为什么在某些情况下不能运行包含futures.ThreadpoolExecutor的类方法？

如何解决为什么在某些情况下不能运行包含futures.ThreadpoolExecutor的类方法？

我有一个名为Downloader的类，我想使用该类来处理来自S3的交易。我想使用并发性尽快从S3下载内容，as per this SO post，但是当我将所有并发性代码放在班级中时，我会得到一些奇怪的行为。

运行下面的代码

import boto3
import botocore
from concurrent import futures

class Downloader:
    def __init__(self,aws_key,aws_id):
        self.s3_client = boto3.resource('s3',aws_access_key_id=aws_id,aws_secret_access_key=aws_key)

    def download_s3(self,key,bucket='bucket'):
        try:
            f = self.s3_client.meta.client.get_object(Bucket=bucket,Key=key)['Body'].read()
        except ClientError as e:
            print(f'Key {key} not found')
            return None
        return np.frombuffer(f)

    def download_s3_parallel(self,keys):

        print(f'executor called...')
        with futures.ThreadPoolExecutor(max_workers=1) as executor:
            print(f'launching threads')
            future_to_key = {executor.submit(self.download_s3,key): key for key in keys}
            print(future_to_key)
        
            print(f'looping through completed threads')
            for future in futures.as_completed(future_to_key):
                key = future_to_key[future]
                exception = future.exception()

                if not exception:
                    yield key,future.result()
                else:
                    yield key,exception


keys = ['key1','key2','key3']
dl = Downloader(<my_secret_key>,<my_aws_id>)
all_objects = dl.download_s3_parallel(keys)

给出无输出，但是当我注释掉for循环时，如下所示：

class Downloader:
    def __init__(self,key): key for key in keys}
            print(future_to_key)
        
            print(f'looping through completed threads')
            # for future in futures.as_completed(future_to_key):
            #     key = future_to_key[future]
            #     exception = future.exception()

            #     if not exception:
            #         yield key,future.result()
            #     else:
            #         yield key,exception

输出变为

executor called...
launching threads
{<Future at 0x7f5f8dd12430 state=running>: 'key1',<Future at 0x7f5f8dd12730 state=pending>: 'key2',<Future at 0x7f5f8dd2c8b0 state=pending>: 'key3'}
looping through completed threads

所以我的问题是：为什么for循环导致整个download_features_s3_parallel函数无法运行？即使在该方法开头的print语句也不会被调用。发生这种情况的原因是什么？ 如果是因为for循环引发错误，如何找出错误所在？