That fact that this is so is implicit in the name multi*process*ing and
the documented restrictions of the id function.  That is, it is the purpose of the module is to manage computation across multiple processes.  Since different processes have distinct memory spaces, you cannot depend on object identity between processes, by the definition of object identity (it is constant only for the lifetime of the object in memory, and the different processes have different memory spaces, therefore the object id may be different in the different processes).  By construction this applies also to any multiprocessing mechanism that is used to transmit objects, even if the transmission turns out to be to the same process in a particular case.  You can't *depend* on the id in that case, because the transmission mechanism must be free to change the object identity in order to work in the general case.

Should we document this explicitly?  Perhaps so.  Maybe in the multiprocessing introduction?