[Tutor] Doubly linked list getting "AttributeError: 'NoneType' object has no attribute 'element'"

[Sudhier Batara]

Thanks for the reply Steve, I solved it myself.I was not returning 'newest'
Node in parent class.

--sudhier

On Mon, 21 Sep 2015 11:43 pm Steven D'Aprano <steve at pearwood.info> wrote:

> Hi Sudhier, and welcome,
>
> Unfortunately I'm about to step away from the computer, so I can't
> really answer your question in detail, but since it's been 2 or 3 days
> since you asked your question, and I haven't seen any replies at all, I
> thought I'd give a quick reply so you know we aren't ignoring you.
>
> (My inbox is suffering from overload at the moment, so it is possible
> that others have answered your question and I haven't noticed. If so, my
> apologies for wasting everyone's time.)
>
> Regards,
>
>
> Steve
>
> On Sat, Sep 19, 2015 at 07:52:07PM +0530, Sudhier Batara wrote:
> > Hi,
> >
> > I have the below code which is a doubly linked list , which returns
> > the Position of the node rather than the node itself.The code is quite
> > self explanatory.In the below code i call following methods ,
> >
> > p = L.add_last(8)
> > print p.element()
> >
> > the above methods works fine adds the element into the List instance
> > and also printing the element shows the element in the list.
> >
> > But when i call the below methods,
> >
> > q = L.add_after(p,5)
> > q.element()
> >
> > throws following error
> >
> >
> > _______________________________________
> > in make position
> > in make position
> > 8
> > inside validate
> > 8
> > in make position
> > Traceback (most recent call last):
> >   File "positional_dlist.py", line 128, in <module>
> >     print q.element()
> >   File "positional_dlist.py", line 45, in element
> >     return self._node.element
> > AttributeError: 'NoneType' object has no attribute 'element'
> > ____________________________________________________
> >
> > Complete code below
> >
> >
> >
> > #!/usr/bin/python
> >
> > class _DoublelinkedBase:
> >     class _Node:
> >         def __init__(self,element,prev,Next):
> >             self.element = element
> >             self._prev = prev
> >             self._Next = Next
> >
> >     def __init__(self):
> >         self._header = self._Node(None,None,None)
> >         self._trailer = self._Node(None,None,None)
> >         self._header._Next = self._trailer
> >         self._trailer._prev = self._header
> >         self._size = 0
> >
> >     def __len__(self):
> >         return self._size
> >
> >     def is_empty(self):
> >         return self._size == 0
> >
> >     def _insert_between(self,element,predecessor,successor):
> >         newest = self._Node(element,predecessor,successor)
> >         predecessor._Next = newest
> >         successor._prev = newest
> >         self._size += 1
> >
> >     def _delete_node(self,node):
> >         predecessor = node._prev
> >         successor = node._Next
> >         predecessor._Next = successor
> >         successor._prev = predecessor
> >         element = node.element
> >         node.element = node._Next = node._prev = None
> >         self._size -= 1
> >
> > class PositionalList(_DoublelinkedBase):
> >     class Position:
> >         def __init__(self,container,node):
> >             self._container = container
> >             self._node = node
> >
> >         def element(self):
> >             return self._node.element
> >
> >         def __eq__(self,other):
> >             return type(other) is type(self) and other._node is
> self._node
> >
> >         def __ne__(self,other):
> >             return  not (other == self)
> >
> >     def _validate(self, p):
> >         if not isinstance(p,self.Position):
> >             raise TypeError("p is not of type Position")
> >         if p._container is not self:
> >             raise ValueError("p is not the instance of this list")
> >         if p._node._Next is None:
> >             print p._node
> >             raise ValueError("No longer a valid node")
> >         print "inside validate"
> >         print p.element()
> >         return p._node
> >
> >     def _make_position(self,node):
> >         if node is self._header or node is self._trailer:
> >             print "in make position for none"
> >             return None
> >         else:
> >             print "in make position"
> >             return self.Position(self,node)
> >
> >     def first(self):
> >         return self._make_position(self._header._Next)
> >
> >     def last(self):
> >         return self._make_position(self._trailer._prev)
> >
> >     def before(self,p):
> >         node = self._validate(p)
> >         return self._make_position(node._prev)
> >
> >     def after(self,p):
> >         node = self._validate(p)
> >         return self._make_position(node._Next)
> >
> >     def __iter__(self):
> >        cursor = self.first()
> >        while cursor is not None:
> >            yield  cursor.element()
> >            cursor = self.after(cursor)
> >
> >     def _insert_between(self,e,predecessor,successor):
> >         #node =
> > super(_DoublelinkedBase,self)._insert_between(e,predecessor,successor)
> >         node =
> _DoublelinkedBase()._insert_between(e,predecessor,successor)
> >         return self._make_position(node)
> >
> >     def add_first(self,e):
> >         return self._insert_between(e,self._header,self._header._Next)
> >
> >     def add_last(self,e):
> >         return self._insert_between(e,self._trailer._prev,self._trailer)
> >
> >     def add_before(self,p,e):
> >         original = self._validate(p)
> >         return self._insert_between(e,original._prev,original)
> >
> >     def add_after(self,p,e):
> >         original = self._validate(p)
> >         return self._insert_between(e,original,original._Next)
> >
> >     def delete_node(self,p):
> >         original = self._validate(p)
> >         return self._delete_node(original)
> >
> >     def replace(self,p,e):
> >         original = self._validate(p)
> >         old_value = original.element
> >         original.element = e
> >         return old_value
> >
> >
> > L = PositionalList()
> > p = L.add_last(8)
> > p = L.first()
> > print p.element()
> > q = L.add_after(p,5)
> > print q.element()
> > _______________________________________________
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