  
          # Instance can be edited w/out re-uploading the file and existing file should be preserved.
  
          f = TextFileForm(
                  data={'description': 'New Description'},
                  instance=instance)
          f.fields['file'].required = False
          self.assertEqual(f.is_valid(), True)
          instance = f.save()
          self.assertEqual(instance.description, 'New Description')
          self.assertEqual(instance.file.name, 'tests/test3.txt')
 
</s> remove                 data={'description': 'Assistance'},
                files={'file': SimpleUploadedFile('test3.txt', b'hello world')}, instance=instance)
 </s> add             data={'description': 'Assistance'},
            files={'file': SimpleUploadedFile('test3.txt', b'hello world')}, instance=instance) </s> remove                 data={'description': 'New Description'},
                instance=instance)
 </s> add             data={'description': 'New Description'},
            instance=instance) </s> remove                 data={'description': 'Assistance'},
                instance=instance)
 </s> add             data={'description': 'Assistance'},
            instance=instance) </s> remove                 data={'description': 'Assistance'},
                files={'file': SimpleUploadedFile('test2.txt', b'hello world')}, instance=instance)
 </s> add             data={'description': 'Assistance'},
            files={'file': SimpleUploadedFile('test2.txt', b'hello world')}, instance=instance) </s> remove                 data={'description': 'Assistance'},
                files={'file': SimpleUploadedFile('test3.txt', b'hello world')})
 </s> add             data={'description': 'Assistance'},
            files={'file': SimpleUploadedFile('test3.txt', b'hello world')})
            data={'description': 'New Description'},
            instance=instance)
  
          # Instance can be edited w/out re-uploading the file and existing file should be preserved.
  
          f = TextFileForm(
            data={'description': 'New Description'},
            instance=instance)
            data={'description': 'New Description'},
            instance=instance)
          f.fields['file'].required = False
          self.assertEqual(f.is_valid(), True)
          instance = f.save()
          self.assertEqual(instance.description, 'New Description')
          self.assertEqual(instance.file.name, 'tests/test3.txt')
keep keep keep keep replace replace keep keep keep keep keep
 Fixed #21288 -- Fixed E126 pep8 warnings 
tests/model_forms/tests.py