I have a build_bot running (yeah me!), and was surprised to see test_zlib fail on AIX. There is not an issue with test_zlib, but I do have a suggestion. I was getting an error with test_flushes(). On python2-2.7.15 the test passes and in python3-3.8 (and earlier I expect) the test fails. The difference is the addition of the two modes 'Z_PARTIAL_FLUSH' and 'Z_BLOCK' sync_opt = ['Z_NO_FLUSH', 'Z_SYNC_FLUSH', 'Z_FULL_FLUSH', 'Z_PARTIAL_FLUSH', 'Z_BLOCK'] On default AIX installs zlib is, sadly, still at version 1.2.3 and Z_BLOCK was 'improved' in version 1.2.5. And it fails on Z_BLOCK. My suggestion is to enhance the test_library_version() so that it verifies that the minor (or smaller?) number is at least '5' When I static link python with zlib-1.2.11 then the test_zlib passes all sub-tests. Again, not a python bug - but a suggestion for improving what is tested. I can open an issue, and with a bit of assistance/interest from others I'll even do a PR. Michael