def test_only(self):
    c = Collection([1, 2, 3, 4, 5])
    self.assertEqual([2, 4], c.only(1, 3).all())
def test_only(self):
    c = Collection([1, 2, 3, 4, 5])
    self.assertEqual([2, 4], c.only(1, 3).all())