Source code for textattack.search_methods.particle_swarm_optimization


Particle Swarm Optimization

Reimplementation of search method from Word-level Textual Adversarial
Attacking as Combinatorial Optimization by Zang et.

import copy

import numpy as np

from textattack.goal_function_results import GoalFunctionResultStatus
from textattack.search_methods import PopulationBasedSearch, PopulationMember
from textattack.shared import utils
from textattack.shared.validators import transformation_consists_of_word_swaps

[docs]class ParticleSwarmOptimization(PopulationBasedSearch): """Attacks a model with word substiutitions using a Particle Swarm Optimization (PSO) algorithm. Some key hyper-parameters are setup according to the original paper: "We adjust PSO on the validation set of SST and set ω_1 as 0.8 and ω_2 as 0.2. We set the max velocity of the particles V_{max} to 3, which means the changing probability of the particles ranges from 0.047 (sigmoid(-3)) to 0.953 (sigmoid(3))." Args: pop_size (:obj:`int`, optional): The population size. Defaults to 60. max_iters (:obj:`int`, optional): The maximum number of iterations to use. Defaults to 20. post_turn_check (:obj:`bool`, optional): If `True`, check if new position reached by moving passes the constraints. Defaults to `True` max_turn_retries (:obj:`bool`, optional): Maximum number of movement retries if new position after turning fails to pass the constraints. Applied only when `post_movement_check` is set to `True`. Setting it to 0 means we immediately take the old position as the new position upon failure. """ def __init__( self, pop_size=60, max_iters=20, post_turn_check=True, max_turn_retries=20 ): self.max_iters = max_iters self.pop_size = pop_size self.post_turn_check = post_turn_check self.max_turn_retries = 20 self._search_over = False self.omega_1 = 0.8 self.omega_2 = 0.2 self.c1_origin = 0.8 self.c2_origin = 0.2 self.v_max = 3.0 def _perturb(self, pop_member, original_result): """Perturb `pop_member` in-place. Replaces a word at a random in `pop_member` with replacement word that maximizes increase in score. Args: pop_member (PopulationMember): The population member being perturbed. original_result (GoalFunctionResult): Result of original sample being attacked Returns: `True` if perturbation occured. `False` if not. """ # TODO: Below is very slow and is the main cause behind memory build up + slowness best_neighbors, prob_list = self._get_best_neighbors( pop_member.result, original_result ) random_result = np.random.choice(best_neighbors, 1, p=prob_list)[0] if random_result == pop_member.result: return False else: pop_member.attacked_text = random_result.attacked_text pop_member.result = random_result return True def _equal(self, a, b): return -self.v_max if a == b else self.v_max def _turn(self, source_text, target_text, prob, original_text): """ Based on given probabilities, "move" to `target_text` from `source_text` Args: source_text (PopulationMember): Text we start from. target_text (PopulationMember): Text we want to move to. prob (np.array[float]): Turn probability for each word. original_text (AttackedText): Original text for constraint check if `self.post_turn_check=True`. Returns: New `Position` that we moved to (or if we fail to move, same as `source_text`) """ assert len(source_text.words) == len( target_text.words ), "Word length mismatch for turn operation." assert len(source_text.words) == len( prob ), "Length mismatch for words and probability list." len_x = len(source_text.words) num_tries = 0 passed_constraints = False while num_tries < self.max_turn_retries + 1: indices_to_replace = [] words_to_replace = [] for i in range(len_x): if np.random.uniform() < prob[i]: indices_to_replace.append(i) words_to_replace.append(target_text.words[i]) new_text = source_text.attacked_text.replace_words_at_indices( indices_to_replace, words_to_replace ) indices_to_replace = set(indices_to_replace) new_text.attack_attrs["modified_indices"] = ( source_text.attacked_text.attack_attrs["modified_indices"] - indices_to_replace ) | ( target_text.attacked_text.attack_attrs["modified_indices"] & indices_to_replace ) if "last_transformation" in source_text.attacked_text.attack_attrs: new_text.attack_attrs[ "last_transformation" ] = source_text.attacked_text.attack_attrs["last_transformation"] if not self.post_turn_check or (new_text.words == source_text.words): break if "last_transformation" in new_text.attack_attrs: passed_constraints = self._check_constraints( new_text, source_text.attacked_text, original_text=original_text ) else: passed_constraints = True if passed_constraints: break num_tries += 1 if self.post_turn_check and not passed_constraints: # If we cannot find a turn that passes the constraints, we do not move. return source_text else: return PopulationMember(new_text) def _get_best_neighbors(self, current_result, original_result): """For given current text, find its neighboring texts that yields maximum improvement (in goal function score) for each word. Args: current_result (GoalFunctionResult): `GoalFunctionResult` of current text original_result (GoalFunctionResult): `GoalFunctionResult` of original text. Returns: best_neighbors (list[GoalFunctionResult]): Best neighboring text for each word prob_list (list[float]): discrete probablity distribution for sampling a neighbor from `best_neighbors` """ current_text = current_result.attacked_text neighbors_list = [[] for _ in range(len(current_text.words))] transformed_texts = self.get_transformations( current_text, original_text=original_result.attacked_text ) for transformed_text in transformed_texts: diff_idx = next( iter(transformed_text.attack_attrs["newly_modified_indices"]) ) neighbors_list[diff_idx].append(transformed_text) best_neighbors = [] score_list = [] for i in range(len(neighbors_list)): if not neighbors_list[i]: best_neighbors.append(current_result) score_list.append(0) continue neighbor_results, self._search_over = self.get_goal_results( neighbors_list[i] ) if not len(neighbor_results): best_neighbors.append(current_result) score_list.append(0) else: neighbor_scores = np.array([r.score for r in neighbor_results]) score_diff = neighbor_scores - current_result.score best_idx = np.argmax(neighbor_scores) best_neighbors.append(neighbor_results[best_idx]) score_list.append(score_diff[best_idx]) prob_list = normalize(score_list) return best_neighbors, prob_list def _initialize_population(self, initial_result, pop_size): """ Initialize a population of size `pop_size` with `initial_result` Args: initial_result (GoalFunctionResult): Original text pop_size (int): size of population Returns: population as `list[PopulationMember]` """ best_neighbors, prob_list = self._get_best_neighbors( initial_result, initial_result ) population = [] for _ in range(pop_size): # Mutation step random_result = np.random.choice(best_neighbors, 1, p=prob_list)[0] population.append( PopulationMember(random_result.attacked_text, random_result) ) return population
[docs] def check_transformation_compatibility(self, transformation): """The genetic algorithm is specifically designed for word substitutions.""" return transformation_consists_of_word_swaps(transformation)
@property def is_black_box(self): return True
[docs] def extra_repr_keys(self): return ["pop_size", "max_iters", "post_turn_check", "max_turn_retries"]
[docs]def normalize(n): n = np.array(n) n[n < 0] = 0 s = np.sum(n) if s == 0: return np.ones(len(n)) / len(n) else: return n / s