import settings

class MySQL:
  __instance = None

  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

  @classmethod
  def from_conf(cls):
    if cls.__instance is None:
      cls.__instance = cls(settings.IP,settings.PORT)
    return cls.__instance

obj1 = MySQL.from_conf()
obj2 = MySQL.from_conf()
obj3 = MySQL.from_conf()
print(obj1)
print(obj2)
print(obj3)

import settings

def singleton(cls):
  _instance = cls(settings.IP, settings.PORT)

  def wrapper(*args, **kwargs):
    if args or kwargs:
      obj = cls(*args, **kwargs)
      return obj
    return _instance

  return wrapper

@singleton
class MySQL:
  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

obj1 = MySQL()
obj2 = MySQL()
obj3 = MySQL()
print(obj1)
print(obj2)
print(obj3)

import settings

class Mymeta(type):
  def __init__(self, class_name, class_bases, class_dic):
    self.__instance = self(settings.IP, settings.PORT)

  def __call__(self, *args, **kwargs):
    if args or kwargs:
      obj = self.__new__(self)
      self.__init__(obj, *args, **kwargs)
      return obj
    else:
      return self.__instance

class MySQL(metaclass=Mymeta):
  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

obj1 = MySQL()
obj2 = MySQL()
obj3 = MySQL()
print(obj1)
print(obj2)
print(obj3)

def f1():
  from singleton import instance
  print(instance)

def f2():
  from singleton import instance,MySQL
  print(instance)
  obj = MySQL('1.1.1.1', '3389')
  print(obj)

f1()
f2()


singleton.py文件里内容:
import settings

class MySQL:
  print('run...')

  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

instance = MySQL(settings.IP, settings.PORT)