bpo-27144: concurrent.futures as_complete and map iterators do not keep reference to returned object by grzgrzgrz3 · Pull Request #1560 · python/cpython (original) (raw)

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If we do not remove waiter, second result set on future will trigger waiter and cause KeyError, future is already returned and reference cleared.

For example:

>>> from concurrent.futures import Future, as_completed
>>> fs_finished = Future()
>>> fs = Future()
>>> fs_finished.set_result("")
>>> for x in as_completed([fs_finished, fs, Future()]):
...     fs.set_result(None)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/grzegorz/cpython/Lib/concurrent/futures/_base.py", line 235, in as_completed
    pending.remove(future)
KeyError: <Future at 0x7f566ff6fc20 state=finished returned NoneType>

This error occurs in both version.

However docstring for Future.set_result and Future.set_exception says:

Should only be used by Executor implementations and unit tests.

So maybe we should ignore this case?