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Date: Fri, 7 Aug 2015 10:46:03 +1000
Subject: Re: except block isn't catching exception
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Fri, Aug 7, 2015 at 10:34 AM,  <sohcahtoa82@gmail.com> wrote:
> Despite my "except KeyboardInterrupt", the KeyboardInterrupt forced by th=
e thread.interrupt_main() in the worker thread isn't being caught.
>
> Other things worth noting is that the exception takes about 3 seconds aft=
er the call to thread.interrupt_main().  It appears to not actually happen =
until the sock.accept() times out.  If the call to sock.settimeout(5) is re=
moved, the KeyboardInterrupt never appears and flow is still blocked on the=
 sock.accept().
>
> My Python version is 2.7.3.  I know its out of date, but I don't really h=
ave a choice.

As far as I know, the only effect of thread.interrupt_main() is to set
a flag saying "Hey main thread, next time you go checking for these
things, there's a KeyboardInterrupt waiting for you". It doesn't
actually send an OS-level signal, which means it cannot interrupt a
blocking call. So you have a few choices:

1) Shorten the timeout on sock.accept() to an (if you'll excuse the
pun) acceptable delay, and then simply check a flag. Something like
this:

interrupt =3D False
while not interrupt:
    try:
        connection, _ =3D sock.accept()
    except socket.timeout:
        # On timeout, check flag and keep sleeping
        pass

If you wish to also have an actual timeout, you could monitor that separate=
ly.

2) Instead of blocking on sock.accept directly, have another event
which the other thread can raise, which will wake the main thread. You
could use select.select() for this.

3) Bury the details of select.select() away behind a nice framework
like asyncio, merge your two threads, and run everything through a
back-end event loop.

4) Use separate processes, and signal the interrupt using an OS-level
notification (on Unix systems, SIGINT; on Windows, there's an
equivalent called BreakSignal or something). This will break out of
the underlying system call that handles accept().

thread.interrupt_main() is similar to just setting a global that you
periodically check, except that the periodic check is handled for you
by the system. That's really all.

ChrisA