class Mylist:
    def __init__(self, lst=None):
        if lst is None:
            self.items = []
        else:
            self.items = lst
    
    def __add__(self, other):
        if isinstance(other, list):
            return self.items.extend(other)
        elif isinstance(other, (int, str)):
            return self.items.append(other)
        else:
            return NotImplemented
    
    def __repr__(self):
        return repr(self.items)

my_list = Mylist([1, 2, 3])
my_list + [4, 5, 6]
print(my_list)
my_list + 7
print(my_list)