`numpy` arrays cannot be serialized when used in `pmap` context

[ma-sadeghi]

Affects: JuliaCall

Describe the bug
I'm trying to call a Julia function (from Python) that returns a vector of objects generated via pmap on multiple workers. (Not sure if the pmap is even relevant, just in case)

Reproduce the bug

import numpy as np
from juliacall import Main as jl

script = """using Distributed
# a = Array(a)
n = 2
_workers = WorkerPool(addprocs(n))
results = pmap(_ -> sum(a), _workers, 1:n)
println(results)
"""
script = ";".join([s for s in script.split("\n") if not s.startswith("#")])


def fn_parallel():
    jl.a = np.random.rand(5)
    jl.seval(script)


fn_parallel()

More context
The issue is related to passing numpy arrays (no error when passing say a = 1). If the input is explicitly converted to Array, it runs fine (uncomment # a = Array). See #454 for the original issue.

Error message

Python: TypeError: cannot pickle 'PyCapsule' object   core.py:432
Python stacktrace: none                                                                     
Stacktrace:                                                                                 
[1] pythrow()                                                                             
    @ PythonCall ~/.julia/packages/PythonCall/wXfah/src/err.jl:94                           
[2] errcheck                                                                              
    @ ~/.julia/packages/PythonCall/wXfah/src/err.jl:10 [inlined]                            
[3] pycallargs                                                                            
    @ ~/.julia/packages/PythonCall/wXfah/src/abstract/object.jl:210 [inlined]               
[4] pycall(f::Py, args::Py; kwargs::@Kwargs{})                                            
    @ PythonCall ~/.julia/packages/PythonCall/wXfah/src/abstract/object.jl:228              
[5] pycall                                                                                
    @ ~/.julia/packages/PythonCall/wXfah/src/abstract/object.jl:218 [inlined]               
[6] Py                                                                                    
    @ ~/.julia/packages/PythonCall/wXfah/src/Py.jl:341 [inlined]                            
[7] serialize_py(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Py)              
    @ PythonCall                                                                            
~/.julia/packages/PythonCall/wXfah/src/compat/serialization.jl:9                            
[8] serialize(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Py)                 
    @ PythonCall                                                                            
~/.julia/packages/PythonCall/wXfah/src/compat/serialization.jl:25                           
[9] serialize_any(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)            
    @ Serialization                                                                         
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Serializa            
tion/src/Serialization.jl:676                                                               
[10] serialize(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)                
    @ Serialization                                                                         
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Serializa            
tion/src/Serialization.jl:655                                                               
[11] serialize_any(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)            
    @ Serialization                                                                         
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Serializa            
tion/src/Serialization.jl:676                                                               
[12] serialize(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)                
    @ Serialization                                                                         
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Serializa            
tion/src/Serialization.jl:655                                                               
[13] serialize_msg(s::Distributed.ClusterSerializer{Sockets.TCPSocket},                    
o::Distributed.CallMsg{:call_fetch})                                                        
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/messages.jl:78                                                                       
[14] #invokelatest#2                                                                       
    @ ./essentials.jl:887 [inlined]                                                         
[15] invokelatest                                                                          
    @ ./essentials.jl:884 [inlined]                                                         
[16] send_msg_(w::Distributed.Worker, header::Distributed.MsgHeader,                       
msg::Distributed.CallMsg{:call_fetch}, now::Bool)                                           
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/messages.jl:181                                                                      
[17] send_msg                                                                              
    @                                                                                       
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/messages.jl:122 [inlined]                                                            
[18] remotecall_fetch(f::Function, w::Distributed.Worker, args::Int64;                     
kwargs::@Kwargs{})                                                                          
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/remotecall.jl:460                                                                    
[19] remotecall_fetch(f::Function, w::Distributed.Worker, args::Int64)                     
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/remotecall.jl:454                                                                    
[20] remotecall_fetch(f::Function, id::Int64, args::Int64)                                 
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/remotecall.jl:492                                                                    
[21] remotecall_pool(rc_f::Function, f::Function, pool::Distributed.WorkerPool,            
