homepage

When I did some tests involving unittest.mock.sentinel and multiprocessing.Queue, I noticed that multiprocessing.Queue changes the id of the sentinel.
This behaviour is definitely surprising and not documented.

See http://stackoverflow.com/a/925241/5069869
Apparently multiprocessing.Queue uses pickle to serialize the objects in the queue, which explains the change of identity, but is absolutely unclear from the documentation.

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?

My first thought was that Queue was implemented using shared memory.
I guess from the fact that the "Shared memory" section is separate in the multiprocessing documentation I should have known better, though.
So I guess some clarification in the documentation would be helpful.

Yeah, that's why I said "in the general case". Making it clear in the overview seems reasonable to me.

All communication between processes in multiprocessing has consistently used pickle to serialize the data being communicated (this includes what is described in the "Shared memory" section of the docs). The documentation has not done a great job of making this clear, instead only describing the requirement that data be pickleable in select places. For example, in the section on Queues:
 Note: When an object is put on a queue, the object is pickled and a
 background thread later flushes the pickled data to an underlying pipe.
Though it only applies to 3.6+, issue28053 still needs its own documentation improvement to make clear that the mechanism for communicating data defaults to serialization by pickle but that this can be replaced by alternatives.
I agree that the documentation around the use of pickle in multiprocessing deserves improvement.

There is a note mentioning pickle in this section: https://docs.python.org/3/library/multiprocessing.html#pipes-and-queues
It starts with "When an object is put on a queue, the object is pickled and..."
A comment about the object ids can be added there.

Mentioning ids would be pretty much redundant with mentioning pickle. If it is pickled its id is going to change. I think Davin was suggesting that while the use of serialization is documented, it is not documented *consistently*. Everywhere serialization happens it should be mentioned in the docs.
Regardless, a proposed doc PR is the way forward here.