def test_append():
    u"""Assert .append() adds a value to the t of the doubly linked list."""
    doubly_linked = DoublyLinked()
    for val in VALUES:
        doubly_linked.append(val)
        assert doubly_linked.tail.value == val
    assert doubly_linked.tail.value == VALUES[-1]
def test_pop():
    u"""Assert .pop() removes and returns the h value from the list."""
    doubly_linked = DoublyLinked()
    for val in VALUES:
        doubly_linked.append(val)
    for val in VALUES:
        assert doubly_linked.pop() == val
    assert doubly_linked.pop() is None