There are 18 public writable attributes in shlex object, and unthinking setting some of them can make shlex to produce incorrect results or create an infinite loop. For example there is nothing to prevent your from setting the eof attribute. Only 14 of 18 attributes is documented and posix is not in this set.

One of solutions is just does nothing. We are all consenting adults here.

More protective solution is to make undocumented attributes (filestack, posix, pushback, state) private. For backward compatibility we can temporary add properties that will emit deprecation warnings.