先来看一段 demo 代码：

from concurrent import futures
import time
import random

def returnNumber(number: int)->int:
    print("start return number {}".format(number))
    time.sleep(random.randint(10, 20))
    print("end return number {}".format(number))
    return number

if __name__ == '__main__':
    with futures.ThreadPoolExecutor(5) as executor:
        to_do = []
        for number in range(0, 10):
            future = executor.submit(returnNumber, number)   # 添加到futures的任务队列，等待排队执行
            print(future)
            to_do.append(future)
        print("-----middle-----")
        for future in futures.as_completed(to_do):          # 当某一个future任务执行完毕后，执行下面代码。会阻塞，等待线程完成后执行
            res = future.result()                           # 获取线程的返回结果
            print(res)
            
    print("-----end-----")

执行结果解释：

start return number 0
<Future at 0x1b8b4728390 state=running>
start return number 1
<Future at 0x1b8b4ffff98 state=running>
start return number 2
<Future at 0x1b8b5022160 state=running>
start return number 3
<Future at 0x1b8b5022390 state=running>
start return number 4
<Future at 0x1b8b50225c0 state=running>
<Future at 0x1b8b50227b8 state=pending>             # 由于线程池是5个线程，其他的任务将会标记为padding状态等待执行
<Future at 0x1b8b5022898 state=pending>
<Future at 0x1b8b50227f0 state=pending>
<Future at 0x1b8b50229b0 state=pending>
<Future at 0x1b8b5022a90 state=pending>
-----middle-----                                    # 打印完middle将等待第一个线程结束
end return number 2                                 # 第一个线程结束
end return number 0
start return number 5
start return number 6
2                                                   # 此时as_completed方法执行
0
end return number 4
start return number 7
4
end return number 1
start return number 8
1
end return number 3
start return number 9
3
end return number 5
5
end return number 6
6
end return number 7
7
end return number 8
8
end return number 9
9
-----end-----                                        # 所有future执行完，程序结束