python3 sorted 如何实现自定义排序标准

def cmp(a, b):
 # 如果逻辑上认为 a < b ，返回 -1
 # 如果逻辑上认为 a > b , 返回 1
 # 如果逻辑上认为 a == b, 返回 0 
 pass

a = [2,3,1,2]
a = sorted(a, cmp)

import functools
def cmp(a, b):
  if b < a:
    return -1
  if a < b:
    return 1
  return 0
a = [1, 2, 5, 4]
print(sorted(a, key=functools.cmp_to_key(cmp)))

def cmp_to_key(mycmp):
  """Convert a cmp= function into a key= function"""
  class K(object):
    __slots__ = ['obj']
    def __init__(self, obj):
      self.obj = obj
    def __lt__(self, other):
      return mycmp(self.obj, other.obj) < 0
    def __gt__(self, other):
      return mycmp(self.obj, other.obj) > 0
    def __eq__(self, other):
      return mycmp(self.obj, other.obj) == 0
    def __le__(self, other):
      return mycmp(self.obj, other.obj) <= 0
    def __ge__(self, other):
      return mycmp(self.obj, other.obj) >= 0
    __hash__ = None
  return K

k1 = K(1)
k2 = K(2)

# 问题，k1，k2 谁是 self，谁是 other
# k1 是 self， k2 是 other
print(k1 < k2)