def test_push():
colors = DoubleLinkedList()
colors.push("Pthalo Blue")
colors._invariant()
assert colors.count() == 1
colors.push("Ultramarine Blue")
assert colors.count() == 2
colors._invariant()
def test_pop():
colors = DoubleLinkedList()
colors.push("Magenta")
colors._invariant()
colors.push("Alizarin")
colors.push("Van Dyke")
colors._invariant()
assert colors.pop() == "Van Dyke"
colors._invariant()
assert colors.get(1) == "Alizarin"
assert colors.pop() == "Alizarin"
assert colors.pop() == "Magenta"
colors._invariant()
assert colors.pop() is None
def test_remove():
colors = DoubleLinkedList()
colors.push("Cobalt")
colors.push("Zinc White")
colors.push("Nickle Yellow")
colors.push("Perinone")
assert colors.remove("Cobalt") == 0
colors._invariant()
colors.dump("before perinone")
assert colors.remove("Perinone") == 2
colors._invariant()
colors.dump("after perinone")
assert colors.remove("Nickle Yellow") == 1
colors._invariant()
assert colors.remove("Zinc White") == 0
colors._invariant()