Author Martin.Teichmann
Recipients Martin.Teichmann
Date 2018-05-02.18:39:17
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asyncio.gather() returns a _GatheringFuture, which inherits from asyncio.Future. This is weird in current asyncio, as futures are supposed to be created with loop.create_future(). So I tried to reimplement gather() without this weird special future. I succeeded, yet I stumbled over weird inconsistencies with cancellation. There are three cases:

- coroutines have no special notion of cancellation, they treat CancelledError as any other exception

- futures have a clear distinction between exceptions and cancellation: future.set_exception(CancelledError()) is different from future.cancel(), as only for the latter future.cancelled() is True. This is used in the _GatheringFuture: it is cancelled() only if it got cancelled via future.cancel(), if its children gets cancelled it may set_exception(CancelledError()), but it will not be cancelled itself.

- Tasks consider raising a CancelledError always as a cancellation, whether it actually got cancelled or the wrapped coroutine raised CancelledError for whatever other reason. There is one exception: if the coroutine manages to return immediately after being cancelled, it raises a CancelledError, but task.cancelled() is false. So if a coroutine ends in


the current task raises a CancelledError, but task.cancelled() is false.

I consider the last exception actually a bug, but it allows me to make my inheritance-free gather() look to the outside exactly like it used to be.
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