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

[Antoine Pitrou]

Change by Antoine Pitrou <pitrou at free.fr>:


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resolution:  -> fixed
versions: +Python 3.7, Python 3.8 -Python 3.6

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