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Trying to streamline the URL retrieval example in the concurrent.futures docs were severely hampered by the fact the URLError thrown by urlopen doesn't tell you *what URL* it was trying to retrieve when it failed.
This is OK in synchronous code (where you probably still have the URL around), but quite a pain in asynchronous code (such as that using concurrent.futures) where the original context may have been lost by the time the future is signalled.