Why is BeautifulSoup related to 'Task exceptio

I want to use the coroutine to crawl and parse webpages. I write a sample and test. The program could run well in python 3.5 in ubuntu 16.04 and it will quit when all the works have been done. The source code is below.

import aiohttp
import asyncio
from bs4 import BeautifulSoup

async def coro():
    coro_loop = asyncio.get_event_loop()
    url = u'https://www.python.org/'
    for _ in range(4):
        async with aiohttp.ClientSession(loop=coro_loop) as coro_session:
            with aiohttp.Timeout(30, loop=coro_session.loop):
                async with coro_session.get(url) as resp:
                    print('get response from url: %s' % url)
                    source_code = await resp.read()
                    soup = BeautifulSoup(source_code, 'lxml')

def main():
    loop = asyncio.get_event_loop()
    worker = loop.create_task(coro())
    try:
        loop.run_until_complete(worker)
    except KeyboardInterrupt:
        print ('keyboard interrupt')
        worker.cancel()
    finally:
        loop.stop()
        loop.run_forever()
        loop.close()

if __name__ == '__main__':
    main()

While testing, I find when I shut down the program by 'Ctrl+C', there will be a error 'Task exception was never retrieved'.

^Ckeyboard interrupt
Task exception was never retrieved
future: <Task finished coro=<coro() done, defined at ./test.py:8> exception=KeyboardInterrupt()>
Traceback (most recent call last):
  File "./test.py", line 23, in main
    loop.run_until_complete(worker)
  File "/usr/lib/python3.5/asyncio/base_events.py", line 375, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 345, in run_forever
    self._run_once()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1312, in _run_once
    handle._run()
  File "/usr/lib/python3.5/asyncio/events.py", line 125, in _run
    self._callback(*self._args)
  File "/usr/lib/python3.5/asyncio/tasks.py", line 307, in _wakeup
    self._step()
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "./test.py", line 17, in coro
    soup = BeautifulSoup(source_code, 'lxml')
  File "/usr/lib/python3/dist-packages/bs4/__init__.py", line 215, in __init__
    self._feed()
  File "/usr/lib/python3/dist-packages/bs4/__init__.py", line 239, in _feed
    self.builder.feed(self.markup)
  File "/usr/lib/python3/dist-packages/bs4/builder/_lxml.py", line 240, in feed
    self.parser.feed(markup)
  File "src/lxml/parser.pxi", line 1194, in lxml.etree._FeedParser.feed (src/lxml/lxml.etree.c:119773)
  File "src/lxml/parser.pxi", line 1316, in lxml.etree._FeedParser.feed (src/lxml/lxml.etree.c:119644)
  File "src/lxml/parsertarget.pxi", line 141, in lxml.etree._TargetParserContext._handleParseResult (src/lxml/lxml.etree.c:137264)
  File "src/lxml/parsertarget.pxi", line 135, in lxml.etree._TargetParserContext._handleParseResult (src/lxml/lxml.etree.c:137128)
  File "src/lxml/lxml.etree.pyx", line 324, in lxml.etree._ExceptionContext._raise_if_stored (src/lxml/lxml.etree.c:11090)
  File "src/lxml/saxparser.pxi", line 499, in lxml.etree._handleSaxData (src/lxml/lxml.etree.c:131013)
  File "src/lxml/parsertarget.pxi", line 88, in lxml.etree._PythonSaxParserTarget._handleSaxData (src/lxml/lxml.etree.c:136397)
  File "/usr/lib/python3/dist-packages/bs4/builder/_lxml.py", line 206, in data
    def data(self, content):
KeyboardInterrupt

I looked through the offical docs of python but haven't got a clue. I try to capture the Keyboard Interrupt in coro().

try:
    soup = BeautifulSoup(source_code, 'lxml')
except KeyboardInterrupt:
    print ('capture exception')
    raise

Everytime the 'try/except' around BeautifulSoup() capture the KeyboardInterrupt, the error will occur. It seems that BeautifulSoup contribute to the error. But how to tackle it?

When you call task.cancel() this function doesn't actually cancel task, it just "mark" task to be cancelled. Actual process of cancelling task would be started when task will resume it's execution. asyncio.CancelledError will be immediately raised inside task forcing it to be actually cancelled. Task will finish it's execution with this exception.

On the other hand asyncio warns you if some of your tasks finished with exception silently (if you didn't check result of task execution).

To avoid problems you should await task cancellation receiving asyncio.CancelledError (and probably suppressing since you don't need it then):

import asyncio
from contextlib import suppress


async def coro():
    # ...

def main():
    loop = asyncio.get_event_loop()
    worker = asyncio.ensure_future(coro())
    try:
        loop.run_until_complete(worker)
    except KeyboardInterrupt:
        print('keyboard interrupt')

        worker.cancel()
        with suppress(asyncio.CancelledError):
            loop.run_until_complete(worker)  # await task cancellation.
    finally:
        loop.close()

if __name__ == '__main__':
    main()