code: def clean(cls, cpf):
          u
          if isinstance(cpf, six.string_types):
              cpf = int(re.sub('[^0-9]', '', cpf))
  
          return '{0:011d}'.format(cpf)
docs: u"""
          Retorna apenas os dígitos do CPF.
  
          >>> CPF.clean('581.194.436-59')
          '58119443659'