import zlib import zlib_into a = b'abcde' * 5000 b = bytearray(5000) res = zlib_into.compress_into(a, b, level=9) print("res", res) print(b[:res]) print("Roundtrip", zlib.decompress(b[:res]) == a) c = bytearray(25001) res2 = zlib_into.decompress_into(b[:res], c) print(f"Decompressed {res2} bytes") print("Roundtrip within zlib_into", bytes(c[:res2]) == a)