Returning first element of a double linked list

Question

I have made a Node and Deque class to represent a double linked list. I wrote a function to return the first item in the linked list, but was getting an IndexError - an error that I have raised in my code in case the linked list is empty.

My expected output:

my_list.push_front(1)
my_list.push_front(2)
my_list.push_front(3)
linked list is [3, 2, 1]
print(my_list.peek_front()) --> 3

My linked list function file:

class Node:
    """
    Initialize empty node
    """
    def __init__(self, data=None, prev = None, next = None):
        self.data = data
        self.next = next
        self.prev = prev

class Deque:
    """
    A double-ended queue
    """
    def __init__(self):
        """
        Initializes an empty Deque
        """
        self.head = Node()

    def push_front(self, e): #should work fine
        """
        Inserts an element at the front of the Deque
        :param e: An element to insert
        """
        new_head = Node(data = e, next = self.head)

        if self.head:
            self.head.prev = new_head
        self.head = new_head

    def peek_front(self): #FUNCTION WITH ERROR
        """
        Looks at, but does not remove, the first element
        :return: The first element
        """
        if self.head.data == None: #if it is empty, raise error
            raise IndexError
        else:
            return self.head.data

    def listprint(self, node):
        """
        Prints each element of the node front to back
        :param node:
        """
        while (node is not None):
            print(node.data)
            last = node
            node = node.next

My main file:

def main():

    my_list = Deque()

    my_list.push_front(1)
    my_list.push_front(2)
    my_list.push_front(3)

    my_list.listprint(my_list.head) #print elements in the list
    print(my_list.peek_front())

My error message:

IndexError (error that was unintentionally raised by me)

Answer 1

Not getting your error (and I tried on python2 and python3, though out of laziness I just ran it as scripts without def main() ).

However, the output lists a None at the end of the listprint:

$ python test.py
3
2
1
None
3

This is because what your __init__ creates is not an empty deque but rather a deque with a single node that has empty data. Maybe that's your way of implementing an empty deque (not a good idea if you ask me), but then you shouldn't be surprised that this empty-data node persists as you add new stuff onto the deque (after all, you never remove it).

If you want to fix this behavior, replace self.head = Node() by self.head = None in the __init__ , and replace if self.head.data == None by if self.head is None in the peek_front (note: is None is better Python than == None , since the identity check is enough and the equality check involves indirection). I think the rest should work, but I haven't checked.