# Lime¶

class captum.attr.LimeBase(forward_func, interpretable_model, similarity_func, perturb_func, perturb_interpretable_space, from_interp_rep_transform, to_interp_rep_transform)[source]

Lime is an interpretability method that trains an interpretable surrogate model by sampling points around a specified input example and using model evaluations at these points to train a simpler interpretable ‘surrogate’ model, such as a linear model.

LimeBase provides a generic framework to train a surrogate interpretable model. This differs from most other attribution methods, since the method returns a representation of the interpretable model (e.g. coefficients of the linear model). For a similar interface to other perturbation-based attribution methods, please use the Lime child class, which defines specific transformations for the interpretable model.

LimeBase allows sampling points in either the interpretable space or the original input space to train the surrogate model. The interpretable space is a feature vector used to train the surrogate interpretable model; this feature space is often of smaller dimensionality than the original feature space in order for the surrogate model to be more interpretable.

If sampling in the interpretable space, a transformation function must be provided to define how a vector sampled in the interpretable space can be transformed into an example in the original input space. If sampling in the original input space, a transformation function must be provided to define how the input can be transformed into its interpretable vector representation.

More details regarding LIME can be found in the original paper: https://arxiv.org/abs/1602.04938

Parameters
• forward_func (callable) – The forward function of the model or any modification of it. If a batch is provided as input for attribution, it is expected that forward_func returns a scalar representing the entire batch.

• interpretable_model (Model) –

Model object to train interpretable model. A Model object provides a fit method to train the model, given a dataloader, with batches containing three tensors:

• interpretable_inputs: Tensor [2D num_samples x num_interp_features],

• expected_outputs: Tensor [1D num_samples],

• weights: Tensor [1D num_samples]

The model object must also provide a representation method to access the appropriate coefficients or representation of the interpretable model after fitting. Some predefined interpretable linear models are provided in captum._utils.models.linear_model including wrappers around SkLearn linear models as well as SGD-based PyTorch linear models.

Note that calling fit multiple times should retrain the interpretable model, each attribution call reuses the same given interpretable model object.

• similarity_func (callable) –

Function which takes a single sample along with its corresponding interpretable representation and returns the weight of the interpretable sample for training interpretable model. Weight is generally determined based on similarity to the original input. The original paper refers to this as a similarity kernel.

The expected signature of this callable is:

>>> similarity_func(
>>>    original_input: Tensor or tuple of Tensors,
>>>    perturbed_input: Tensor or tuple of Tensors,
>>>    perturbed_interpretable_input:
>>>        Tensor [2D 1 x num_interp_features],
>>>    **kwargs: Any
>>> ) -> float or Tensor containing float scalar


perturbed_input and original_input will be the same type and contain tensors of the same shape (regardless of whether or not the sampling function returns inputs in the interpretable space). original_input is the same as the input provided when calling attribute.

All kwargs passed to the attribute method are provided as keyword arguments (kwargs) to this callable.

• perturb_func (callable) –

Function which returns a single sampled input, generally a perturbation of the original input, which is used to train the interpretable surrogate model. Function can return samples in either the original input space (matching type and tensor shapes of original input) or in the interpretable input space, which is a vector containing the intepretable features.

The expected signature of this callable is:

>>> perturb_func(
>>>    original_input: Tensor or tuple of Tensors,
>>>    **kwargs: Any
>>> ) -> Tensor or tuple of Tensors


All kwargs passed to the attribute method are provided as keyword arguments (kwargs) to this callable.

Returned sampled input should match the input type (Tensor or Tuple of Tensor and corresponding shapes) if perturb_interpretable_space = False. If perturb_interpretable_space = True, the return type should be a single tensor of shape 1 x num_interp_features, corresponding to the representation of the sample to train the interpretable model.

All kwargs passed to the attribute method are provided as keyword arguments (kwargs) to this callable.

• perturb_interpretable_space (bool) – Indicates whether perturb_func returns a sample in the interpretable space (tensor of shape 1 x num_interp_features) or a sample in the original space, matching the format of the original input. Once sampled, inputs can be converted to / from the interpretable representation with either to_interp_rep_transform or from_interp_rep_transform.

