Creating symbols (and expressions) in different processes

[Momchil Peychev]

Hello,

I have the following issue: when I try to create symbols (and by extension expressions) in different processes, SymPy somehow does not detect that the symbols are the same even though they have the same name and assumptions.

As an example, consider the following code snippet and the respective output:

import multiprocessing as mp
import sympy as sp

VAR_X = sp.Symbol('x', real=True, nonzero=True)

def process():
    return sp.Symbol('x', real=True, nonzero=True)

if __name__ == '__main__':
    a1 = sp.Symbol('a', real=True, nonzero=True)
    a2 = sp.Symbol('a', real=True, nonzero=True)
    print(a1, a2, a1 == a2, a1 - a2, '\n')

    pool = mp.Pool(4)
    jobs = []
    for _ in range(5):
        jobs.append(pool.apply_async(process))
    symbols = []
    for job in jobs:
        symbols.append(job.get())
    pool.close()

    for s in symbols:
        print(s, ' | ', VAR_X, ' | ', s - VAR_X, ' | ', sp.simplify(s - VAR_X))

Output:

a a True 0

x  |  x  |  -x + x  |  -x + x
x  |  x  |  -x + x  |  -x + x
x  |  x  |  -x + x  |  -x + x
x  |  x  |  -x + x  |  -x + x
x  |  x  |  -x + x  |  -x + x

Has anyone seen this before? Can anyone explain me what is going on?

Thank you,

Momchil

[Oscar Benjamin]

On Thu, 18 Mar 2021 at 06:26, Momchil Peychev <momchil...@gmail.com> wrote:
>
> I have the following issue: when I try to create symbols (and by extension expressions) in different processes, SymPy somehow does not detect that the symbols are the same even though they have the same name and assumptions.

Maybe there's a problem with pickling and unpickling the symbol.
Working in the same process creating two different symbols returns the
exact same object:

In [5]: x1 = Symbol('x')

In [6]: x2 = Symbol('x')

In [7]: x1 is x2
Out[7]: True

Perhaps pickling creates a distinct object that does not then compare
equal in some way because the equality testing uses is for symbols.

I'm not sure what causes this but I think it's a bug so can you open
an issue on GitHub?

Oscar

[Momchil Peychev]

Dear Oscar,

Thank you for the reply.

I opened the issue here: https://github.com/sympy/sympy/issues/21121

and propose to move any further discussion there.

Thanks,

Momchil