问如何在扩展dict (在python3中)时正确覆盖方法键()

EN

class Measurands(MutableSet, MutableMapping):
    @property
    def measurands(self):
        return self._measurands
    
    
    @measurands.setter
    def measurands(self, val):
        self._measurands = []
        
        for el in val:
            self.add(el)
    
    
    def __init__(self, arg=None):
        if arg is None:
            self.measurands = []
        else:
            try:
                measurands = arg.measurands
            except AttributeError:
                if isinstance(arg, Iterable):
                    measurands = list(arg)
                else:
                    measurands = [arg]
            
            self.measurands = measurands
            
    
    def __iter__(self):
        return iter(self.measurands)
    
    
    def __getitem__(self, key):
        for m in self:
            if m.id == key:
                return m
    
        raise KeyError(key)
    
    
    def __contains__(self, el_x):
        is_in = False
        
        for el in self:
            if el_x.id == el.id:
                is_in = True
                break
        
        return is_in
    
    
    def add(self, el):
        try:
            self[el.id].session_ids |= el.session_ids
        except KeyError:
            self[el.id] = Measurand(el)
    
    
    def _discard(self, key):
        res = False
        
        for i, m in enumerate(self):
            if m.id == key:
                del self.measurands[i]
                res = True
                break
        
        return res
    
    
    def __delitem__(self, key):
        res = self._discard(self, key)
        
        if not res:
            raise KeyError(key)
    
    def discard(self, key):
        self._discard(self, key)
    
    
    def remove(self, key):
        self.__delitem__(self, key)
    
    
    def __len__(self):
        return len(self.measurands)
    
    
    def __setitem__(self, key, value):
        res = False
        value_true = Measurand(value)
        
        for i, m in enumerate(self):
            if m.id == key:
                res = True
                
                if value.id == key:
                    self.measurands[i] = value_true
                else:
                    raise KeyError(key)
                
                break
        
        if not res:
            self.measurands.append(value_true)
    
    
    def __str__(self):
        string = "Measurands({"
        
        for m in self:
            string += str(m)
            string += ", "
        
        if string:
            string = string[:-2]
        
        string += "})"
        
        return string