textattack-nlp-transformer/textattack/constraints/pre_transformation_constraint.py

from abc import ABC, abstractmethod

from textattack.shared.utils import default_class_repr


class PreTransformationConstraint(ABC):
    """ 
    An abstract class that represents constraints which are applied before
    the transformation. These restrict which words are allowed to be modified
    during the transformation. For example, we might not allow stopwords to be
    modified.
    """

    def __call__(self, current_text, transformation):
        """ 
        Returns the word indices in ``current_text`` which are able to be modified. 
        First checks compatibility with ``transformation`` then calls ``_get_modifiable_indices``\.

        Args:
            current_text: The ``AttackedText`` input to consider.
            transformation: The ``Transformation`` which will be applied.
        """
        if not self.check_compatibility(transformation):
            return set(range(len(current_text.words)))
        return self._get_modifiable_indices(current_text)

    @abstractmethod
    def _get_modifiable_indices(current_text):
        """
        Returns the word indices in ``current_text`` which are able to be modified. 
        Must be overridden by specific pre-transformation constraints.

        Args:
            current_text: The ``AttackedText`` input to consider.
        """
        raise NotImplementedError()

    def check_compatibility(self, transformation):
        """ 
        Checks if this constraint is compatible with the given transformation.
        For example, the ``WordEmbeddingDistance`` constraint compares the embedding of
        the word inserted with that of the word deleted. Therefore it can only be
        applied in the case of word swaps, and not for transformations which involve
        only one of insertion or deletion.

        Args:
            transformation: The ``Transformation`` to check compatibility with.
        """
        return True

    def extra_repr_keys(self):
        """Set the extra representation of the constraint using these keys.
        
        To print customized extra information, you should reimplement
        this method in your own constraint. Both single-line and multi-line
        strings are acceptable.
        """
        return []

    def _check_constraint(self):
        raise RuntimeError(
            "PreTransformationConstraints do not support `_check_constraint()`."
        )

    __str__ = __repr__ = default_class_repr