question about ctrl-d and atexit with threads

[Darren Dale]

On Mar 6, 1:32 pm, rdmur... at bitdance.com wrote:
> Darren Dale <dsdal... at gmail.com> wrote:
> >On Mar 5, 6:27 pm, "Gabriel Genellina" <gagsl-... at yahoo.com.ar> wrote:
> >> En Thu, 05 Mar 2009 15:26:18 -0200, Darren Dale <dsdal... at gmail.com>
> >> escribi :
>
> >> > On Mar 5, 12:02 pm, s... at pobox.com wrote:
> >> >> What happens if you simply call
>
> >> >>    my_thread.setDaemon(True)
>
> >> >> (or in Python 2.6):
>
> >> >>    my_thread.daemon = True
>
> >> >> ? That is the documented way to exit worker threads when you want the
> >> >> application to exit. From the threading module docs:
>
> >> >> "The entire Python program exits when no alive non-daemon threads
> >> >> are left."
>
> >> > Thank you Skip, that solves the problem. I'm still curious what the
> >> > difference is between python's handling of sys.exit and EOF, but its
> >> > academic at this point.
>
> >> Some applications open a new window for each document they're handling.
> >> When you close the last window, the application exits ("close" does an
> >> implicit "quit"). Note that this does *not* happen when you close the
> >> first, original document you opened, but when there are no more documents
> >> open. The first document is not special in this regard.
>
> >> Python threads work the same way; a thread may finish, but as long as
> >> there are other threads alive, the process continues running. Only after
> >> the last thread has finished, the application quits. The main thread *is*
> >> special sometimes, but not in this aspect,
> >> Setting daemon=True is like telling Python "I don't care about this
> >> thread; don't wait for it if that's the only thing you have to do".
>
> >> Calling sys.exit() is an explicit statement: "I want this program to
> >> finish now" (or as soon as possible). It doesn't wait for the remaining
> >> threads (unless you explicitely do so, like in your code).
>
> >Right, I understand all that. I don't understand how calling sys.exit
> >at the python command line is different from invoking ctrl-D. They
> >should both trigger the same mechanism if they are advertised as
> >equivalent mechanisms for exiting the interpreter, shouldnt they?
>
> First, just to make sure we are on the same page, I assume you
> understand that 'ctlr-D' at the python interpreter prompt is
> completely equivalent to your example file that does not call
> sys.exit before the end of the script file.  That is, ctrl-D is
> "end of file" for the 'script' you are creating at the
> interactive interpreter prompt.
>
> When Gabriel says "as long as there are other threads alive, the
> process continues running", that is the key to your question.
> The main thread has exited (either the interpreter or your main
> script file) but another thread is still running (your child
> thread), so Python keeps executing that thread.  sys.exit is
> _not_ called at the end of the "main" script file, but only after
> all threads have exited.
>
> Unless, that is, you call sys.exit explicitly, or set daemon = True.

OK, I understand now. Thank you both for the clarification, I learned
something important.