Not able to write a iterator for a Linked List created in python [duplicate]

Have tried to create a python Single Linked list , but i'm not able to create a iterator. Here is my code :

class LinkedList: def __init__(self): self._head=self self._tail=self self._size=0 def __iter__(self): print 'Calling Iterator\n\n' _ListIterator(self._head) class ListObj: def __init__(self,value): self._data=value self._pointingTo=None class _ListIterator: def __init__(self,listHead): LIST=None self._curNode=listHead print dir(self._curNode) def __next__(self): if self._curNode._pointingTo is None: raise StopIteration else: item=self._curNode._data self._curNode=self._curNode._pointingTo return item 

This iterator is failing by throwing an error as

TypeError: __iter__ returned non-iterator of type 'NoneType'