def test_validate_when_only_digits_do_nothing(self):
     contains_only_digits('1234567890')
    def test_validate_when_value_contains_letter_do_nothing(self):
        with self.assertRaises(ValidationError) as context:
            contains_only_digits('123a')

        self.assertIsNotNone(context.exception)
 def test_validate_when_empty_do_nothing(self):
     contains_only_digits('')
 def test_validate_whenOnlyDigits_shouldDoNothing(self):
     contains_only_digits('123')
    def test_validate_whenValueContainsALetter_shouldRaise(self):
        with self.assertRaises(ValidationError) as context:
            contains_only_digits('a')

        self.assertIsNotNone(context.exception)
 def test_validate_whenValueIsEmpty_shouldDoNothing(self):
     contains_only_digits('')