Saving SymPy objects on disk with pickle

Question

I am solving a system of linear equations with simpy, this takes some time and I need to use the result several times for several different applications. For this reason I would like to store the results of the computation and load them when needed.

I developed this method:

    @property
    def solutions(self) -> list[Expr]:
        directory = SYMPY_DATA_PATH
        filename = os.path.join(directory, f'{self.__class__.__name__}.pkl')
        if not os.path.exists(directory):
            os.makedirs(directory)
        if os.path.exists(filename):
            with open(filename, 'rb') as f:
                print(f"Loading {filename}")
                solution_set = pickle.load(f)
        else:
            solution_set = together(linsolve(self.equations, self.pins))
            print(f"Saving {filename}")
            with open(filename, 'wb') as f:
                pickle.dump(list(solution_set)[0], f)
        return list(solution_set)[0]

    def solution(self, pin) -> Expr:
        return self.solutions[pin]

but I get the error:

TypeError: 'One' object is not subscriptable

Previously what I did was:

    @property
    def solutions(self) -> list[Expr]:
        solution_set = together(linsolve(self.equations, self.pins))
        return list(solution_set)[0]

and the code worked as I was expecting.