On Sat, Mar 31, 2012 at 3:33 AM, Michael Foord <fuzzyman@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 switching

  def __exit__(self, ...):

    sys.setswitchinterval(self._orig_switchinterval)

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.

-gps