A SQLAlchemy field to process "tags"

fields.py

from unittest import TestCase

from sqlalchemy.types import TypeDecorator, VARCHAR


class Tags(TypeDecorator):
    impl = VARCHAR

    def process_bind_param(self, value, dialect):
        if value is None:
            return None
        return ",".join(value)

    def process_result_value(self, value, dialect):
        if value is None:
            return None
        return value.split(",")


class FieldTests(TestCase):
    def setUp(self):
        self.tags = Tags()
        self.value = ["hi", "there"]
        self.expected_response = "hi,there"

    def test_set_none_returns_none(self):
        val = self.tags.process_bind_param(None, "")
        self.assertEqual(val, None)

    def test_set_list_returns_string(self):
        val = self.tags.process_bind_param(self.value, "")
        self.assertEqual(val, self.expected_response)

    def test_list_not_string_raises_type_error(self):
        with self.assertRaises(TypeError):
            self.tags.process_bind_param([1, 2, 3], "")

    def test_get_none_returns_none(self):
        val = self.tags.process_result_value(None, "")
        self.assertEqual(val, None)

    def test_get_string_returns_list(self):
        val = self.tags.process_result_value(self.expected_response, "")
        self.assertEqual(val, self.value)

    def test_get_not_string_raises_attribute_error(self):
        with self.assertRaises(AttributeError):
            self.tags.process_result_value(4, "")