New submission from Andrew Svetlov <andrew.svet...@gmail.com>:
Suppose multiple `task.cancel(msg)` with different messages are called on the same event loop iteration. What message (`cancel_exc.args[0]`) should be sent on the next loop iteration? As of Python 3.10 it is the message from the *last* `task.cancel(msg)` call. The main branch changed it to the *first* message (it is a subject for discussion still). Both choices are equally bad. The order of tasks execution at the same loop iteration is weak and depends on very many circumstances. Effectively, first-cancelled-message and last-cancelled-message are equal to random-message. This makes use of cancellation message not robust: a task can be cancelled by many sources, task-groups adds even more mess. Guido van Rossum suggested that messages should be collected in a list and raised altogether. There is a possibility to do it in a backward-compatible way: construct the exception as `CancelledError(last_msg, tuple(msg_list))`. args[0] is args[1][-1]. Weird but works. `.cancel()` should add `None` to the list of cancelled messages. The message list should be cleared when a new CancelledError is constructed and thrown into cancelling task. Working with exc.args[0] / exc.args[1] is tedious and error-prone. I propose adding `exc.msgs` property. Not sure if the last added message is worth a separate attribute, a robust code should not rely on messages order as I described above. The single message is not very useful but a list of messages can be used in timeouts implementation as cancellation count alternative. I don't have a strong preference now but want to carefully discuss possible opportunities before making the final decision. ---------- components: asyncio messages: 413749 nosy: ajoino, alex.gronholm, asvetlov, chris.jerdonek, dreamsorcerer, gvanrossum, iritkatriel, jab, njs, tinchester, yselivanov priority: normal severity: normal status: open title: Confusing CancelError message if multiple cancellations are scheduled versions: Python 3.11 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue46829> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
