[issue33110] Adding a done callback to a concurrent.futures Future once it has already completed, may raise an exception, contrary to docs

[Sam Martin]

Change by Sam Martin <nivekkas at gmail.com>:


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keywords: +patch
pull_requests: +13054
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33110>
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