class A:
    def __init__(self, rid, title):
        self.rid = rid
        self.title = title
        self.b = []

    def __add__(self, other):
        if type(other) == B:
            self.b += [other]

    def __str__(self):
        return self.rid + ' - ' + self.title

    def __repr__(self):
        return str(self)

class B:
    def __init__(self, rid, title):
        self.rid = rid
        self.title = title

b = B('123', 'abc')
a = A('345', 'cde')

print(a)

a += b

print(a)

345 - cde

None