args::Int64; kwargs::@Kwargs{})                                                             
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/workerpool.jl:126                                                                    
[22] remotecall_pool                                                                       
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/workerpool.jl:123 [inlined]                                                          
[23] remotecall_fetch                                                                      
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/workerpool.jl:232 [inlined]                                                          
[24] #208                                                                                  
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/workerpool.jl:288 [inlined]                                                          
[25]                                                                                       
(::Base.var"#1023#1028"{Distributed.var"#208#210"{Distributed.var"#208#209#211"{            
Distributed.WorkerPool, EquivalentCircuits.var"#177#178"{Int64, Int64, String,              
Int64, Float64, Nothing, Float64, Nothing, PyArray{ComplexF64, 1, true, true,               
ComplexF64}, PyArray{Float64, 1, true, true,                                                
Float64}}}}})(r::Base.RefValue{Any}, args::Tuple{Int64})                                    
    @ Base ./asyncmap.jl:94                                                                 
[26]                                                                                       
(::Base.var"#1039#1040"{Base.var"#1023#1028"{Distributed.var"#208#210"{Distribut            
ed.var"#208#209#211"{Distributed.WorkerPool,                                                
EquivalentCircuits.var"#177#178"{Int64, Int64, String, Int64, Float64, Nothing,             
Float64, Nothing, PyArray{ComplexF64, 1, true, true, ComplexF64},                           
PyArray{Float64, 1, true, true, Float64}}}}}, Channel{Any}, Nothing})()                     
    @ Base ./asyncmap.jl:228                                                                
Stacktrace:                                                                                 
[1] (::Base.var"#1033#1035")(x::Task)                                                     
    @ Base ./asyncmap.jl:171                                                                
[2] foreach(f::Base.var"#1033#1035", itr::Vector{Any})                                    
    @ Base ./abstractarray.jl:3094                                                          
[3] maptwice(wrapped_f::Function, chnl::Channel{Any},                                     
worker_tasks::Vector{Any}, c::UnitRange{Int64})                                             
    @ Base ./asyncmap.jl:171                                                                
[4] wrap_n_exec_twice                                                                     
    @ ./asyncmap.jl:147 [inlined]                                                           
[5] #async_usemap#1018                                                                    
    @ ./asyncmap.jl:97 [inlined]                                                            
[6] kwcall(::NamedTuple, ::typeof(Base.async_usemap), f::Any, c::Vararg{Any})             
    @ Base ./asyncmap.jl:78 [inlined]                                                       
[7] #asyncmap#1017                                                                        
    @ ./asyncmap.jl:75 [inlined]                                                            
[8] asyncmap                                                                              
    @ ./asyncmap.jl:74 [inlined]                                                            
[9] pmap(f::Function, p::Distributed.WorkerPool, c::UnitRange{Int64};                     
distributed::Bool, batch_size::Int64, on_error::Nothing,                                    
retry_delays::Vector{Any}, retry_check::Nothing)                                            
    @ Distributed                                                                           
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/pmap.jl:126                                                                          
[10] pmap                                                                                  
    @                                                                                       
~/.julia/juliaup/julia-1.10.0+0.x64.linux.gnu/share/julia/stdlib/v1.10/Distribut            
ed/src/pmap.jl:99 [inlined]                                                                 
[11] circuit_evolution_batch(measurements::PyArray{ComplexF64, 1, true, true,              
ComplexF64}, frequencies::PyArray{Float64, 1, true, true, Float64};                         
generations::Int64, population_size::Int64, terminals::String, head::Int64,                 
cutoff::Float64, initial_population::Nothing, convergence_threshold::Float64,               
bounds::Nothing, numprocs::Int64, iters::Int64, quiet::Bool)                                
    @ EquivalentCircuits                                                                    
~/Code/EquivalentCircuits.jl/src/CircuitEvolution.jl:381                                    
[12] pyjlany_call(self::typeof(circuit_evolution_batch), args_::Py,                        
kwargs_::Py)                                                                                
    @ PythonCall ~/.julia/packages/PythonCall/wXfah/src/jlwrap/any.jl:34                    
[13] _pyjl_callmethod(f::Any, self_::Ptr{PythonCall.C.PyObject},                           
args_::Ptr{PythonCall.C.PyObject}, nargs::Int64)                                            
    @ PythonCall ~/.julia/packages/PythonCall/wXfah/src/jlwrap/base.jl:69                   
[14] _pyjl_callmethod(o::Ptr{PythonCall.C.PyObject},                                       
args::Ptr{PythonCall.C.PyObject})                                                           
        @ PythonCall.C ~/.julia/packages/PythonCall/wXfah/src/cpython/jlwrap.jl:47      

Your system
Please provide detailed information about your system:

• The operating system: Ubuntu 22.04
• Julia 1.10, Python 3.10 (by Miniforge), JuliaCall 0.9.15

