[Python-Dev] Last chance! (original) (raw)

Christian Tismer tismer at tismer.com
Mon Dec 22 05:36:07 EST 2003

Tim Peters wrote:

[Tim]

Still, the possibility to switch threads across generator resumptions seems darned hard to view as "a feature". I'd advise people not to rely on it . [Christian Tismer] Well, this might only work for generators if it is guaranteed that the thread switch doesn't happen while the generator is executing. I think this will give an exception. But most probably the result will be hard to predict.

Explaining that: I was talking about freely using the generator in another thread, which would most likely resume it while it is active, and that's an exception.

I'm not sure I'm following, but nothing in Python now stops you from resuming a suspended generator from another thread. Here's an extreme example of that:

[XTreme example]

Then the body of the single generator is executed by 3 different threads, more or less randomly, although never by the thread that created the generator.

If the body of the generator were relying on thread-local state (like, e.g., the mass of settings in the decimal arithmetic package's per-thread default "numeric context" object), the result would sure be unpredictable. It wouldn't crash Python, it would just be a bitch to debug. Then again, lots of bad threading practices lead to bitch-to-debug code, so I'm not really inclined to offer extra protection here.

I agree, sure, cross-thread generators need to be designed for that.

[tasklets]

If a tasklet can be affected by thread-local state (like numeric context -- Python has little of this nature now, but it's bound to increase), then a tasklet will also start to care "who's in charge". So I expect a patch to reintroduce ftstate in about a year .

It is already there, not in the frames, but in the tasklet. A tasklet can be thread-bound or not. In that case, it keeps a reference to the thread state. But only once, since that information is not local for frames. Being thead bound is either enforced in the case that the tasklet is carrying non-trivial C state, otherwise it can be set as a user option (conservative default, of course). "Nomad threads" might be nice for some background monitoring.

This is all theory, I haven't tried any real threads with Stackless so far. But people are mixing tasklets with threads already, so I have to care somehow.

ciao - chris

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