Python properties

test_properties.py

# Tested with Python 3.5
# Fails on 2.x

from unittest import TestCase, main


class Point:

    def __init__(self, x, y):
        self._x = x
        self._y = y

    def get_double_x(self):
        return self._x * 2

    def get_y(self):
        return self._y

    def set_y(self, y):
        self._y = y

    xx = property(get_double_x)
    y = property(get_y, set_y)

    @property
    def property_using_decorator(self):
        return self._y


class TestProperties(TestCase):

    def test_can_construct(self):
        point = Point(y=2, x=4)
        self.assertEqual(point.y, 2)

    def test_can_set_property_with_setter(self):
        point = Point(y=2, x=4)
        point.y = 42
        self.assertEqual(point.y, 42)

    def test_can_get_property_with_computing_setter(self):
        point = Point(y=2, x=4)
        self.assertEqual(point.xx, 8)

    def test_set_property_without_setter__yay_python(self):
        point = Point(y=2, x=4)
        with self.assertRaises(AttributeError):
            point.xx = 42

    def test_cannot_set_decorator_based_property_without_setter(self):
        point = Point(y=2, x=4)
        with self.assertRaises(AttributeError):
            point.property_using_decorator = 42


if __name__ == '__main__':
    main()