>I think they at least should be supported on Linux then. 

And what happens if some Unix flavor (i.e. Solaris) adds new flag, say
O_NONLINUXSYNC and it has the same value as linux's O_ASYNC? And then
FreeBSD adds O_BSDSYNC flag with the same number and so on. Then Python
will have to have separate system-specific module for each *nix system
that uses Python. Why do you think Python maintainers would want that?

>It does work if you use the value itself anyway.

It's the way fcntl works -- it's just a thin layer on top of fcntl() and
ioctl() calls. But that's not a good reason for including non-posix
flags to 'posix' module