Simple python Linked List implementation

linked_list.py

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


class LinkedList:
    storage = []

    def __init__(self):
        self.first = None
        self.last = None
        self.length = 0

    def __str__(self):
        self.values = []
        self.iter(lambda _, current: self.values.append(str(current.value)))
        if self.length:
            return ' -> '.join(self.values)
        else:
            return 'Empty list'

    def iter(self, callback):
        prev = None
        current = self.first
        while True:
            callback(prev, current)
            if current.next != None:
                prev = current
                current = current.next
            else:
                break
        return current

    def add(self, value):
        self.length += 1
        if not self.first:
            self.first = self.last = Node(value)
        else:
            item = self.first
            while item.next != None:
                item = item.next
            item.next = self.last = Node(value)

    def remove(self, value):
        def remove(prev, current):
            self.length -= 1
            if current.value == value:
                if prev:
                    prev.next = current.next
                else:
                    if current.next:
                        self.first = current.next
                    else:
                        self.first = self.last = None
        self.iter(remove)


a = LinkedList()
a.add(1)
a.add(2)
a.add(3)
a.add(10)
print a