base
  
  
  class PngTestCase(base.ShellParserTestCase, unittest.TestCase):
      extension = 'tiff'
  
subject: test tiff not png