Return value fail - python-3.x

Question

I am working on a doubly linked list and I am having trouble getting returning the value I remove an item from the middle of the list. I am not really sure why and would appreciate the a second pair of eyes look this through.
If I append three numbers and then remove the number at index 1, it should return the value at index 1.
class Linked_List:
class __Node:
def __init__(self, val):
self.val = val
self.next = None
self.prev = None
return
def __init__(self):
self.__header = self.__Node(None)
self.__trailer = self.__Node(None)
self.__header.next = self.__trailer
self.__trailer.prev = self.__header
self.__size = 0
def __len__(self):
return self.__size
def remove_element_at(self, index):
if index > self.__size or index < 0 or index == self.__size:
raise IndexError
cur = self.__header.next
element_to_return = cur
if index == 0:
self.__header.next = self.__header.next.next
else:
for i in range(0, index):
cur = cur.next
element_to_return = cur.val
cur.next = cur.next.next
self.__size -= 1
return str(element_to_return.val)

Answer 1

I added the addValue() and a debug() method (tip: read How to debug small programs (#1) ).
I modified the start/end node to have a value for clarity. I reduced/restructured (rewrote) your the remove method - it had some unneeded overhead and pointer errors. For double linked lists you have to take care of both the folloing and preceeding element and change all the pointers correctly to keep the integrity of the chain.
class Linked_List:
class __Node:
def __init__(self, val):
self.val = val
self.next = None
self.prev = None
return
def __init__(self):
self.__header = self.__Node('Start') # start node
self.__trailer = self.__Node('End') # end node
self.__header.next = self.__trailer
self.__trailer.prev = self.__header
self.__size = 0
def debug(self):
cur = self.__header
print("prev.val cur.val next.val")
i = 0
while cur:
print(" " * i, f'{cur.prev.val if cur.prev else None} {cur.val} {cur.next.val if cur.next else None}')
i+=1
cur = cur.next
def addValue(self, val):
"""Adds the str of val at the end."""
n = self.__Node(str(val))
n.next = self.__trailer
n.prev = self.__trailer.prev
self.__trailer.prev.next = n
self.__trailer.prev = n
self.__size += 1
def remove_element_at(self, index):
if not (0 <= index < self.__size ): # simplified
# better message on error
raise IndexError (f'Index {index} out of bounds [0,{self.__size-1}]')
cur = self.__header.next # simplified, element_to_return not needed
# there is no need for special cases for 0 or size - this will handle all
for i in range(0, index):
cur = cur.next # go ever deeper, if we are at 0 or self.__size not entering
cur.prev.next = cur.next # repointer prev's next to our next
cur.next.prev = cur.prev # repointer next's prev to our prev
self.__size -= 1 # decrease size
return str(cur.val) # return our value
k = Linked_List()
for n in range(5):
k.addValue(n)
k.debug()
removed = k.remove_element_at(4)
k.debug()
print(f'removed: {removed}')
Output:
prev.val cur.val next.val
None Start 0
Start 0 1
0 1 2
1 2 3
2 3 4
3 4 End
4 End None
None Start 0
Start 0 1
0 1 2
1 2 3
2 3 End
3 End None
removed: 4
Edit:
My add method is self-build. Make sure youres gets the pointers correctly - use the debug method to assure that cur.prev.next points to cur and cur.next.prev is cur as well.