inheritance - Python : calling __new__ method of superclass -

i have 2 python classes defined follows :

class a(object) :       def __init__(self, param) :          print('a.__init__ called')         self.param = param      def __new__(cls, param) :         print('a.__new__ called')          x = object.__new__(a)         x._initialize()    # initialization code         return x  class b(a) :       def __init__(self, different_param) :          print('b.__init__ called')      def __new__(cls, different_param) :          print('b.__new__ called')         # call __new__ of class a, passing "param" parameter         # , initialized instance of class b         # below         b = object.__new__(b)         param = cls.convert_param(different_param)         return super(b, cls).__new__(b, param)    # expecting                                                   # similar      @staticmethod     def convert_param(param) :          return param  

class b subclass of class a. difference between 2 classes parameters passed class b in different format compared expected class a. so, convert_param method of classb called convert parameters compatible __new__ method of class a.

i stuck @ part wish call __new__ method of class a __new__ method of class b, since there lot of initialisation takes place in there, , initialised instance of class b.

i having difficult time figuring out. please help.

convert_param should either staticmethod or classmethod , don't want calling object.__new__ b (otherwise, you're trying create 2 new instances of b instead of one). if convert_param staticmethod or classmethod, can parameter conversion before have instance (e.g. before __new__ has been called on superclass):

class b(a):     @staticmethod     def convert_param(params):         # magic         return params      def __new__(cls, params):         params = cls.convert_params(params)         return super(b, cls).__new__(cls, params) 

additionally, you'll need change a's __new__ not hard-code type of instance returned object.__new__:

class a(object) :      def __new__(cls, param) :         print('a.__new__ called')          x = super(a, cls).__new__(cls)         x._initialize()    # initialization code         return x