Replicability across function evaluations in PSO

[andrealati]

Hello,

First, thank you very much for developing and maintaining this library is very useful.

I am using the PSO algorithm to estimate a model with 17 parameters. While experimenting with the algorithm we noticed a replicability issue that we haven't been able to solve. Specifically, we noted a discrepancy between the objective function's values: using identical parameters produces different results inside versus outside the solver.

More precisely we do the following:

1. at every function evaluation of pymoo_minimize we print the values of the parameters and the associated error.
2. we then run our objective function at the values printed during the evaluation step in pymoo_minimize and we get a different value for the error, sometimes significantly different.

Is this expected behaviour ? If yes, is there a setting in the solver we can use to address this ?

We tried to replicate the same issue using scipy.minimize but in that case the evaluations within the solver and outside are consistent.

Thank you very much for your help and sorry if there's an obvious solution to this.

Our implementation of the algorithm is as follows:

class PymooProblem(ElementwiseProblem):
	def __init__(self):
		super().__init__(n_var=smm_init.shape[0],
					n_obj=1,
					xl=np.array([bound[0] for bound in p_bounds]),
					xu=np.array([bound[1] for bound in p_bounds])
					)

	def _evaluate(self,x,out,*args,**kwargs):
		print("Inputs:", ["{:.11f}".format(val) for val in x])
		f1 = Objective(x, **kwargs)
		print("F val:", f1)
		out["F"] = [f1]

algorithm = PSO(pop_size=20)
problem_abs = PymooProblem()
res = pymoo_minimize(problem_abs,
					algorithm,
					seed=1,
					verbose=True,
					save_history=True)