Example of failure to pickle:

>>> from gi.repository import GLib
>>> import pickle
>>> pickle.dumps(GLib.Error("Example"))
Traceback (most recent call last):

+ Trace 238226

Thanks. Can you provide a small example showing the problem with subprocess and queue?

Created attachment 365030 [details]
Example showing my attempt to return an exception from a 'multiprocessing' subprocess via a queue

Sure, here's an example -- slightly contrived but shows a scenario where I would want to be able to pass a GLib.Error from subprocess to a main process, and demonstrates that it doesn't work.

It's contrived in part because the main process doesn't try to handle the exception, so it could be "fixed" by just printing the traceback in the subprocess. But there are many cases where you want to do more with the exception in the main process than to just re-raise it, and none of that is easily possible if we can't get the exception into a Queue object.

Thanks, to summarize:

* Using multiprocessing.Queue one can return exception instances to the parent process.
* Normal Python exceptions are picklable
* GLib.Error is an "exception" and is not picklable

Sounds reasonable.

----

import multiprocessing
from gi.repository import GLib

def add_queue(e):
    q = multiprocessing.Queue()
    q.put(e)
    print(repr(q.get()))

try:
    raise ValueError("foo")
except Exception as e:
    add_queue(e)

try:
    GLib.file_get_contents("")
except Exception as e:
    add_queue(e)

Thanks for summarizing a bit more clearly than I could :-)

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