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Thu Oct 1 10:58:59 CEST 2009 · https://mail.python.org/pipermail/python-dev/2009-October/092435.html

2009/9/30 Robert Kern <robert.kern at gmail.com>:
> I am blissfully unaware of the problems Paul mentioned about Windows GUI-mode programs.

:-)

> I'm not sure what would make a program "GUI-mode" or not. Certainly, I have written
> Python programs that use wxPython and PyQt on Windows that print to stdout/stderr,
> and they appear to work fine.

It's the difference between using python.exe and pythonw.exe. There's
a flag in the executable header saying whether the executable is
"console mode" or "gui mode".

GUI mode programs are run (by the OS) without a console (or if run
from a console prompt, they automatically detach from that console,
much like Unix programs which fork themselves to leave the terminal
group (did I get the terminology right?) but done by the OS). As a
result, the program has no valid stdin/out/err handles. Any attempt to
write to them causes the program to crash.

Traceback (most recent call last):
  File "hello.py", line 13, in <module>
    main()
  File "hello.py", line 7, in main
    sys.stdout.flush()
IOError: [Errno 9] Bad file descriptor

(Question - is it *ever* possible for a Unix program to have invalid
file descriptors 0,1 and 2? At startup - I'm assuming anyone who does
os.close(1) knows what they are doing!)

Paul.

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[Python-Dev] PEP 389: argparse - new command line parsing module