I don’t see any violation or anything needing fixing in the documentation or code. Can you elaborate?

The fcntl() documentation says “The values used for ‘cmd’ [F_DUPFD, etc] are operating system dependent, and are available as constants in the ‘fcntl’ module, using the same names as used in the relevant C header files.” The newest version of Posix specifies both the F_DUPFD and F_DUPFD_CLOEXEC. It would be misleading for Python to use one when the user specified the other.

If you want a best-effort version of F_DUPFD_CLOEXEC for a single-threaded program, why not use os.dup()? I presume this uses F_DUPFD_CLOEXEC where appropriate, and uses some non-atomic fallback if necessary.