bpo-22005: Document the reality of pickle compatibility. by gpshead · Pull Request #11054 · python/cpython (original) (raw)
People still seem surprised that pickle can't magically send bytes/str/unicode values back and forth from Python 2 and Python 3 processes without a lot of hand holding despite the underlying fundamental types of the language changing. This should clarify that there is no such free lunch.
It has always has been, impossible to guarantee such compatibility. That the docs were not updated to reflect this in 3.0 was an oversight.