gistfile1.py

# main/models.py
from django.core.validators import MaxValueValidator


class Product(WithSlug):
    ...
    # all the other fields
    tax_rate = models.PositiveIntegerField(
        default=25, 
        help_text=_("Tax rate"),
        validators=[MaxValueValidator(100),],
    )
    # all other methods
    @property
    def tax_amount(self):
        return round(self.price * (self.tax_rate / 100))
 
 
 #tests.py
 class ModelsProductTest(TestCase):
     def setUp(self):
         self.t_objs = [
             (100, 25, 25),
             (50, 10, 5),
             (2, 50, 1),
         ]
         
     def test_tax_rate(self):
         for price, tax_rate, expected in self.t_objs:
             p = ProductFacroty(price=price, tax_rate=tax_rate)
             
             self.assertEqual(expected, p.tax_amount)