Problem:
Your task is to implement a double linked list.  Write a program which performs the following operations:    * insert x: insert an element with key x into the front of the list.   * delete x: delete the first element which has the key of x from the list. If there is not such element, you need not do anything.   * deleteFirst: delete the first element from the list.    * deleteLast: delete the last element from the list.

Incorrect submission (verdict: Wrong Answer):
import sys

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

class DoublyLinkedList:
	def __init__(self):
		self.head = None
		self.tail = None
		self.size = 0
	
	def get_size(self):
		return self.size
	
	def is_empty(self):
		return self.size == 0
	
	def appendToHead(self, value):
		node = Node(value)
		if self.head == None and self.tail == None:
			self.head = self.tail = node
		else:
			node.next = self.head
			self.head.prev = node
			self.head = node
		self.size += 1
	
	def removeFirst(self):
		if self.is_empty(): return
		if self.size == 1:
			self.head = self.head = None
		else:			
			self.head = self.head.next
			self.head.prev = None
		self.size -= 1
		
	def removeLast(self):
		if self.is_empty(): return 
		if self.size == 1:
			self.head = self.tail = None
		else:
			self.tail = self.tail.prev
			self.tail.next = None
		self.size -= 1

	def remove(self, value):
		if self.is_empty(): return
		if self.head.value == value:
			self.removeFirst()
		else:
			node = self.head.next
			while node != None:
				if node.value == value:
					node.prev.next = node.next
					if node.next != None:
						node.next.prev = node.prev
					self.size -= 1
					break
				else:
					node = node.next
		
	def print(self):
			node = self.head
			while node != None:
				sys.stdout.write(str(node.value))
				node = node.next
				if node != None:
					sys.stdout.write(' ')
	
n = int(input())
dll = DoublyLinkedList()
for i in range(n):
	cmd = str(input())
	if cmd == 'deleteFirst':
		dll.removeFirst()
	elif cmd == 'deleteLast':
		dll.removeLast()
	else:
		cmd, value = cmd.split()
		if cmd == 'insert':
			dll.appendToHead(int(value))
		elif cmd == 'delete':
			dll.remove(int(value))
		else:
			print('Unknown command:' + cmd)
	
dll.print()


Fixed, accepted submission:
class Node(object):
    def __init__(self, num, prv = None, nxt = None):
        self.num = num
        self.prv = prv
        self.nxt = nxt
 
 
class DoublyLinkedList(object):
    def __init__(self):
        self.start = self.last = None
 
    def insert(self, num):
        new_elem = Node(num)
 
        if self.start is None:
            self.start = self.last = new_elem
        else:
            new_elem.nxt = self.start
            self.start.prv = new_elem
            self.start = new_elem
 
    def delete_num(self, target):
        it = self.start
        while it is not None:
            if it.num == target:
                if it.prv is None and it.nxt is None:
                    self.start = self.last = None
                else:
                    if it.prv is not None:
                        it.prv.nxt = it.nxt
                    else:
                        self.start = self.start.nxt
 
                    if it.nxt is not None:
                        it.nxt.prv = it.prv
                    else:
                        self.last = self.last.prv
                break
            it = it.nxt
 
    def delete_start(self):
        if self.start is self.last:
            self.start = self.last = None
        else:
            self.start.nxt.prv = None
            self.start = self.start.nxt
 
    def delete_last(self):
        if self.start is self.last:
            self.start = self.last = None
        else:
            self.last.prv.nxt = None
            self.last = self.last.prv
 
    def get_content(self):
        ret = []
        it = self.start
 
        while it is not None:
            ret.append(it.num)
            it = it.nxt
 
        return ' '.join(ret)
 
 
def _main():
    from sys import stdin
 
    n = int(input())
 
    lst = DoublyLinkedList()
     
    for _ in range(n):
        cmd = stdin.readline().strip().split()
        if cmd[0] == 'insert':
            lst.insert(cmd[1])
        elif cmd[0] == 'delete':
            lst.delete_num(cmd[1])
        elif cmd[0] == 'deleteFirst':
            lst.delete_start()
        elif cmd[0] == 'deleteLast':
            lst.delete_last()
 
    print(lst.get_content())
 
 
if __name__ == '__main__':
    _main()