textattack-nlp-transformer/textattack/constraints/grammaticality/language_models/language_model_constraint.py

import math
import torch

from textattack.constraints import Constraint


class LanguageModelConstraint(Constraint):
    """ 
    Determines if two sentences have a swapped word that has a similar
    probability according to a language model.
        
    Args:
        max_log_prob_diff (float): the maximum difference in log-probability
            between x and x_adv
    """

    def __init__(self, max_log_prob_diff=None):
        if max_log_prob_diff is None:
            raise ValueError("Must set max_log_prob_diff")
        self.max_log_prob_diff = max_log_prob_diff

    def get_log_probs_at_index(self, text_list, word_index):
        """ Gets the log-probability of items in `text_list` at index 
            `word_index` according to a language model.
        """
        raise NotImplementedError()

    def _check_constraint(self, transformed_text, current_text, original_text=None):
        try:
            indices = transformed_text.attack_attrs["newly_modified_indices"]
        except KeyError:
            raise KeyError(
                "Cannot apply language model constraint without `newly_modified_indices`"
            )

        for i in indices:
            probs = self.get_log_probs_at_index((current_text, transformed_text), i)
            if len(probs) != 2:
                raise ValueError(
                    f"Error: get_log_probs_at_index returned {len(probs)} values for 2 inputs"
                )
            cur_prob, transformed_prob = probs
            if self.max_log_prob_diff is None:
                cur_prob, transformed_prob = math.log(p1), math.log(p2)
            if abs(cur_prob - transformed_prob) > self.max_log_prob_diff:
                return False

        return True

    def extra_repr_keys(self):
        return ["max_log_prob_diff"]