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@marcotcr @mrkaiser @echo66 @kriselle @imatiach-msft @igorbrigadir
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"""
Contains abstract functionality for learning locally linear sparse model.
"""
from __future__ import print_function
import numpy as np
import scipy as sp
from sklearn.linear_model import Ridge, lars_path
from sklearn.utils import check_random_state
class LimeBase(object):
"""Class for learning a locally linear sparse model from perturbed data"""
def __init__(self,
kernel_fn,
verbose=False,
random_state=None):
"""Init function
Args:
kernel_fn: function that transforms an array of distances into an
array of proximity values (floats).
verbose: if true, print local prediction values from linear model.
random_state: an integer or numpy.RandomState that will be used to
generate random numbers. If None, the random state will be
initialized using the internal numpy seed.
"""
self.kernel_fn = kernel_fn
self.verbose = verbose
self.random_state = check_random_state(random_state)
@staticmethod
def generate_lars_path(weighted_data, weighted_labels):
"""Generates the lars path for weighted data.
Args:
weighted_data: data that has been weighted by kernel
weighted_label: labels, weighted by kernel
Returns:
(alphas, coefs), both are arrays corresponding to the
regularization parameter and coefficients, respectively
"""
x_vector = weighted_data
alphas, _, coefs = lars_path(x_vector,
weighted_labels,
method='lasso',
verbose=False)
return alphas, coefs
def forward_selection(self, data, labels, weights, num_features):
"""Iteratively adds features to the model"""
clf = Ridge(alpha=0, fit_intercept=True, random_state=self.random_state)
used_features = []
for _ in range(min(num_features, data.shape[1])):
max_ = -100000000
best = 0
for feature in range(data.shape[1]):
if feature in used_features:
continue
clf.fit(data[:, used_features + [feature]], labels,
sample_weight=weights)
score = clf.score(data[:, used_features + [feature]],
labels,
sample_weight=weights)
if score > max_:
best = feature
max_ = score
used_features.append(best)
return np.array(used_features)
def feature_selection(self, data, labels, weights, num_features, method):
"""Selects features for the model. see explain_instance_with_data to
understand the parameters."""
if method == 'none':
return np.array(range(data.shape[1]))
elif method == 'forward_selection':
return self.forward_selection(data, labels, weights, num_features)
elif method == 'highest_weights':
clf = Ridge(alpha=0.01, fit_intercept=True,
random_state=self.random_state)
clf.fit(data, labels, sample_weight=weights)
coef = clf.coef_
if sp.sparse.issparse(data):
coef = sp.sparse.csr_matrix(clf.coef_)
weighted_data = coef.multiply(data[0])
# Note: most efficient to slice the data before reversing
sdata = len(weighted_data.data)
argsort_data = np.abs(weighted_data.data).argsort()
# Edge case where data is more sparse than requested number of feature importances
# In that case, we just pad with zero-valued features
if sdata < num_features:
nnz_indexes = argsort_data[::-1]
indices = weighted_data.indices[nnz_indexes]
num_to_pad = num_features - sdata
indices = np.concatenate((indices, np.zeros(num_to_pad, dtype=indices.dtype)))
indices_set = set(indices)
pad_counter = 0
for i in range(data.shape[1]):
if i not in indices_set:
indices[pad_counter + sdata] = i
pad_counter += 1
if pad_counter >= num_to_pad:
break
else:
nnz_indexes = argsort_data[sdata - num_features:sdata][::-1]
indices = weighted_data.indices[nnz_indexes]
return indices
else:
weighted_data = coef * data[0]
feature_weights = sorted(
zip(range(data.shape[1]), weighted_data),
key=lambda x: np.abs(x[1]),
reverse=True)
return np.array([x[0] for x in feature_weights[:num_features]])
elif method == 'lasso_path':
weighted_data = ((data - np.average(data, axis=0, weights=weights))
* np.sqrt(weights[:, np.newaxis]))
weighted_labels = ((labels - np.average(labels, weights=weights))
* np.sqrt(weights))
nonzero = range(weighted_data.shape[1])
_, coefs = self.generate_lars_path(weighted_data,
weighted_labels)
for i in range(len(coefs.T) - 1, 0, -1):
nonzero = coefs.T[i].nonzero()[0]
if len(nonzero) <= num_features:
break
used_features = nonzero
return used_features
elif method == 'auto':
if num_features <= 6:
n_method = 'forward_selection'
else:
n_method = 'highest_weights'
return self.feature_selection(data, labels, weights,
num_features, n_method)
def explain_instance_with_data(self,
neighborhood_data,
neighborhood_labels,
distances,
label,
num_features,
feature_selection='auto',
model_regressor=None):
"""Takes perturbed data, labels and distances, returns explanation.
Args:
neighborhood_data: perturbed data, 2d array. first element is
assumed to be the original data point.
neighborhood_labels: corresponding perturbed labels. should have as
many columns as the number of possible labels.
distances: distances to original data point.
label: label for which we want an explanation
num_features: maximum number of features in explanation
feature_selection: how to select num_features. options are:
'forward_selection': iteratively add features to the model.
This is costly when num_features is high
'highest_weights': selects the features that have the highest
product of absolute weight * original data point when
learning with all the features
'lasso_path': chooses features based on the lasso
regularization path
'none': uses all features, ignores num_features
'auto': uses forward_selection if num_features <= 6, and
'highest_weights' otherwise.
model_regressor: sklearn regressor to use in explanation.
Defaults to Ridge regression if None. Must have
model_regressor.coef_ and 'sample_weight' as a parameter
to model_regressor.fit()
Returns:
(intercept, exp, score, local_pred):
intercept is a float.
exp is a sorted list of tuples, where each tuple (x,y) corresponds
to the feature id (x) and the local weight (y). The list is sorted
by decreasing absolute value of y.
score is the R^2 value of the returned explanation
local_pred is the prediction of the explanation model on the original instance
"""
weights = self.kernel_fn(distances)
labels_column = neighborhood_labels[:, label]
used_features = self.feature_selection(neighborhood_data,
labels_column,
weights,
num_features,
feature_selection)
if model_regressor is None:
model_regressor = Ridge(alpha=1, fit_intercept=True,
random_state=self.random_state)
easy_model = model_regressor
easy_model.fit(neighborhood_data[:, used_features],
labels_column, sample_weight=weights)
prediction_score = easy_model.score(
neighborhood_data[:, used_features],
labels_column, sample_weight=weights)
local_pred = easy_model.predict(neighborhood_data[0, used_features].reshape(1, -1))
if self.verbose:
print('Intercept', easy_model.intercept_)
print('Prediction_local', local_pred,)
print('Right:', neighborhood_labels[0, label])
return (easy_model.intercept_,
sorted(zip(used_features, easy_model.coef_),
key=lambda x: np.abs(x[1]), reverse=True),
prediction_score, local_pred)
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