[docs] [issue12760] Add create mode to open()

[Charles-François Natali]

Charles-François Natali <neologix at free.fr> added the comment:

> Yeah, but I think "exclusively" is quite misleading since it does not
> perform any locking of any kind.

It might be misleading, but I find it clear enough, and this name has been endorsed by POSIX.

Furthermore, there's an added bonus: actually, with the old I/O layer, one can already pass an 'x' flag to open, since it just calls fopen:
"""
cf at neobox:~$ strace -e open python -c "open('/tmp/foo', 'wx')"
[...]
open("/tmp/foo", O_WRONLY|O_CREAT|O_EXCL|O_TRUNC|O_LARGEFILE, 0666) = 3
cf at neobox:~$ strace -e open python -c "open('/tmp/foo', 'wx')"
[...]
open("/usr/lib/pymodules/python2.6/<string>", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)
IOError: [Errno 17] File exists: '/tmp/foo'
"""

I don't know if it's documented behavior, but the OP in issue 12105 was using it with python 2.
Changing it to 'x' would make such code backward-compatible.

Finally, when I read open('/tmp/foo', 'wx'), it's immediately clear to me what's going on, while I'd have to look at open()'s documentation to find out what the 'c' flag does.

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<http://bugs.python.org/issue12760>
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