I have had a look at this in more detail (Python C interfacing is actually pretty easy :-).

I dunno whether to go into details here, but I have noticed that inflate is being called with Z_NO_FLUSH when the zlib docs suggest Z_SYNC_FLUSH or Z_FINISH. However, the zlib implementation only treats Z_FINISH differently, so this should not make a difference (but it might in future versions of zlib).

As you have said, .unused_data will only contain data if something is appended to a complete compressed object in the stream fed to a decompressor. Perhaps the docs need something to clarify this, perhaps in the form of an example?

I am going to write some test progs to see if it's in zlib. At this stage I suspect that it is, and I'm hoping over christmas to get a patch done.