Leetcode: Sort List @Python

class Solution:
    def sortList(self, head):
        # QuickSort
        len = 0
        tail = head
        while tail != None:
            len += 1
            tail = tail.next
        pivot = head
        return self.partition(head, len)

    def partition(self, head, len):
        if len<2:
            return head

        pivot = head
        r_pivot = pivot
        tail  = head
        head  = ListNode(0)
        left  = head
        l = 0
        lr = 0
        stall = 0
        l_len = 0
        r_len = len-1

        while tail.next != None and l<len-1:
            if tail.next.val < pivot.val:
                left.next = tail.next
                left = left.next
                tail.next = tail.next.next
                r_len -= 1
                l_len += 1
                lr += 1
                left.next = pivot
            elif tail.next.val > pivot.val:
                tail = tail.next
            elif tail.next.val == pivot.val:
                self.swap_nodes(r_pivot, r_pivot.next, tail, tail.next)
                lr += 1
                stall = l-lr
                if r_pivot == tail:
                    tail = r_pivot.next
                r_pivot = r_pivot.next
                r_len -= 1

            if stall>0:
                stall -= 1
            else:
                l += 1

        left.next = pivot

        head_l = self.partition(head.next,  l_len)
        head_r = self.partition(r_pivot.next, r_len)

        head.next  = head_l
        r_pivot.next = head_r

        return head.next

    def swap_nodes(self, x, a, y, b):
        x.next = b
        y.next = a
        temp = a.next
        a.next = b.next
        b.next = temp