Python列表是单链接还是双链接列表？

I'm wondering what the order of complexity for a Python v2.7 list being built up using append() is? Is a Python list doubly linked and thus it is constant complexity or is it singly linked and thus linear complexity? If it is singly linked, how can I in linear time build up a list from an iteration that provides the values of the list in the order of from beginning to end?I'm wondering what the order of complexity for