'The highest context which has the given key is used.'

def test_set_upward_with_push(self):

    c = Context({'a': 1})
    c.push({'a': 2})
    c.set_upward('a', 3)
    self.assertEqual(c.get('a'), 3)
    c.pop()
    self.assertEqual(c.get('a'), 1)
