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Well, yes and no.
I agree that it is annoying to have to add another parameter to get a 
returning tuple. But you have to weigh that against adding another 
conveinence API.
On the other hand, I think the offset argument is quite convenient. If 
we were to add a "decompress2" function, I would like to keep the 
offset because it removes the extra overhead of having to create a 
temporary buffer object, at no extra cost.
Also, note that your example code becomes more complex, for a loop. 
You would need to:
results = []
used =0
while used<len(source):
 r, u = zlib.decompress2(buffer(source, used))
 used += u
 results.append(r)
as opposed to:
 r, used = zlib.decompress2(source, used)
 results.append(r)
You have to make sure you add the buffer's hidden offset to the output 
offset.
There is a third option. The "offset" could be a list containg the 
offset in its 0th position, a sort of "byref" passing, but that is not 
in the general Python spirit, is it?
This would give us the loop:
o = [0]
while len(source)>o[0]
 results.append(zlib.decompress(source, offset=offsetlist))