Is there a character that never appears in the output of zlib.compress?

[Cameron Simpson]

On 28Jan2020 23:09, Peng Yu <pengyu.ut at gmail.com> wrote:
>I'd like to tell what part is zlib.compress data in an input stream.
>One way is to use some characters that never appear in zlib.compress
>output to denote the boundary. Are there such characters? Thanks.

If you mean: is there a byte which never appears, then apparently not:

  [~]fleet*1> python3 testzlib.py
  ........

where testzlib.py contains this code:

from random import randint
import sys
from zlib import compress

unseen = set(range(256))

while unseen:
  sys.stdout.write('.')
  sys.stdout.flush()
  block = bytes(randint(0,255) for _ in range(256))
  cdata = compress(block)
  for c in cdata:
    unseen.discard(c)

sys.stdout.write('\n')