      def test_abstract_inherited_unique(self):
          title = 'Boss'
          isbn = '12345'
          DerivedBook.objects.create(title=title, author=self.writer, isbn=isbn)
          form = DerivedBookForm({'title': 'Other', 'author': self.writer.pk, 'isbn': isbn})
          self.assertFalse(form.is_valid())
          self.assertEqual(len(form.errors), 1)
          self.assertEqual(form.errors['isbn'], ['Derived book with this Isbn already exists.'])
  
      def test_abstract_inherited_unique_together(self):
 
</s> remove                 fields = '__all__'
 </s> add                 fields = ('title', 'date_published', 'mode', 'category')
        form = DerivedBookForm({
            'title': 'Other', 'author': self.writer.pk, 'isbn': isbn,
            'suffix1': '1', 'suffix2': '2',
        })
      def test_abstract_inherited_unique(self):
          title = 'Boss'
          isbn = '12345'
          DerivedBook.objects.create(title=title, author=self.writer, isbn=isbn)
        form = DerivedBookForm({
            'title': 'Other', 'author': self.writer.pk, 'isbn': isbn,
            'suffix1': '1', 'suffix2': '2',
        })
          self.assertFalse(form.is_valid())
          self.assertEqual(len(form.errors), 1)
          self.assertEqual(form.errors['isbn'], ['Derived book with this Isbn already exists.'])
  
      def test_abstract_inherited_unique_together(self):
keep keep keep keep replace keep keep keep keep keep
 Fixed #27039 -- Fixed empty data fallback to model field default in model forms. 
tests/model_forms/tests.py