"""
Language Models Constraint
---------------------------
"""
from abc import ABC, abstractmethod
from textattack.constraints import Constraint
[docs]class LanguageModelConstraint(Constraint, ABC):
"""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 decrease in log-probability
in swapped words from `x` to `x_adv`
compare_against_original (bool): If `True`, compare new `x_adv` against the original `x`.
Otherwise, compare it against the previous `x_adv`.
"""
def __init__(self, max_log_prob_diff=None, compare_against_original=True):
if max_log_prob_diff is None:
raise ValueError("Must set max_log_prob_diff")
self.max_log_prob_diff = max_log_prob_diff
super().__init__(compare_against_original)
[docs] @abstractmethod
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, reference_text):
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((reference_text, transformed_text), i)
if len(probs) != 2:
raise ValueError(
f"Error: get_log_probs_at_index returned {len(probs)} values for 2 inputs"
)
ref_prob, transformed_prob = probs
if transformed_prob <= ref_prob - self.max_log_prob_diff:
return False
return True