>> os.open("/mnt/dozer", os.O_DIRECTORY)
3
>>> os.open("/dev/fd/3", os.O_DIRECTORY)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NotADirectoryError: [Errno 20] Not a directory: '/dev/fd/3'
Manpage states:
     Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR	are
     ignored.
Ideally, it would be nice to have O_DIRECTORY support.

The reason for this preference is that there are various places where Samba needs to be able to convert descriptors opened with O_PATH to regular opens, and it perform these opens with O_DIRECTORY if it is expected to be a directory.