• from_interp_rep_transform (callable) –

Function which takes a single sampled interpretable representation (tensor of shape 1 x num_interp_features) and returns the corresponding representation in the input space (matching shapes of original input to attribute).

This argument is necessary if perturb_interpretable_space is True, otherwise None can be provided for this argument.

The expected signature of this callable is:

>>> from_interp_rep_transform(
>>>    curr_sample: Tensor [2D 1 x num_interp_features]
>>>    original_input: Tensor or Tuple of Tensors,
>>>    **kwargs: Any
>>> ) -> Tensor or tuple of Tensors


Returned sampled input should match the type of original_input and corresponding tensor shapes.

All kwargs passed to the attribute method are provided as keyword arguments (kwargs) to this callable.

• to_interp_rep_transform (callable) –

Function which takes a sample in the original input space and converts to its interpretable representation (tensor of shape 1 x num_interp_features).

This argument is necessary if perturb_interpretable_space is False, otherwise None can be provided for this argument.

The expected signature of this callable is:

>>> to_interp_rep_transform(
>>>    curr_sample: Tensor or Tuple of Tensors,
>>>    original_input: Tensor or Tuple of Tensors,
>>>    **kwargs: Any
>>> ) -> Tensor [2D 1 x num_interp_features]


curr_sample will match the type of original_input and corresponding tensor shapes.

All kwargs passed to the attribute method are provided as keyword arguments (kwargs) to this callable.

attribute(inputs, target=None, additional_forward_args=None, n_perturb_samples=50, perturbations_per_eval=1, **kwargs)[source]

This method attributes the output of the model with given target index (in case it is provided, otherwise it assumes that output is a scalar) to the inputs of the model using the approach described above. It trains an interpretable model and returns a representation of the interpretable model.

It is recommended to only provide a single example as input (tensors with first dimension or batch size = 1). This is because LIME is generally used for sample-based interpretability, training a separate interpretable model to explain a model’s prediction on each individual example.

A batch of inputs can be provided as inputs only if forward_func returns a single value per batch (e.g. loss). The interpretable feature representation should still have shape 1 x num_interp_features, corresponding to the interpretable representation for the full batch, and perturbations_per_eval must be set to 1.

Parameters
• inputs (tensor or tuple of tensors) – Input for which LIME is computed. If forward_func takes a single tensor as input, a single input tensor should be provided. If forward_func takes multiple tensors as input, a tuple of the input tensors should be provided. It is assumed that for all given input tensors, dimension 0 corresponds to the number of examples, and if multiple input tensors are provided, the examples must be aligned appropriately.

• target (int, tuple, tensor or list, optional) –

Output indices for which surrogate model is trained (for classification cases, this is usually the target class). If the network returns a scalar value per example, no target index is necessary. For general 2D outputs, targets can be either:

• a single integer or a tensor containing a single integer, which is applied to all input examples

• a list of integers or a 1D tensor, with length matching the number of examples in inputs (dim 0). Each integer is applied as the target for the corresponding example.

For outputs with > 2 dimensions, targets can be either:

• A single tuple, which contains #output_dims - 1 elements. This target index is applied to all examples.

• A list of tuples with length equal to the number of examples in inputs (dim 0), and each tuple containing #output_dims - 1 elements. Each tuple is applied as the target for the corresponding example.

Default: None

• additional_forward_args (any, optional) – If the forward function requires additional arguments other than the inputs for which attributions should not be computed, this argument can be provided. It must be either a single additional argument of a Tensor or arbitrary (non-tuple) type or a tuple containing multiple additional arguments including tensors or any arbitrary python types. These arguments are provided to forward_func in order following the arguments in inputs. For a tensor, the first dimension of the tensor must correspond to the number of examples. For all other types, the given argument is used for all forward evaluations. Note that attributions are not computed with respect to these arguments. Default: None

• n_perturb_samples (int, optional) – The number of samples of the original model used to train the surrogate interpretable model. Default: 50 if n_perturb_samples is not provided.

