Returning symbolic variable from modeul

General Usage
,
1

The following code works if you run it directly in REPL

module MyM
using SymPy
export beta, return_beta
beta = symbols("β")
function return_beta()
    return -beta
end
end
MyM.return_beta()

But if you put MyM in a file MyM.jl and use it like this

using MyM; return_beta()

you will see an error

ERROR: ArgumentError: ref of NULL PyObject
Stacktrace:
 [1] getproperty(::PyCall.PyObject, ::String) at /home/xing/.julia/packages/PyCall/zqDXB/src/PyCall.jl:296
 [2] getproperty(::PyCall.PyObject, ::Symbol) at /home/xing/.julia/packages/PyCall/zqDXB/src/PyCall.jl:306
 [3] getproperty(::SymPy.Sym, ::Symbol) at /home/xing/.julia/packages/SymPy/JaxDZ/src/types.jl:119
 [4] -(::SymPy.Sym) at /home/xing/.julia/packages/SymPy/JaxDZ/src/mathops.jl:10
 [5] return_beta() at /home/xing/Dropbox/Research/2020/Bak-Sneppen/MyM.jl:6
 [6] top-level scope at REPL[1]:1

I also tried similar code with SymEngine.jl, exactly the same problem. Is there something wrong about my code?

2

The problem with beta is that the underlying memory is freed when you load it from file. You need to assign beta in the __init__ function of your module.
See here how SymEngine.jl handles this.

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