<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def format(self, format_spec='f'):
 u""" Formata o CPF. Códigos de formatação: r -> raw f -> formatado '58119443659' '581.194.436-59' """

        if format_spec == '' or format_spec == 'f':
            cpf = CPF.clean(self._cpf)
            return '{0}.{1}.{2}-{3}'.format(cpf[0:3], cpf[3:6], cpf[6:9], cpf[9:11])
        if format_spec == 'r':
            return self._cpf
        raise ValueError(
            "Unknown format code '{0}' for object of type '{1}'".format(format_spec, CPF.__name__))