• perturbations_per_eval (int, optional) – Allows multiple samples to be processed simultaneously in one call to forward_fn. Each forward pass will contain a maximum of perturbations_per_eval * #examples samples. For DataParallel models, each batch is split among the available devices, so evaluations on each available device contain at most (perturbations_per_eval * #examples) / num_devices samples. If the forward function returns a single scalar per batch, perturbations_per_eval must be set to 1. Default: 1

• **kwargs (Any, optional) – Any additional arguments necessary for sampling and transformation functions (provided to constructor). Default: None

Returns

• **interpretable model representation* (Any):

A representation of the interpretable model trained. The return type matches the return type of train_interpretable_model_func. For example, this could contain coefficients of a linear surrogate model.

Return type

interpretable model representation

Examples:

>>> # SimpleClassifier takes a single input tensor of
>>> # float features with size N x 5,
>>> # and returns an Nx3 tensor of class probabilities.
>>> net = SimpleClassifier()
>>>
>>> # We will train an interpretable model with the same
>>> # features by simply sampling with added Gaussian noise
>>> # to the inputs and training a model to predict the
>>> # score of the target class.
>>>
>>> # For interpretable model training, we will use sklearn
>>> # linear model in this example. We have provided wrappers
>>> # around sklearn linear models to fit the Model interface.
>>> # Any arguments provided to the sklearn constructor can also
>>> # be provided to the wrapper, e.g.:
>>> # SkLearnLinearModel("linear_model.Ridge", alpha=2.0)
>>> from captum._utils.models.linear_model import SkLearnLinearModel
>>>
>>>
>>> # Define similarity kernel (exponential kernel based on L2 norm)
>>> def similarity_kernel(
>>>     original_input: Tensor,
>>>     perturbed_input: Tensor,
>>>     perturbed_interpretable_input: Tensor,
>>>     **kwargs)->Tensor:
>>>         # kernel_width will be provided to attribute as a kwarg
>>>         kernel_width = kwargs["kernel_width"]
>>>         l2_dist = torch.norm(original_input - perturbed_input)
>>>
>>>
>>> # Define sampling function
>>> # This function samples in original input space
>>> def perturb_func(
>>>     original_input: Tensor,
>>>     **kwargs)->Tensor:
>>>         return original_input + torch.randn_like(original_input)
>>>
>>> # For this example, we are setting the interpretable input to
>>> # match the model input, so the to_interp_rep_transform
>>> # function simply returns the input. In most cases, the interpretable
>>> # input will be different and may have a smaller feature set, so
>>> # an appropriate transformation function should be provided.
>>>
>>> # Generating random input with size 2 x 5
>>> input = torch.randn(2, 5)
>>> # Defining LimeBase interpreter
>>> lime_attr = LimeBase(net,
SkLearnLinearModel("linear_model.Ridge"),
similarity_func=similarity_kernel,
perturb_func=perturb_func,
perturb_interpretable_space=False,
from_interp_rep_transform=None,
to_interp_rep_transform=lambda x: x)
>>> # Computes interpretable model, returning coefficients of linear
>>> # model.
>>> attr_coefs = lime_attr.attribute(input, target=1, kernel_width=1.1)

has_convergence_delta()[source]

This method informs the user whether the attribution algorithm provides a convergence delta (aka an approximation error) or not. Convergence delta may serve as a proxy of correctness of attribution algorithm’s approximation. If deriving attribution class provides a compute_convergence_delta method, it should override both compute_convergence_delta and has_convergence_delta methods.

Returns

Returns whether the attribution algorithm provides a convergence delta (aka approximation error) or not.

Return type

bool

class captum.attr.Lime(forward_func, interpretable_model=None, similarity_func=None, perturb_func=None)[source]

Lime is an interpretability method that trains an interpretable surrogate model by sampling points around a specified input example and using model evaluations at these points to train a simpler interpretable ‘surrogate’ model, such as a linear model.

