Want to build a binary header block

[Bob Greschke]

This is the idea

   Block = pack("240s", "")
   Block[0:4] = pack(">H", W)
   Block[4:8] = pack(">H", X)
   Block[8:12] = pack(">B", Y)
   Block[12:16] = pack(">H", Z))

but, of course, Block, a str, can't be sliced.  The real use of this is a 
bit more complicated such that I can't just fill in all of the "fields" 
within Block in one giant pack().  I need to build it on the fly.  For 
example the pack(">H", X) may be completely skipped some times through the 
function.  There are many more fields in Block than in this example and they 
are all kinds of types not just H's and B's.  What I really want is a C 
struct union. :)

How would I do this?

Thanks!

Bob