[Python-ideas] Asynchronous exception handling around with/try statement borders

[Chris Angelico]

On Tue, Sep 25, 2018 at 1:10 AM Erik Bray <erik.m.bray at gmail.com> wrote:
>
> On Fri, Sep 21, 2018 at 12:58 AM Chris Angelico <rosuav at gmail.com> wrote:
> >
> > On Fri, Sep 21, 2018 at 8:52 AM Kyle Lahnakoski <klahnakoski at mozilla.com> wrote:
> > > Since the java.lang.Thread.stop() "debacle", it has been obvious that
> > > stopping code to run other code has been dangerous.  KeyboardInterrupt
> > > (any interrupt really) is dangerous. Now, we can probably code a
> > > solution, but how about we remove the danger:
> > >
> > > I suggest we remove interrupts from Python, and make them act more like
> > > java.lang.Thread.interrupt(); setting a thread local bit to indicate an
> > > interrupt has occurred.  Then we can write explicit code to check for
> > > that bit, and raise an exception in a safe place if we wish.  This can
> > > be done with Python code, or convenient places in Python's C source
> > > itself.  I imagine it would be easier to whitelist where interrupts can
> > > raise exceptions, rather than blacklisting where they should not.
> >
> > The time machine strikes again!
> >
> > https://docs.python.org/3/c-api/exceptions.html#signal-handling
>
> Although my original post did not explicitly mention
> PyErr_CheckSignals() and friends, it had already taken that into
> account and it is not a silver bullet, at least w.r.t. the exact issue
> I raised, which had to do with the behavior of context managers versus
> the
>
> setup()
> try:
>     do_thing()
> finally:
>     cleanup()
>
> pattern, and the question of how signals are handled between Python
> interpreter opcodes.  There is a still-open bug on the issue tracker
> discussing the exact issue in greater details:
> https://bugs.python.org/issue29988

To be fair, your post not only didn't mention CheckSignals, but it
almost perfectly described its behaviour. So I stand by my response.
:) I don't think the system needs to be replaced; it ought to be
possible to resolve the context manager issue without tossing out the
existing code.

ChrisA