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I'm trying to understand how to work with aiohttp and asyncio. The code below retrieves all websites in urls and prints out the "size" of each response.

  • Is the error handling within the fetch method correct?
  • Is it possible to remove the result of a specific url from results in case of an exception - making return (url, '') unnecessary?
  • Is there a better way than ssl=False to deal with a potential ssl.SSLCertVerificationError?
  • Any additional advice on how i can improve my code quality is highly appreciated
    import asyncio
    import aiohttp
    
    
    async def fetch(session, url):
        try:
            async with session.get(url, ssl=False) as response:
                return url, await response.text()
        except aiohttp.client_exceptions.ClientConnectorError as e:
            print(e)
            return (url, '')
    
    
    async def main():
        tasks = []
        urls = [
            'http://www.python.org',
            'http://www.jython.org',
            'http://www.pypy.org'
        ]
        async with aiohttp.ClientSession() as session:
            while urls:
                tasks.append(fetch(session, urls.pop()))
    
            results = await asyncio.gather(*tasks)
            [print(f'{url}: {len(result)}') for url, result in results]
    
    
    if __name__ == '__main__':
        loop = asyncio.get_event_loop()
        loop.run_until_complete(main())
        loop.close()
    

    Update

    • Is there a way how i can add tasks to the list from within the "loop"? e.g. add new urls while scraping a website and finding new subdomains to scrape.
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    4
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    tasks = []
    while urls:
        tasks.append(fetch(session, urls.pop()))
    

    can be largely simplified to

    tasks = [fetch(session, url) for url in urls]
    

    Is it possible to remove the result of a specific url from results in case of an exception - making return (url, '') unnecessary?

    Yes, somewhat. asyncio.gather accept a return_exceptions parameters. Set it to True to avoid a single exception failing the gather call. You must filter them out afterwards anyway:

    import asyncio
    import aiohttp
    
    
    async def fetch(session, url):
        async with session.get(url, ssl=False) as response:
            return await response.text()
    
    
    async def main():
        urls = [
            'http://www.python.org',
            'http://www.jython.org',
            'http://www.pypy.org'
        ]
        async with aiohttp.ClientSession() as session:
            tasks = [fetch(session, url) for url in urls]
            results = await asyncio.gather(*tasks)
    
        for url, result in zip(urls, results):
            if not isinstance(result, Exception):
                print(f'{url}: {len(result)}')
            else:
                print(f'{url} FAILED')
    
    
    if __name__ == '__main__':
        loop = asyncio.get_event_loop()
        loop.run_until_complete(main())
        loop.close()
    
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    • \$\begingroup\$ Not sure if your code example is meant to catch exceptions or if you just wanted to point out how to deal with exceptions in the results list. I only manage to have a working code if i put try... except within the fetch method. If put return e in except your code works. \$\endgroup\$ – RandomDude Jul 24 '18 at 19:46

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