"""
Max Modification Rate
-----------------------------
"""
import math
from textattack.constraints import PreTransformationConstraint
[docs]class MaxModificationRate(PreTransformationConstraint):
"""A constraint that prevents modifying words beyond certain percentage of
total number of words.
Args:
max_rate (:obj:`float`):
Percentage of words that can be modified. For example, given text of 20 words, `max_rate=0.1` will allow at most 2 words to be modified.
min_threshold (:obj:`int`, optional, defaults to :obj:`1`):
The minimum number of words that can be perturbed regardless of `max_rate`. For example, given text of 20 words and `max_rate=0.1`,
setting`min_threshold=4` will still allow 4 words to be modified even though `max_rate=0.1` only allows 2 words. This is useful since
text length can vary a lot between samples, and a `N%` modification limit might not make sense for very short text.
"""
def __init__(self, max_rate, min_threshold=1):
assert isinstance(max_rate, float), "`max_rate` must be a float."
assert max_rate >= 0 and max_rate <= 1, "`max_rate` must between 0 and 1."
assert isinstance(min_threshold, int), "`min_threshold` must an int"
self.max_rate = max_rate
self.min_threshold = min_threshold
def _get_modifiable_indices(self, current_text):
"""Returns the word indices in current_text which are able to be
modified."""
threshold = max(
math.ceil(current_text.num_words * self.max_rate), self.min_threshold
)
if len(current_text.attack_attrs["modified_indices"]) >= threshold:
return set()
else:
return set(range(len(current_text.words)))