[GRASS-user] Python 3 Error using MultiModule and ParallelModuleQueue

[Anika Bettge]

Hello,


I try to write a GRASS addon, which does some preprocessing and
classifies with r.learn.ml on different region parallel. For this, I
want to use MultiModule and ParallelModuleQueue.

I tested the examples on
https://grass.osgeo.org/grass77/manuals/libpython/pygrass.modules.interface.html?highlight=parallelmodulequeue#pygrass.modules.interface.module.ParallelModuleQueue
with Python 3.

The example "Check MultiModule approach with three by two processes
running in a background process" leads to 

    ...

    Traceback (most recent call last):
      File "/usr/lib/python3.6/multiprocessing/queues.py", line 235, in
    _feed
        obj = _ForkingPickler.dumps(obj)
      File "/usr/lib/python3.6/multiprocessing/reduction.py", line 52,
    in dumps
        cls(buf, protocol).dump(obj)
    TypeError: can't pickle _thread.lock objects

    ...

The same error appears in the MultiModule example "Asynchronous module
run, setting finish = False and using temporary region".

Using python 2 all works fine.

Has someone a solution for the error or an idea how I can write a Grass
addon with parallelization on different regions?


Thanks and best regards,

Anika


P.S. I used this versions on Linux Mint 19 Tara:

python3 --version
Python 3.6.7

python --version
Python 2.7.15rc1

grass77 --config svn_revision version
74074M
7.7.svn


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