    def test_snow(self):
        """
        Check that Snow works.
        """
        # Check that Snow randomly updates the Screen every 3rd frame.
        screen = MagicMock(spec=Screen, colours=8, unicode_aware=False)
        canvas = Canvas(screen, 10, 40, 0, 0)
        effect = Snow(canvas)
        effect.reset()
        self.assert_blank(canvas)
        my_buffer = [[(32, 7, 0, 0) for _ in range(40)] for _ in range(10)]
        for i in range(60):
            effect.update(i)
            self.assertEqual(self.check_canvas(
                canvas,
                my_buffer,
                lambda value: self.assertIn(chr(value[0]), ".+* ,;#@")),
                i % 3 == 0)

        # Check there is no stop frame by default.
        self.assertEqual(effect.stop_frame, 0)

        # This effect should ignore events.
        event = object()
        self.assertEqual(event, effect.process_event(event))
Check that Snow works.
tests/test_effects.py