[New-bugs-announce] [issue22630] `concurrent.futures.Future.set_running_or_notify_cancel` does not notify cancel

Ben Mather report at bugs.python.org
Tue Oct 14 11:05:54 CEST 2014

New submission from Ben Mather:

The documentation for the `set_running_or_notify_cancel` method on `concurrent.futures.Future` states that it will notify waiting threads and trigger callbacks after the `Future` has been cancelled, however currently this is done immediately by the call to `cancel`.

Oddly waiters (private interface used to implement `wait` and `as_completed`) do follow the behaviour documented for callbacks (they are called in `set_running_or_notify_cancel`) which means that they are not equivalent to setting a callback on each future.

I have attached three possible patches:
 1) "change-callbacks.patch" - this changes the behaviour to match the documentation.
 2) "change-docs.patch" - this fixes the documentation to match the current behaviour.
 3) "change-docs-and-waiters.patch" - in addition to fixing the documentation, this also fixes the inconsistency between waiters and callbacks by invoking waiters' `add_cancelled` method in `cancel`.

I believe moving to the documented behaviour (1) would be a mistake as currently `set_running_or_notify_cancel` and the `RUNNING` state can be skipped entirely allowing Futures to be used just as a way to send results.

Should mention that I have a separate patch (which I will submit separately (or here?)) that re-implements `wait` and `as_completed` using only publicly exposed methods.  This makes it possible to extend or replace `Future` without having to preserve its private interface.  Unfortunately this slightly breaks compatibility due to the difference between waiters and callbacks.  I thought it would be best to discuss this issue first as I don't believe the current behaviour is as intended.

assignee: docs at python
components: Documentation, Library (Lib)
files: change-callbacks.patch
keywords: patch
messages: 229282
nosy: bwhmather, docs at python
priority: normal
severity: normal
status: open
title: `concurrent.futures.Future.set_running_or_notify_cancel` does not notify cancel
versions: Python 3.2, Python 3.3, Python 3.4, Python 3.5
Added file: http://bugs.python.org/file36908/change-callbacks.patch

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