Let me be more precise: 

My suggestion is not to remove `unconsumed_tail` entirely, but I think its value needs to be defined only when the end of the compressed stream has been reached.

In other words, you could still do:

while not decomp.eof
  # ...
if decomp.unconsumed_tail:
    raise RuntimeError('unexpected data after end of compressed stream')

but as long as decomp.eof is True, decomp.unconsumed_tail could (as far as I can tell) be None, no matter if there is uncompressed data in the internal buffer or not.