如果采用FIRST_COMPLETED參數(shù)，程序并不會等到線程池里面所有的任務(wù)都完成。
ziwenxie :: ~ » python example5.pyDoneAndNotDoneFutures(done={<Future at 0x7f84109edb00 state=finished returned str>,<Future at 0x7f840e2e9320 state=finished returned str>,<Future at 0x7f840f25ccc0 state=finished returned str>},not_done={<Future at 0x7f840e2e9ba8 state=running>,<Future at 0x7f840e2e9940 state=running>})