[Python-ideas] Thread stopping

Gregory P. Smith greg at krypto.org
Sun Apr 1 05:50:17 CEST 2012

On Sat, Mar 31, 2012 at 3:33 AM, Michael Foord <fuzzyman at gmail.com> wrote:

> An "uninterruptable context manager" would be nice - but would probably
> need extra vm support and isn't essential.

I'm not so sure that would need much vm support for an uninterruptable
context manager, at least in CPython 3.2 and 3.3:

Isn't something like this about it; assuming you were only talking about
uninteruptable within the context of native Python code rather than
whatever other extension modules or interpreter embedding code may be
running on their own in C/C++/Java/C# thread land:

class UninterruptableContext:
  def __enter__(self, ...):
    self._orig_switchinterval = sys.getswitchinterval()
    sys.setswitchinterval(1000000000)   # 31 years with no Python thread

  def __exit__(self, ...):

the danger with that of course is that you could be saving an obsolete
switch interval value.  but I suspect it is rare to change that other than
near process start time. you could document the caveat and suggest that the
switch interval be set to its desired setting before using any of these
context managers.  or monkeypatch setswitchinterval out with a dummy when
this library is imported so that it becomes the sole user and owner of that
api.  all of which are pretty evil-hacks to expunge from ones memory and
pretend you didn't read.

the _other_ big caveat to the above is that if you do any blocking
operations that release the GIL within such a context manager I think you
just voluntarily give up your right to not be interrupted.  Plus it depends
on setswitchinterval() which is an API that we could easily discard in the
future with different threading and GIL implementations.

brainstorming... its what python-ideas is for.

I have zero use cases for the uninterruptable context manager within
Python.  for tiny sections of C code, sure. Within a high level language...
not so much.  Please use finer grained locks.  An uninterruptible context
manager is essentially a context manager around the GIL.

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