Simple double linked list with constant time list reversal.

constant_reverses.py

class List:
    _head: 'Node|None'
    _tail: 'Node|None'

    def __init__(self):
        self.reversed = False
        self._head = None
        self._tail = None

    def append(self, value):
        node = Node(self, value)
        if not self._head:
            # assume self._tail is None too
            self._head = node
            self._tail = node
            return

        self._tail.next = node
        node.prev = self._tail
        self._tail = node

    def appendleft(self, value):
        node = Node(self, value)
        if not self._head:
            # assume self._tail is None too
            self._head = node
            self._tail = node
            return

        self._head.prev = node
        node.next = self._head
        self._head = node

    def reverse(self):  # O(1)
        self._head, self._tail = self._tail, self._head
        self.reversed = not self.reversed

    def __repr__(self):
        current = self._head
        vals = []
        while current:
            vals.append(current.value)
            current = current.next
        sval = str(vals)[1:-1]
        return f'List({sval})'


class Node:
    _list: List

    def __init__(self, _list, value):
        self.value = value
        self._list = _list
        self.left = None
        self.right = None

    @property
    def next(self):
        if self._list.reversed:
            return self.left
        return self.right

    @next.setter
    def next(self, value):
        if self._list.reversed:
            self.left = value
        else:
            self.right = value

    @property
    def prev(self):
        if self._list.reversed:
            return self.right
        return self.left

    @prev.setter
    def prev(self, value):
        if self._list.reversed:
            self.right = value
        else:
            self.left = value


def main():
    lst = List()
    lst.append(1)
    lst.append(2)
    lst.append(3)
    lst.appendleft(4)
    lst.appendleft(5)
    lst.append(6)
    lst.append(7)

    print(lst)
    lst.reverse()
    print(lst)
    lst.append(10)
    print(lst)
    lst.reverse()
    print(lst)


if __name__ == '__main__':
    main()