how to simplify computed attributes

computed attributes

class mytest():                                       

    def init(self):                                                                                                            
        pass                                                                                                            

    def __getattr__(self, attr):                                                                                               
        print("looking for ", attr)                                                                                              
        print("keys: ", self.__dict__.keys())                                                                            
        if not ("_"+attr) in self.__dict__.keys():                                                                     
            if attr in ['a', 'b']:          # would be nice to automate the next lines with a decorator on do_calc   
                self.do_calc1()                                                                                                  
            if attr in ['c', 'd']:                                                                                             
                self.do_calc2()                                                                                             
        return self.__dict__["_"+attr]                                                                                          
  
    def do_calc1(self):                                                                                                        
        print("calc a, b")                                                                                
        self._a = 2                                                                                      
        self._b = 3                                                                                             
 
    def do_calc2(self):                                                                                             
        print("calc c, d")                                                                                                         
        self._c = 4                                                                                                        
        self._d = 5                                                                                                          
  
a = mytest()                                                                                                      
 
a.a                                                                                                                                        
a.b                                                                                                                                        
a.c                                                                                                                                        
a.d