❯ pip list
Package                       Version      Editable project location
----------------------------- ------------ -------------------------
alabaster                     0.7.13
altair                        5.2.0
arviz                         0.17.0
asttokens                     2.4.1
attrs                         23.2.0
autoeis                       0.0.19       /home/amin/Code/AutoEIS
Babel                         2.13.0
beautifulsoup4                4.12.2
click                         8.1.7
comm                          0.2.1
contourpy                     1.2.0
cycler                        0.12.1
debugpy                       1.8.0
decorator                     5.1.1
dill                          0.3.8
docutils                      0.20.1
exceptiongroup                1.2.0
executing                     2.0.1
fonttools                     4.47.2
h5netcdf                      1.3.0
h5py                          3.10.0
imagesize                     1.4.1
impedance                     1.7.1
iniconfig                     2.0.0
ipykernel                     6.29.2
ipython                       8.20.0
ipywidgets                    8.1.1
jax                           0.4.23
jaxlib                        0.4.23
jedi                          0.19.1
Jinja2                        3.1.2
joblib                        1.3.2
jsonschema                    4.20.0
jsonschema-specifications     2023.12.1
julia                         0.6.2
juliacall                     0.9.15
juliapkg                      0.1.10
jupyter_client                8.6.0
jupyter_core                  5.7.1
jupyterlab-widgets            3.0.9
kiwisolver                    1.4.5
markdown-it-py                3.0.0
MarkupSafe                    2.1.3
matplotlib                    3.8.2
matplotlib-inline             0.1.6
mdurl                         0.1.2
ml-dtypes                     0.3.2
mpire                         2.9.0
multipledispatch              1.0.0
multiprocess                  0.70.16
nest-asyncio                  1.6.0
numpy                         1.26.3
numpyro                       0.13.2
opt-einsum                    3.3.0
packaging                     23.2
pandas                        2.1.4
parso                         0.8.3
pexpect                       4.9.0
pillow                        10.2.0
pip                           23.3.2
platformdirs                  4.2.0
pluggy                        1.4.0
prompt-toolkit                3.0.43
psutil                        5.9.8
ptyprocess                    0.7.0
pure-eval                     0.2.2
Pygments                      2.16.1
pyparsing                     3.1.1
pytest                        8.0.0
python-dateutil               2.8.2
pytz                          2023.3.post1
pyzmq                         25.1.2
referencing                   0.32.1
rich                          13.7.0
rpds-py                       0.17.1
ruff                          0.2.1
scikit-learn                  1.4.0
scipy                         1.11.4
seaborn                       0.13.1
semantic-version              2.10.0
setuptools                    69.0.3
six                           1.16.0
snowballstemmer               2.2.0
soupsieve                     2.5
Sphinx                        7.2.6
sphinx-basic-ng               1.0.0b2
sphinxcontrib-applehelp       1.0.7
sphinxcontrib-devhelp         1.0.5
sphinxcontrib-htmlhelp        2.0.4
sphinxcontrib-jsmath          1.0.1
sphinxcontrib-qthelp          1.0.6
sphinxcontrib-serializinghtml 1.1.9
stack-data                    0.6.3
threadpoolctl                 3.2.0
tomli                         2.0.1
toolz                         0.12.0
tornado                       6.4
tqdm                          4.66.1
traitlets                     5.14.1
typing_extensions             4.9.0
tzdata                        2023.4
wcwidth                       0.2.13
wheel                         0.42.0
widgetsnbextension            4.0.9
xarray                        2023.12.0
xarray-einstats               0.6.0

>>> juliacall.Pkg.status()
Status `~/mambaforge/envs/autoeis/julia_env/Project.toml`
  [da5bd070] EquivalentCircuits v0.3.1 `~/Code/EquivalentCircuits.jl`
  [6099a3de] PythonCall v0.9.15

[cjdoris]

This is now fixed on main - the underlying issue being that PyArray was not serializable. Though unless you really do need it to be a PyArray I'd recommend just converting it to Array as you had done.

[ma-sadeghi]

Great, thanks! Just a quick question: Do you see #424 being worked on in the forseeable future? Thanks!

[cjdoris]

I have no particular plans to do it - there are other more pressing issues for PythonCall. But would be quite simple if you want to give it a go.

[ma-sadeghi]

Sure, I'd be happy to take a crack at it. Any pointers where to look/start?

[cjdoris]

Take a look at serialization.jl. Pretty much you'd need to add a setting to PythonCall, and if that setting is set you'd import dill instead of pickle.