Further comments...

After looking at the C code, a few things became clear; I need to read more about C/Python interfacing, and the "unused_data" attribute will only contain data if additional data is fed to a de-compressor at the end of a complete compressed stream.

The purpose of the "unused_data" attribute is not clear in the documentation, so that should probably be clarified (mind you, I am looking at pre-2.0 docs so maybe it already has?).

The failure to produce all data up to a sync-flush is something else... I'm still looking into it. I'm not sure if it is an inherent limitation of zlib, something that needs to be fixed in zlib, or something that needs to be fixed in the python interface. If it is an inherent limitation, I'd like to characterise it a bit better before documenting it. If it is something that needs to be fixed in either zlib or the python interface, I'd like to fix it.

Unfortunately, this is a bit beyond me at the moment, mainly in time, but also a bit in skill (need to read the python/C interfacing documentation). Maybe over the christmas holidays I'll get a chance to fix it.