Hi,
I don't think it is currently possible (with os.chown
or any other Python library function) to change the
groupId of a file without specifying the userId (like
posix 'chgrp' does), or to change the userId without
specifying the groupId.
I would suggest adding the option of having os.chown
accept None as either the userId or groupId, and having
the function change only the 'non-None' portion of the
file ownership.

Thanks for your consideration.

-g

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the posix (os) module is intended as a thin wrapper around
operating system services.  The chown(2) syscall requires
that owner and group both be specified.

Possibly an enhanced chownonly/chgrp would find a home in
the 'shutil' module.

Possibly translating a parameter of None into -1 (which
means 'no change') would be accepted as a patch.

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Jeff already provided the right analysis: It *is* currently
possibly with os.chown to only change the groupid without
specifying the userid: just pass -1 as the userid. The
requested feature is already there, so I'm rejecting this
request.

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I've create patch #1213031 which includes a note in the docs
about this.