Insertion Sort with a Linked List - sorting

Question

I am trying to create an insertion sort with linked lists. Here is what I have:
def insertion_sort(a):
"""
-------------------------------------------------------
Sorts a list using the Insertion Sort algorithm.
Use: insertion_sort( a )
-------------------------------------------------------
Preconditions:
a - linked list of comparable elements (?)
Postconditions:
Contents of a are sorted.
-------------------------------------------------------
"""
unsorted = a._front
a._front = None
while unsorted is not None and unsorted._next is not None:
current = unsorted
unsorted = unsorted._next
if current._value < unsorted._value:
current._next = unsorted._next
unsorted._next = current
unsorted = unsorted._next
else:
find = unsorted
while find._next is not None and current._value > find._next._value:
find = find._next
current._next = find._next
current = find._next
a._front = unsorted
return a
I believe what I have is correct in terms of sorting. However when I try to read the list in the main module I get a bunch of None values.
In this case, the insertion sort is not creating a new list when sorting. Rather, it is moving all sorted elements to the 'front'.
To summarize, I have two problems: I am not sure if the insertion sort is correct, and there are problems with the returned list a as it contains None values. Thanks in advance

Answer 1

Not exactly sure about the type of a is, but if you assume a simple:
class Node:
def __init__(self, value, node=None):
self._value = value
self._next = node
def __str__(self):
return "Node({}, {})".format(self._value, self._next)
Then your insertion sort isn't far off, it needs to handle the head case properly:
def insertion_sort(unsorted):
head = None
while unsorted:
current = unsorted
unsorted = unsorted._next
if not head or current._value < head._value:
current._next = head;
head = current;
else:
find = head;
while find and current._value > find._next._value:
find = find._next
current._next = find._next
find._next = current
return head
>>> print(insertion_sort(Node(4, Node(1, Node(3, Node(2))))))
Node(1, Node(2, Node(3, Node(4, None))))