The assignment is to use selection sort by swapping nodes, not the values. When sorting algorithm changes the last node, it raises AttributeError, because it seems that first.next in def min_sort()does not seem to change and is None. Here is the code

class LinkedList:
    class Node:
        def __init__(self, data, next=None):
            self.data, self.next = data, next

    def __init__(self, seq):
        self.front = self.Node(None)
        curr = self.front
        for value in seq:
            curr.next = self.Node(value)
            curr = curr.next

    def swap(self, x, y):
        first = self.front
        while first.next is not None:
            if first.next.data == x.data:
                prevx = first
            if first.next.data == y.data:
                prevy = first
            first = first.next
        x, y = y, x
        prevx.next, prevy.next = x, y
        temp = x.next
        x.next = y.next
        y.next = temp

    def progress(self, first, reverse):
        try:
            solid = first
            while first.next is not None:
                if solid.data > first.next.data and not reverse:
                    self.swap(solid, first.next)
                elif solid.data < first.next.data and reverse:
                    self.swap(solid, first.next)
                first = first.next
        except Exception:
            pass

    def min_sort(self, reverse):
        first = self.front
        while first.next is not None:
            self.progress(first, reverse)
            first = first.next

I don’t know how to change the reference in function min_sort so it would work