code: def _initialize_pop(self, pop_size):
          
          self.toolbox.register("individual", self._generate)
          self.toolbox.register("population", tools.initRepeat,
                                list, self.toolbox.individual)
          self.population = self.toolbox.population(n=pop_size)
          self.assign_fitnesses(self.population)
          self._model_count += len(self.population)
          return
docs: Assigns indices to individuals in population.