Lime provides a more specific implementation than LimeBase in order to expose a consistent API with other perturbation-based algorithms. For more general use of the LIME framework, consider using the LimeBase class directly and defining custom sampling and transformation to / from interpretable representation functions.

Lime assumes that the interpretable representation is a binary vector, corresponding to some elements in the input being set to their baseline value if the corresponding binary interpretable feature value is 0 or being set to the original input value if the corresponding binary interpretable feature value is 1. Input values can be grouped to correspond to the same binary interpretable feature using a feature mask provided when calling attribute, similar to other perturbation-based attribution methods.

One example of this setting is when applying Lime to an image classifier. Pixels in an image can be grouped into super-pixels or segments, which correspond to interpretable features, provided as a feature_mask when calling attribute. Sampled binary vectors convey whether a super-pixel is on (retains the original input values) or off (set to the corresponding baseline value, e.g. black image). An interpretable linear model is trained with input being the binary vectors and outputs as the corresponding scores of the image classifier with the appropriate super-pixels masked based on the binary vector. Coefficients of the trained surrogate linear model convey the importance of each super-pixel.

More details regarding LIME can be found in the original paper: https://arxiv.org/abs/1602.04938

Parameters
• forward_func (callable) – The forward function of the model or any modification of it

• interpretable_model (optional, Model) –

Model object to train interpretable model.

This argument is optional and defaults to SkLearnLasso(alpha=1.0), which is a wrapper around the Lasso linear model in SkLearn. This requires having sklearn version >= 0.23 available.

Other predefined interpretable linear models are provided in captum._utils.models.linear_model.

Alternatively, a custom model object must provide a fit method to train the model, given a dataloader, with batches containing three tensors:

• interpretable_inputs: Tensor [2D num_samples x num_interp_features],

• expected_outputs: Tensor [1D num_samples],

• weights: Tensor [1D num_samples]

The model object must also provide a representation method to access the appropriate coefficients or representation of the interpretable model after fitting.

Note that calling fit multiple times should retrain the interpretable model, each attribution call reuses the same given interpretable model object.

• similarity_func (optional, callable) –

Function which takes a single sample along with its corresponding interpretable representation and returns the weight of the interpretable sample for training the interpretable model. This is often referred to as a similarity kernel.

This argument is optional and defaults to a function which applies an exponential kernel to the consine distance between the original input and perturbed input, with a kernel width of 1.0.

A similarity function applying an exponential kernel to cosine / euclidean distances can be constructed using the provided get_exp_kernel_similarity_function in captum.attr._core.lime.

Alternately, a custom callable can also be provided. The expected signature of this callable is:

>>> def similarity_func(
>>>    original_input: Tensor or tuple of Tensors,
>>>    perturbed_input: Tensor or tuple of Tensors,
>>>    perturbed_interpretable_input:
>>>        Tensor [2D 1 x num_interp_features],
>>>    **kwargs: Any
>>> ) -> float or Tensor containing float scalar


perturbed_input and original_input will be the same type and contain tensors of the same shape, with original_input being the same as the input provided when calling attribute.

• perturb_func (optional, callable) –

Function which returns a single sampled input, which is a binary vector of length num_interp_features.

This function is optional, the default function returns a binary vector where each element is selected independently and uniformly at random. Custom logic for selecting sampled binary vectors can be implemented by providing a function with the following expected signature:

>>> perturb_func(
>>>    original_input: Tensor or tuple of Tensors,
>>>    **kwargs: Any
>>> ) -> Tensor [Binary 2D Tensor 1 x num_interp_features]


attribute(inputs, baselines=None, target=None, additional_forward_args=None, feature_mask=None, n_perturb_samples=25, perturbations_per_eval=1, return_input_shape=True)[source]

This method attributes the output of the model with given target index (in case it is provided, otherwise it assumes that output is a scalar) to the inputs of the model using the approach described above, training an interpretable model and returning a representation of the interpretable model.

