Why not use pickle protocol 4 by default? It allows to pickle some objects which are not pickleable with lower protocols and is more efficient.

Because pickles will not be compatible with Python < 3.4 anymore. People who want maximum efficiency without compatibility can pass the right protocol manually.

Multiprocessing uses default protocol and there is no simple (without hacking the stdlib) way to pass the right protocol manually.

> Multiprocessing uses default protocol

AFAIK, this is so you can interact with processes using another version of Python.

multiprocessing spawns the other processes itself from the same executable used to launch the main process. It's not subprocess. How would a different version of Python get involved?

It is possible to have independent processes communicate together, although that's not the most widely-used feature. See:
https://docs.python.org/3/library/multiprocessing.html#multiprocessing-listeners-clients
Also:
https://docs.python.org/3/library/multiprocessing.html#managers

Can't we "negociate" the protocol automatically?

Closing in deference to the enhancement described in issue28053.

Now that Python 3.3 is dead and 3.4 is soon to follow, it's safe to bump DEFAULT_PROTOCOL to 4.

Should we bump the pickle protocol for shelve?

New changeset c51d8c9ba6211d77db639487501f89aa9b4bcbb1 by Łukasz Langa in branch 'master':
bpo-23403: Bump pickle.DEFAULT_PROTOCOL to 4 (#6355)
https://github.com/python/cpython/commit/c51d8c9ba6211d77db639487501f89aa9b4bcbb1

Is there something left to be done here?

Shelve still uses protocol 3 by default. Should it be bumped too?

> Shelve still uses protocol 3 by default. Should it be bumped too?

That sounds reasonable.  Perhaps open a separate issue for it?

Opened issue34204 for shelve.