doubleList.py

class DoubleList(object):

    head = None
    tail = None
    size = 0

    def append(self, data):
        new_node = Node(data)
        if self.head is None:
            self.head = self.tail = new_node
            self.size = 1
        else:
            new_node.prev = self.tail
            new_node.next = None
            self.tail.next = new_node
            self.tail = new_node
            self.size += 1

    def append_by_node(self, new_node):
            if self.head is None:
                self.head = self.tail = new_node
                self.size = 1
            else:
                new_node.prev = self.tail
                new_node.next = None
                self.tail.next = new_node
                self.tail = new_node
                self.size += 1

    def remove_by_value(self, node_value):
        current_node = self.head

        while current_node is not None:
            if current_node.data == node_value:
                # if it's not the first element
                if current_node.prev is not None:
                    current_node.prev.next = current_node.next
                    current_node.next.prev = current_node.prev
                else:
                    # otherwise we have no prev (it's None), head is the next one, and prev becomes None
                    self.head = current_node.next
                    current_node.next.prev = None
                self.size -= 1

            current_node = current_node.next

    # assuming that node is present
    def remove_by_node(self, node):
        if self.size == 1:
            self.head = None
            self.tail = None
            return

        prev = node.prev
        next = node.next

        if prev is not None:
            prev.next = next

        if next is not None:
            next.prev = prev

    def remove_tail(self):
        if self.size == 0:
            return
        prev = self.tail.prev
        self.size -= 1

        if prev is not None:
            prev.next = None
            self.tail = prev
        else:
            self.head = None
            self.tail = None

    def remove_head(self):
        if self.size == 0:
            return
        next = self.head.next
        self.size -= 1

        if next is not None:
            next.prev = None
            self.head = next
        else:
            self.head = None
            self.tail = None

    def get_size(self):
        return self.size