[Stackless] comments on PEP 219
gmcm at hypernet.com
Tue Mar 13 21:02:14 CET 2001
Can we please get the followups under control? Bernd sent
me a private email. I replied privately. Then he forwarded to
Stackless. So I forwarded my reply to Stackless. Now Jeremy
adds python-dev to the mix.
> >>>>> "BR" == Bernd Rinn <Bernd.Rinn at epost.de> writes:
> BR> On Tue, Mar 13, 2001 at 12:17:39PM -0500, Gordon McMillan
> wrote: >> The one instance I can find on the Stackless list (of
> attempting >> to use a continuation across interpreter
> invocations) was a call >> the uthread.wait() in __init__.
> Arguably a (minor) nuisance, >> arguably bad coding practice
> (even if it worked).
> [explanation of code practice that lead to error omitted]
> BR> So I suspect that you might end up with a rule of thumb:
> BR> """ Don't use classes and libraries that use classes when
> doing BR> IO in microthreaded programs! """
> BR> which might indeed be a problem. Am I overlooking something
> BR> fundamental here?
Synopsis of my reply: this is more a problem with uthreads
than coroutines. In any (real) thread, you're limited to dealing
with one non-blocking IO technique (eg, select) without going
into a busy loop. If you're dedicating a (real) thread to select, it
makes more sense to use coroutines than uthreads.
> A few other variations on the question come to mind:
> If a programmer uses a library implement via coroutines, can
> she call library methods from an __xxx__ method?
Certain situations won't work, but you knew that.
> Can coroutines or microthreads co-exist with callbacks
> invoked by C extensions?
Again, in certain situations it won't work. Again, you knew that.
> Can a program do any microthread IO in an __call__ method?
Considering you know the answer to that one too, you could've
phrased it as a parsable question.
> If any of these are the sort "in theory" problems that the PEP
> alludes to, then we need a full spec for what is and is not
> allowed. It doesn't make sense to tell programmers to follow
> unspecified "reasonable" programming practices.
That's easy. In a nested invocation of the Python interpreter,
you can't use a coroutine created in an outer interpreter.
In the Python 2 documentation, there are 6 caveats listed in
the thread module. That's a couple order of magnitudes
different from the actual number of ways you can screw up
using the thread module.
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