python 3.x - How do I make it possible to subtract and multiply in both directions in a class method -


so trying program can add, subtract , multiply volumes (which consist of magnitude , unit) got __add__ work __rsub__ , __mult__ have not figured out how make them work. want able operations volume * 2 or 2 * volume. advice on how go appreciated! 

def __add__ (self, v2): #adds magnitude using int or volume     if volume.is_valid(self):         if isinstance(v2, volume):             if volume.is_valid(v2): #check if v2 valid                 if self.__units == v2.__units:                     return volume(self.__magnitude + v2.__magnitude,                                    self.__units)                 else:                     if v2.__units == 'ml':                         v3 = v2.customary()                         return volume(self.__magnitude + v3.__magnitude,                                        self.__units)                     elif v2.__units == 'oz':                         v3 = v2.__units                         return volume(self.__magnitude + v3.__magnitude,                                        self.__units)          elif isinstance(v2,(int,float)):             #create new volume same units self             v3 = volume(v2, self.__units)             return volume(self.__magnitude + v3.__magnitude, self.__units)  def __radd__ (self, v2): #adds ability i.e. 2 +     return self.__add__(v2)  def __sub__ (self, v2): #same __add__ subtracts     if volume.is_valid(self):         if isinstance(v2, volume):             if volume.is_valid(v2):                 if self.__units == v2.__units:                     return volume(self.__magnitude - v2.__magnitude,                                    self.__units)                 else:                     if v2.__units == 'ml':                         v3 = v2.customary()                         return volume(self.__magnitude - v3.__magnitude,                                        self.__units)                     elif v2.__units == 'oz':                         v3 = v2.__units                         return volume(self.__magnitude - v3.__magnitude,                                        self.__units)         elif isinstance(v2,(int,float)):             v3 = volume(v2, self.__units)             return volume(self.__magnitude - v3.__magnitude, self.__units)  def __rsub__ (self, v2):     return volume(v2.__magnitude - self.__magnitude, self.__units)  def __mult__ (self, v2):#multiplies volume int (not other vol)     if volume.is_valid(self):         if isinstance(v2, volume):             if volume.is_valid(v2):                 if self.__units == v2.__units:                     return volume(self.__magnitude * v2.__magnitude,                                    self.__units)                 else:                     if v2.__units == 'ml':                         v3 = v2.customary()                         return volume(self.__magnitude * v3.__magnitude,                                        self.__units)                     elif v2.__units == 'oz':                         v3 = v2.__units                         return volume(self.__magnitude * v3.__magnitude,                                        self.__units)         elif isinstance(v2,(int,float)):             v3 = volume(v2, self.__units)             return volume(self.__magnitude * v3.__magnitude, self.__units)         #return volume(self.__magnitude * v2, self.__units)  def __rmult__ (self, v2):     return self.__mult__(v2)

the magic method multiplying __mul__, not __mult__.

__rsub__ doesn't appear work __sub__. perhaps like:

def __rsub__(self, v2):     return volume(v2) - self

would better.


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