It is recommended to only provide a single example as input (tensors with first dimension or batch size = 1). This is because LIME is generally used for sample-based interpretability, training a separate interpretable model to explain a model’s prediction on each individual example.

A batch of inputs can also be provided as inputs, similar to other perturbation-based attribution methods. In this case, if forward_fn returns a scalar per example, attributions will be computed for each example independently, with a separate interpretable model trained for each example. Note that provided similarity and perturbation functions will be provided each example separately (first dimension = 1) in this case. If forward_fn returns a scalar per batch (e.g. loss), attributions will still be computed using a single interpretable model for the full batch. In this case, similarity and perturbation functions will be provided the same original input containing the full batch.

The number of interpretable features is determined from the provided feature mask, or if none is provided, from the default feature mask, which considers each scalar input as a separate feature. It is generally recommended to provide a feature mask which groups features into a small number of interpretable features / components (e.g. superpixels in images).

Parameters
• inputs (tensor or tuple of tensors) – Input for which LIME is computed. If forward_func takes a single tensor as input, a single input tensor should be provided. If forward_func takes multiple tensors as input, a tuple of the input tensors should be provided. It is assumed that for all given input tensors, dimension 0 corresponds to the number of examples, and if multiple input tensors are provided, the examples must be aligned appropriately.

• baselines (scalar, tensor, tuple of scalars or tensors, optional) –

Baselines define reference value which replaces each feature when the corresponding interpretable feature is set to 0. Baselines can be provided as:

• a single tensor, if inputs is a single tensor, with exactly the same dimensions as inputs or the first dimension is one and the remaining dimensions match with inputs.

• a single scalar, if inputs is a single tensor, which will be broadcasted for each input value in input tensor.

• a tuple of tensors or scalars, the baseline corresponding to each tensor in the inputs’ tuple can be:

• either a tensor with matching dimensions to corresponding tensor in the inputs’ tuple or the first dimension is one and the remaining dimensions match with the corresponding input tensor.

• or a scalar, corresponding to a tensor in the inputs’ tuple. This scalar value is broadcasted for corresponding input tensor.

In the cases when baselines is not provided, we internally use zero scalar corresponding to each input tensor. Default: None

• target (int, tuple, tensor or list, optional) –

Output indices for which surrogate model is trained (for classification cases, this is usually the target class). If the network returns a scalar value per example, no target index is necessary. For general 2D outputs, targets can be either:

• a single integer or a tensor containing a single integer, which is applied to all input examples

• a list of integers or a 1D tensor, with length matching the number of examples in inputs (dim 0). Each integer is applied as the target for the corresponding example.

For outputs with > 2 dimensions, targets can be either:

• A single tuple, which contains #output_dims - 1 elements. This target index is applied to all examples.

• A list of tuples with length equal to the number of examples in inputs (dim 0), and each tuple containing #output_dims - 1 elements. Each tuple is applied as the target for the corresponding example.

Default: None

• additional_forward_args (any, optional) – If the forward function requires additional arguments other than the inputs for which attributions should not be computed, this argument can be provided. It must be either a single additional argument of a Tensor or arbitrary (non-tuple) type or a tuple containing multiple additional arguments including tensors or any arbitrary python types. These arguments are provided to forward_func in order following the arguments in inputs. For a tensor, the first dimension of the tensor must correspond to the number of examples. It will be repeated for each of n_steps along the integrated path. For all other types, the given argument is used for all forward evaluations. Note that attributions are not computed with respect to these arguments. Default: None

• feature_mask (tensor or tuple of tensors, optional) – feature_mask defines a mask for the input, grouping features which correspond to the same interpretable feature. feature_mask should contain the same number of tensors as inputs. Each tensor should be the same size as the corresponding input or broadcastable to match the input tensor. Values across all tensors should be integers in the range 0 to num_interp_features - 1, and indices corresponding to the same feature should have the same value. Note that features are grouped across tensors (unlike feature ablation and occlusion), so if the same index is used in different tensors, those features are still grouped and added simultaneously. If None, then a feature mask is constructed which assigns each scalar within a tensor as a separate feature. Default: None

• n_perturb_samples (int, optional) – The number of samples of the original model used to train the surrogate interpretable model. Default: 50 if n_perturb_samples is not provided.

