Questions regarding the use of Requests Futures for accessing REST URLs

Question

This is a much simplified version of the real code focusing just on the handling of Futures from Requests Futures.

I have a few questions:

1. I had to implement my own version of as_completed because the data handlers may add more Futures to _pending. Is this a decent way to handle the problem, or is there another approach?
2. Is stop sufficient to handle KeyboardInterrupt in all cases? It has worked well in my limited testing. I found it hard to find a solution via Google.
3. Is my rate limiting solution OK or is there a better approach? It is not about the number of concurrent connections but about the number of connections per second.

import argparse
from concurrent.futures import ThreadPoolExecutor
import requests
from requests_futures.sessions import FuturesSession
import time

def background_callback(sess, resp):
    # parse the json storing the result on the response object
    if resp.status_code == requests.codes.ok:
        resp.data = resp.json()
    else:
        resp.data = None

class JSONRetriever(object):
    def __init__(self):
        self._executor = ThreadPoolExecutor(max_workers=10)
        self._session = FuturesSession(executor=self._executor)
        self._pending = {}

    def fetch(self, url):
        future = self._session.get(url,
                                   background_callback=background_callback)
        self._pending[future] = url

    def drain(self):
        # Look for completed requests by hand because in the real code
        # the responses my trigger further URLs to be retrieved so
        # self._pending is modified. New requests being added really
        # confused as_completed().
        for future in [f for f in self._pending if f.done()]:
            url = self._pending[future]
            del self._pending[future]

            response = future.result()
            response.raise_for_status()
            if response.status_code == requests.codes.ok:
                print response.data
                # real code would handle data possibly adding more requests
            else:
                # the real code is smarter, this is just for CR
                raise Exception("FIXME: unhandle response")

    def finish(self):
        while self._pending:
            self.drain()
            if self._pending:
                time.sleep(1)

    def stop(self):
        for i in self._pending:
            try:
                i.cancel()
            except Exception as e:
                sys.stderr.write("Caught: " + str(e) + "\n")

        self._executor.shutdown()

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="Perform all REST calls")
    parser.add_argument("--delay", type=int, default=0)
    parser.add_argument("urls", nargs="+")
    args = parser.parse_args()

    retriever = JSONRetriever()

    try:
        for url in args.urls:
            retriever.fetch(url)
            if args.delay > 0:  # may need a delay to rate limit requests
                time.sleep(args.delay)
                retriever.drain()  # clear any requests that completed while asleep

        retriever.finish()
    except KeyboardInterrupt:
        retriever.stop()

Answer 1

Nice code, clearly written.

I understand the rate limiting requirement. Having the drain() call within the loop doesn't seem like the caller's responsibility, better to let the BG callback handle it, or defer until finish() as written, which does make sense. Each url fetch could take more or less than the delay time. So this seems to be a bug / wart still lurking within the code.