Calling best_solution() after a run with elitism doesn't return best solution #337

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1. pmguerre commented on Sep 19, 2025

   

   pmguerre

   on Sep 19, 2025

   Last edited by pmguerre

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   Hi.

   No, you cannot remove the command in line 1926, because if you do so and the best solution is found in the last generation, it won't be returned by the best_solution() call.

   However, there is a bug somewhere as you can see if you try this code:

   import numpy as np
   import pygad
   
   def f(ga_instance, x, x_idx):
       return -sum(x**2)          # i'm minimizing the function x^2
   
   pop_size = 100
   crossover_probability = 0.7
   mutation_probability = 0.1
   num_genes = 3
   num_generations = 10
   num_parents_mating = pop_size
   
   ga_instance = pygad.GA(num_generations=num_generations,
                          num_parents_mating=num_parents_mating,
                          fitness_func=f,
                          sol_per_pop=pop_size,
                          num_genes=num_genes,
                          crossover_probability=crossover_probability,
                          mutation_probability=mutation_probability,
                          #save_best_solutions=True,
                          random_seed=42,
                          )
   
   ga_instance.run()
   
   print(f'Worst   f(x): {-np.min(ga_instance.best_solutions_fitness):.6f}')
   print(f'Average f(x): {-np.mean(ga_instance.best_solutions_fitness):.6f}')
   print(f'Best    f(x): {-np.max(ga_instance.best_solutions_fitness):.6f}')
   print("Best solutions found at generation", ga_instance.best_solution_generation)
   print("-"*50)
   
   solution, solution_fitness, solution_idx = ga_instance.best_solution()
   print(f'Best solution : {solution.round(4)}')
   print(f'Best fitness  : {-solution_fitness:.6f}')
   print(f'Best solution index : {solution_idx}')

   The output is:

   Worst   f(x): 0.382537
   Average f(x): 0.199507
   Best    f(x): 0.131763
   Best solutions found at generation 3
   --------------------------------------------------
   Best solution : [ 0.0524 -0.7452 -0.7311]
   Best fitness  : 1.092448
   Best solution index : 83
   

   But the best solution fitness found in the ga_instance.best_solutions_fitness list, and the answer given by the best_solution() are not the same, and they should be.

   However, if you enable the save_best_solutions=True option, the results are the same.

   I think the error is somewhere in the self.last_generation_elitism list (or self.self.last_generation_elitism_indices), as the first solution (in the for loop in best_solution()) it enters the elseif below (lines 1536-1543), and it returns the wrong index of the solution/fitness.

   elif (self.keep_elitism > 0) and (self.last_generation_elitism is not None) and (len(self.last_generation_elitism) > 0) and (list(sol) in last_generation_elitism_as_list):
       # Return the index of the elitism from the elitism array 'self.last_generation_elitism'.
       # This is not its index within the population. It is just its index in the 'self.last_generation_elitism' array.
       elitism_idx = last_generation_elitism_as_list.index(list(sol))
       # Use the returned elitism index to return its index in the last population.
       elitism_idx = self.last_generation_elitism_indices[elitism_idx]
       # Use the elitism's index to return its pre-calculated fitness value.
       fitness = self.previous_generation_fitness[elitism_idx]
2. pmguerre commented on Sep 19, 2025

   

   pmguerre

   on Sep 19, 2025

   More actions

   Just an addon:

   If I call this way:

   solution, solution_fitness, solution_idx = ga_instance.best_solution(pop_fitness=ga_instance.last_generation_fitness)
   

   it returns the correct value, i.e. the same as given by the call to np.max(ga_instance.best_solutions_fitness)

   So, the error is somewhere in the building of the pop_fitness list, after the algorithm is terminated...
3. ahmedfgad

   added

   bugSomething isn't workingSomething isn't working

   on Apr 5, 2026
4. ahmedfgad

   added 2 commits that reference this issue on Apr 7, 2026

   Fix #337

   6fdd7aa

   PyGAD 3.6.0

   ...

   409e27d
5. ahmedfgad

   closed this as completedin 6fdd7aaon Apr 8, 2026