Documentation would be appreciated. I have a project that uses BaseManager, Client, and Listener to create some servers and clients. I would like to update the project to work with Python 3 and would prefer to update the clients and the servers separately (i.e. switch the client to Python 3 while the server is run with Python 2.7). However, BaseManager uses connection.Client which uses connection._ConnectionBase which uses reduction.ForkingPickler without a protocol argument. It seems the default protocol is 3 on Python 3.6 and 2 on Python 2.7 (contrary to the comment above about v2 being used). I just want to set the protocol version to 2 in Python 3.6. Can I do that with the changes added by this patch?

I tried creating pickle2reducer.py like this:

from multiprocessing.reduction import ForkingPickler, AbstractReducer

class ForkingPickler2(ForkingPickler):
    def __init__(self, *args):
        if len(args) > 1:
            args[1] = 2
        else:
            args.append(2)
        super().__init__(*args)

    @classmethod
    def dumps(cls, obj, protocol=2):
        return ForkingPickler.dumps(obj, protocol)


def dump(obj, file, protocol=2):
    ForkingPickler2(file, protocol).dump(obj)


class Pickle2Reducer(AbstractReducer):
    ForkingPickler = ForkingPickler2
    register = ForkingPickler2.register
    dump = dump

and then putting

import pickle2reducer
multiprocessing.reducer = pickle2reducer.Pickle2Reducer()

at the top of my module before

import multiprocessing.connection

but I still see "ValueError: unsupported pickle protocol: 3" on the server when I connect with a Python 3.6 client.