Many to many bidirectional relationship is not JSON serializable about overholt HOT 5 CLOSED

class JsonSerializer(object):
    
    __json_modifiers__ = None


    def to_json(self):

        modifiers = self.__json_modifiers__ or dict()

        rv = dict()

        for key, modifier in modifiers.items():
            value = getattr(self, key)
            rv[key] = modifier(value, self)

        return rv

# My own modified json serializer
class SubredditJsonSerializer(JsonSerializer):
    """
    Extend the JsonSerializer class to change the users attribute from type
    AppenderBaseQuery to type list.
    """
    __json_modifiers__ = {
        'users': lambda users, _: [dict(id=user.id, username=user.username, phone_num=user.phone_num) for user in users]
    }

    def to_json(self):

        # modifiers == {
        #   'users': lambda users, _: [dict(id=user.id, username=user.username, phone_num=user.phone_num) for user in users]
        # }
        modifiers = self.__json_modifiers__ or dict()

        rv = dict()


        # modifiers.items() == [
        #   ('users', lambda users, _: [dict(id=user.id, username=user.username, phone_num=user.phone_num) for user in users])
        # ]
        for key, modifier in modifiers.items():

            # key == 'users'
            # value == lambda users, _: [dict(id=user.id, username=user.username, phone_num=user.phone_num) for user in users]
            value = getattr(self, key)


            ## This is the step I am having trouble understanding. How is modifier being used as a function if it is a key-value pair?

            # modifier == ('users', lambda users, _: [dict(id=user.id, username=user.username, phone_num=user.phone_num) for user in users])
            # rv['users'] = ???
            rv[key] = modifier(value, self)

        return rv