'Shuffle the list. The current client swaps position with its predecessor.
If maintain_index is True the current_index is adjusted,
such that the same client stays current and goes up in list.'

def shuffle_up(self, maintain_index=True):

    idx = self._current_idx
    if (idx > 0):
        (self.clients[idx], self.clients[(idx - 1)]) = (self[(idx - 1)], self[idx])
        if maintain_index:
            self.current_index -= 1
