Merge sort a linked list

merge_sort_list.py

class Node(object):
    def __init__(self, val):
        self.val = val
        self.next = None

def sort(head, tail=None):
    if not head:
        return None

    if head.next == tail:
        return head

    slow = head; fast = head
    prev_slow = None
    while(fast and fast.next != tail):
        prev_slow = slow
        slow = slow.next
        fast = fast.next.next

    if prev_slow:
        prev_slow.next = None

    m = slow

    left = sort(head, None)
    right = sort(m, tail)

    if(left.val < right.val):
        head = left
        left = left.next
    else:
        head = right
        right = right.next

    final_tail = head

    while(left and right):
        if left.val < right.val:
            final_tail.next = left
            left = left.next
        else:
            final_tail.next = right
            right = right.next
        final_tail = final_tail.next

    if left:
        final_tail.next = left
    elif right:
        final_tail.next = right

    return head