[New-bugs-announce] [issue22729] `wait` and `as_completed` depend on private api

[Ben Mather]

New submission from Ben Mather:

Adds `remove_done_callback` to `Future` object, removes `_waiters` attribute, and re-implements `wait` and `as_completed` using callbacks.

This makes it possible to extend or replace `Future` without having to mimic its private `_waiters` interface.

Currently waiters and callbacks are triggered at different points after a cancel (waiters in `set_condition_and_notify_cancel` and callbacks in `cancel`.)  This is a problem as it means that implementing `wait` and `as_completed` using `add_done_callback` will result in a behaviour change unless the behaviour of `add_done_callback` is changed instead. 

I don't believe the current behaviour is as documented anyway so I'm not sure if this is a problem.  See issue 22630.

I am a little uncertain about the O(n) implementation of `remove_done_callback` but can't imagine a situation where it would be a problem.

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components: Library (Lib)
files: non-magic-waiters.patch
keywords: patch
messages: 230016
nosy: bwhmather
priority: normal
severity: normal
status: open
title: `wait` and `as_completed` depend on private api
type: behavior
versions: Python 3.4, Python 3.5, Python 3.6
Added file: http://bugs.python.org/file37016/non-magic-waiters.patch

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue22729>
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