'Get custom value
@param key: the index for the custom field (between 1-5)
@type key: int
@rtype: str'

def get_custom(self, key):

    self._assert_custom_key_valid(key)
    m = rtorrentlib.rpc.Multicall(self)
    field = 'custom{0}'.format(key)
    self.multicall_add(m, 'd.get_{0}'.format(field))
    setattr(self, field, m.call()[(-1)])
    return getattr(self, field)