• perturbations_per_eval (int, optional) – Allows multiple samples to be processed simultaneously in one call to forward_fn. Each forward pass will contain a maximum of perturbations_per_eval * #examples samples. For DataParallel models, each batch is split among the available devices, so evaluations on each available device contain at most (perturbations_per_eval * #examples) / num_devices samples. If the forward function returns a single scalar per batch, perturbations_per_eval must be set to 1. Default: 1

• return_input_shape (bool, optional) – Determines whether the returned tensor(s) only contain the coefficients for each interp- retable feature from the trained surrogate model, or whether the returned attributions match the input shape. When return_input_shape is True, the return type of attribute matches the input shape, with each element containing the coefficient of the corresponding interpretale feature. All elements with the same value in the feature mask will contain the same coefficient in the returned attributions. If return_input_shape is False, a 1D tensor is returned, containing only the coefficients of the trained interpreatable models, with length num_interp_features.

Returns

• attributions (tensor or tuple of tensors):

The attributions with respect to each input feature. If return_input_shape = True, attributions will be the same size as the provided inputs, with each value providing the coefficient of the corresponding interpretale feature. If return_input_shape is False, a 1D tensor is returned, containing only the coefficients of the trained interpreatable models, with length num_interp_features.

Return type

tensor or tuple of tensors of attributions

Examples:

>>> # SimpleClassifier takes a single input tensor of size Nx4x4,
>>> # and returns an Nx3 tensor of class probabilities.
>>> net = SimpleClassifier()

>>> # Generating random input with size 1 x 4 x 4
>>> input = torch.randn(1, 4, 4)

>>> # Defining Lime interpreter
>>> lime = Lime(net)
>>> # Computes attribution, with each of the 4 x 4 = 16
>>> # features as a separate interpretable feature
>>> attr = lime.attribute(input, target=1, n_perturb_samples=200)

>>> # Alternatively, we can group each 2x2 square of the inputs
>>> # as one 'interpretable' feature and perturb them together.
>>> # This can be done by creating a feature mask as follows, which
>>> # defines the feature groups, e.g.:
>>> # +---+---+---+---+
>>> # | 0 | 0 | 1 | 1 |
>>> # +---+---+---+---+
>>> # | 0 | 0 | 1 | 1 |
>>> # +---+---+---+---+
>>> # | 2 | 2 | 3 | 3 |
>>> # +---+---+---+---+
>>> # | 2 | 2 | 3 | 3 |
>>> # +---+---+---+---+
>>> # With this mask, all inputs with the same value are set to their
>>> # baseline value, when the corresponding binary interpretable
>>> # feature is set to 0.
>>> # The attributions can be calculated as follows:
>>> # feature mask has dimensions 1 x 4 x 4
>>>                             [2,2,3,3],[2,2,3,3]]])

>>> # Computes interpretable model and returning attributions
>>> # matching input shape.

captum.attr._core.lime.get_exp_kernel_similarity_function(distance_mode='cosine', kernel_width=1.0)[source]

This method constructs an appropriate similarity function to compute weights for perturbed sample in LIME. Distance between the original and perturbed inputs is computed based on the provided distance mode, and the distance is passed through an exponential kernel with given kernel width to convert to a range between 0 and 1.

The callable returned can be provided as the similarity_fn for Lime or LimeBase.

Parameters
• distance_mode (str, optional) – Distance mode can be either “cosine” or “euclidean” corresponding to either cosine distance or Euclidean distance respectively. Distance is computed by flattening the original inputs and perturbed inputs (concatenating tuples of inputs if necessary) and computing distances between the resulting vectors. Default: “cosine”

• kernel_width (float, optional) – Kernel width for exponential kernel applied to distance. Default: 1.0

Returns

• similarity_fn (Callable):

Similarity function. This callable can be provided as the similarity_fn for Lime or LimeBase.

Return type

Callable