class Setting(str):

    def __new__(cls, value, source):
        return super(Setting, cls).__new__(cls, value)

    def __init__(self, value, source):
        self.value = value
        self.source = source

TypeError: The basestring type cannot be instantiated

class Setting:
    def __init__(self, value, source):
        self.value = value
        self.source = source

    def __add__(self, rhs):
        #keeping the object immutable
        return Setting(self.value + rhs, self.source)

class Setting(object):

    def __new__(cls, value, source):
        #pick the correct base class
        base_class = str if isinstance(value, str) else unicode
        #construct new Setting class with correct base
        new_type = type(cls.__name__, (base_class,), dict(cls.__dict__))
        return base_class.__new__(new_type, value)

    def __init__(self, value, source):
        self.value = value
        self.source = source