I have the following code to download data from the SRA using multithreading in Python. After running this a few times now (for testing purposes), I keep getting denied access to the data. Not sure how to fix this. In particular, cating the output files gives:

<?xml version="1.0" encoding="UTF-8"?><Error><Code>AccessDenied</Code><Message>Access Denied</Message><RequestId>cb0f0e98-cafb-1dd7-9b7b-d8c49756ec52</RequestId><HostId>QeDGVwBXYp61J0B4_OUTn7UsEsiQEec0n18DAeR0kaE</HostId></Error>

Here is my code:

import threading
import requests
import time 

start = time.perf_counter()

class MyThread(threading.Thread):
    def __init__(self, url):
        self.url = url
        self.result = None
        self.filename = url.split('/')[-1]
    def run(self):
        res = requests.get(self.url)
        with open(self.filename, 'wb') as f:
urls = [

threads = [MyThread(url, ) for url in urls]
for thread in threads:
for thread in threads:

finish = time.perf_counter()

print(f'Finished in {round(finish-start, 2)} second(s)')

1 Answer 1


I once needed to download more than 1000 files from SRA, and I found a blog post that is very useful (https://reneshbedre.github.io/blog/fqutil.html). The idea is to use aspera to fetch SRA files. This is fast becasue of aspera. Then use fasterq-dump to convert SRA file to fastq. fasterq-dump is faster than fastq-dump partly because it does not compress the output file. I think compressing is the rate-limiting step here. After this, you can use pigz, which is a multi-threading version of gzip to compress the fastq files. Note that if you want to download files to a computing cluster, you can submit jobs for fasterq-dump and pigz. This makes things much faster. You cannot submit jobs for fetching SRA, because you need to connect to internet.

  • $\begingroup$ Thanks for sharing this. So I tried running prefetch with aspera as explained in the blog, but I get unrecognized option when I include the '-a' option. I do have aspera installed. Did you have this issue by any chance? $\endgroup$ Commented Jul 13, 2020 at 15:59
  • $\begingroup$ seems like aspera is no longer supported github.com/ncbi/sra-tools/issues/255 $\endgroup$ Commented Jul 13, 2020 at 16:32
  • $\begingroup$ however, prefetch is still very fast, thanks for sharing this, update your answer to reflect this and I'll accept it! $\endgroup$ Commented Jul 13, 2020 at 16:33

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