pythonic malloc

[kjockey]

I have some simple UDP code that does:
s.sendto("\xf0\x00\x02\x00rachel\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00",addr)

This is OK, except that I'd like to change the text in the middle ("rachel",
which is the hostname FWIW), and the NUL padding needs to go out to make
the length 25 bytes (so the padding depends on the length of the name).

So I could do something like:
retpacket = "\xf0\x00\x02\x00"+socket.gethostname()
while len(retpacket) < 26:
     retpacket += "\x00"
s.sendto(retpacket, addr)

But that feels "wrong". And Python in a Nutshell says its a bad idea  -
"anti-idiom".

Can anyone show me the true path?

Brad