def clean(cls, cpf):
        u"""
        Retorna apenas os dígitos do CPF.

        >>> CPF.clean('581.194.436-59')
        '58119443659'
        """
        if isinstance(cpf, six.string_types):
            cpf = int(re.sub('[^0-9]', '', cpf))

        return '{0:011d}'.format(cpf)