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You both still fail to convince me and I still don't see
need for action. The only case ATM where this addition makes
sense (in your opinion) is the Windows OS when using the
NTFS filesystem and certain conditions are met. NTFS has a
preallocation algorithm to deal with this. We don't know if
there is any advantage on FAT filesystems.

On Linux for example there is a plethora of supported
filesystems. Some of them may take advantage, others may
not. Who knows? We can't even detect which filesystem type
we are currently writing to. Apart from that, the behaviour
of truncate(arg) with arg > filesize seems to be
system-dependent.

So, IMO this is a very special optimization targeted at a
single platform. The TarFile class is easily subclassable,
just override the makefile() method and add the two lines of
code. I think that's what ActiveState's Python Cookbook is for.

BTW, I like my files to grow bit by bit. In case of an
error, I can detect if a file was not extracted completely
by comparing the file sizes. Furthermore, a file that grows
is more common and more what a programmer who uses this
module might expect.