I first make multiple copies of a PyObject, and then access them in multiple threads. Finally, I meet this error. julia -t5 using PyCall py""" class A(): x = 0.0 a = A() """ alist = [deepcopy(py"a") for i in 1:10] # @show alist for j in 1:100 Threads.@threads for i in 1:10 alist…

I find a similar question about this https://juliacommunity.github.io/zulip-archive/stream/274208-helpdesk-(published)/topic/using.20PyCall.20object.20in.20threads.html , which gives some reason but does not provide a possible solution.

Can I suggest using PythonCall.jl? It seems to be the more mature option for calling Python from Julia, and it has some notes on multithreading FAQ & Troubleshooting · PythonCall & JuliaCall ?

Is PythonCall/JuliaCall thread safe? No. Some rules if you are writing multithreaded code: Only call Python functions from the first thread. Thanks for your suggestion. It seems that it also does not realize this.

I mean, Python itself has a global interpreter lock and cannot handle more than one thread in parallel… There is not much anybody can do about it… Either you serialize the calls to Python, or you create multiple Julia processes calling multiple instances of the Python interpreter…

What is serialize the calls to Python? Can you give me some examples or links? Thanks.

Writing reentrant and threadsafe code In single-threaded processes, only one flow of control exists. The code executed by these processes thus need not be reentrant or threadsafe. In multithreaded programs, the same functions and the same resources may be accessed concurrently by severa…

Stumbled across this very problem and after some searching found the following solution: using PythonCall using ThreadPools macro pythread(expr) quote fetch(@tspawnat 1 begin $(esc(expr)) end) end end function babel_convert(input, informat = "cdxml", outformat …

Just tried to use @pythread in a more complicated scenario and found that it essential to call PythonCall.GC.isable() from the Main thread as well. As this is not always easy to do, it may be favorable to define @pythread as follows: macro pythread(expr) quote fetch(@tspawnat 1 begin …

After updating to Julia 1.12.4, the julia kernel is crashing inconsistently on some PyPlot calls with the error: Fatal Python error: PyThreadState_Get: the function must be called with the GIL held, but the GIL is released (the current Python thread state is NULL) Python runtime state: initialized …

Julia crashed when using PyCall in multiple threads

General Usage

ufechner7 January 21, 2026, 4:12am 12

You have three options:

use PythonCall/ PythonPlot instead of PyCall/PyPlot (now threadsafe, not in the past)
run Julia with julia -t 1,0 (single threaded)
do the plotting in a separate, single-threaded process

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Julia crashed when using PyCall in multiple threads

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