[Python-Dev] Last chance!
tim.one at comcast.net
Sun Dec 21 21:52:48 EST 2003
>> 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 <wink>.
> 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.
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:
next = myid().next
mut = threading.Lock()
for i in range(3):
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
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.
> Other for my "tasklet" tiny threads. Their first purpose is
> to have concurrent small tasks without interaction (or if
> with interaction, then via channels and blocking). A tasklet
> being interrupted by a thread switch will not be auto-schedulable
> until the thread is resumed, but other tasklets might be, and
> they don't need to care about which real thread is actually in
> charge. This is one of my reasons why I want to get rid of
> f_tstate -- it does not really apply to most of my frames.
> Being tied to a thread is a property of the tasklet, and it
> does not apply all the time, only when I have to keep C state.
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 f_tstate in about a year <wink>.
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