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_shift():
colors = DoubleLinkedList()
colors.shift("Cadmium Orange")
assert colors.count() == 1
colors.shift("Carbazole Violet")
assert colors.count() == 2
assert colors.pop() == "Cadmium Orange"
assert colors.count() == 1
assert colors.pop() == "Carbazole Violet"
assert colors.count() == 0