Deep copy of a Doubly Linked List in Python

Question

I'm having trouble implementing a deep copy method for a DoublyLinkedList class. A deep copy is supposed to return a new, original Doubly Linked List that does not reference the original DLL (unlike a shallow copy).

Here's what I have so far:

class EmptyCollection(Exception):
    pass


class DoublyLinkedList:
    class Node:
        def __init__(self, data=None, next=None, prev=None):
            self.data = data
            self.next = next
            self.prev = prev

        def disconnect(self):
            self.data = None
            self.next = None
            self.prev = None

    def __init__(self):
        self.header = DoublyLinkedList.Node()
        self.trailer = DoublyLinkedList.Node()
        self.header.next = self.trailer
        self.trailer.prev = self.header
        self.size = 0

    def __len__(self):
        return self.size

    def is_empty(self):
        return (len(self) == 0)

    def first_node(self):
        if (self.is_empty()):
            raise EmptyCollection("List is empty")
        return self.header.next

    def last_node(self):
        if (self.is_empty()):
            raise EmptyCollection("List is empty")
        return self.trailer.prev

    def add_first(self, elem):
        return self.add_after(self.header, elem)

    def add_last(self, elem):
        return self.add_after(self.trailer.prev, elem)

    def add_after(self, node, elem):
        prev = node
        succ = node.next
        new_node = DoublyLinkedList.Node()
        new_node.data = elem
        new_node.prev = prev
        new_node.next = succ
        prev.next = new_node
        succ.prev = new_node
        self.size += 1
        return new_node

    def add_before(self, node, elem):
        return self.add_after(node.prev, elem)

    def delete(self, node):
        prev = node.prev
        succ = node.next
        prev.next = succ
        succ.prev = prev
        self.size -= 1
        data = node.data
        node.disconnect()
        return data

    def __iter__(self):
        if(self.is_empty()):
            return
        cursor = self.first_node()
        while(cursor is not self.trailer):
            yield cursor.data
            cursor = cursor.next

    def __str__(self):
        return '[' + '<-->'.join([str(elem) for elem in self]) + ']'

    def __repr__(self):
        return str(self)




def deepCopy(lnk_lst):
    currenthead = lnk_lst.first_node()
    temp = DoublyLinkedList()
    while currenthead is not lnk_lst.trailer:
        temp.add_last(currenthead.data)
        currenthead = currenthead.next
    return temp


lnk_lst1 = DoublyLinkedList()
elem1 = DoublyLinkedList()
elem1.add_last(1)
elem1.add_last(2)
lnk_lst1.add_last(elem1)
elem2 = 3
lnk_lst1.add_last(elem2)
lnk_lst2 = deepCopy(lnk_lst1)
e1 = lnk_lst1.first_node()
e1_1 = e1.data.first_node()
e1_1.data = 10
e2 = lnk_lst2.first_node()
e2_1 = e2.data.first_node()
print(e2_1.data) #should print 1

My deep copy method seems to return a shallow copy. The output of the program should be 1 (since lnk_lst2 should not reference any elements in lnk_lst1.)

Can someone explain how I can modify my deep copy method to produce a deep copy of the linked list and not a shallow copy?

I cannot use python's built in deep or shallow copy for this. Any help would be appreciated.

Answer 1

Many basic objects can be copied with type(obj)(obj). E.g. dict(dct) or list(lst) will create a copy. Immutable types will return the same object, and that's fine. int(42) is 42 and str("string") is "string", etc.

The following solution will be limited to such types.

So let's make a use of that and let's add the DoublyLinkedList to the set of types creating a copy (in our case a deep copy but only at the first level of nesting) this way. Modified __init__:

def __init__(self, iterable=()):
    self.header = DoublyLinkedList.Node()
    self.trailer = DoublyLinkedList.Node()
    self.header.next = self.trailer
    self.trailer.prev = self.header
    self.size = 0 
    for item in iterable:
        self.add_last(type(item)(item))

Now we do not need the deepCopy() any longer. The only change left to do is to replace:

lnk_lst2 = deepCopy(lnk_lst1)

by:

lnk_lst2 = DoublyLinkedList(lnk_lst1)

Answer 2

To perform a deep copy, you need to copy the embedded linked lists:

def deepCopy(lnk_lst):
    currenthead = lnk_lst.first_node()
    temp = DoublyLinkedList()
    while currenthead is not lnk_lst.trailer:
        if isinstance(currenthead.data, DoublyLinkedList):
            temp.add_last(deepCopy(currenthead.data))
        else:
            temp.add_last(currenthead.data)
        currenthead = currenthead.next
    return temp