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# -*- coding: utf-8 -*-
"""Generic feature selection mixin"""
# Authors: G. Varoquaux, A. Gramfort, L. Buitinck, J. Nothman
# License: BSD 3 clause
from abc import ABCMeta, abstractmethod
from warnings import warn
import numpy as np
from scipy.sparse import issparse, csc_matrix
from ..base import TransformerMixin
from ..utils import check_array, safe_mask
from ..externals import six
class SelectorMixin(six.with_metaclass(ABCMeta, TransformerMixin)):
"""
Transformer mixin that performs feature selection given a support mask
This mixin provides a feature selector implementation with `transform` and
`inverse_transform` functionality given an implementation of
`_get_support_mask`.
"""
def get_support(self, indices=False):
"""
Get a mask, or integer index, of the features selected
Parameters
----------
indices : boolean (default False)
If True, the return value will be an array of integers, rather
than a boolean mask.
Returns
-------
support : array
An index that selects the retained features from a feature vector.
If `indices` is False, this is a boolean array of shape
[# input features], in which an element is True iff its
corresponding feature is selected for retention. If `indices` is
True, this is an integer array of shape [# output features] whose
values are indices into the input feature vector.
"""
mask = self._get_support_mask()
return mask if not indices else np.where(mask)[0]
@abstractmethod
def _get_support_mask(self):
"""
Get the boolean mask indicating which features are selected
Returns
-------
support : boolean array of shape [# input features]
An element is True iff its corresponding feature is selected for
retention.
"""
def transform(self, X):
"""Reduce X to the selected features.
Parameters
----------
X : array of shape [n_samples, n_features]
The input samples.
Returns
-------
X_r : array of shape [n_samples, n_selected_features]
The input samples with only the selected features.
"""
X = check_array(X, dtype=None, accept_sparse='csr')
mask = self.get_support()
if not mask.any():
warn("No features were selected: either the data is"
" too noisy or the selection test too strict.",
UserWarning)
return np.empty(0).reshape((X.shape[0], 0))
if len(mask) != X.shape[1]:
raise ValueError("X has a different shape than during fitting.")
return X[:, safe_mask(X, mask)]
def inverse_transform(self, X):
"""
Reverse the transformation operation
Parameters
----------
X : array of shape [n_samples, n_selected_features]
The input samples.
Returns
-------
X_r : array of shape [n_samples, n_original_features]
`X` with columns of zeros inserted where features would have
been removed by `transform`.
"""
if issparse(X):
X = X.tocsc()
# insert additional entries in indptr:
# e.g. if transform changed indptr from [0 2 6 7] to [0 2 3]
# col_nonzeros here will be [2 0 1] so indptr becomes [0 2 2 3]
it = self.inverse_transform(np.diff(X.indptr).reshape(1, -1))
col_nonzeros = it.ravel()
indptr = np.concatenate([[0], np.cumsum(col_nonzeros)])
Xt = csc_matrix((X.data, X.indices, indptr),
shape=(X.shape[0], len(indptr) - 1), dtype=X.dtype)
return Xt
support = self.get_support()
X = check_array(X, dtype=None)
if support.sum() != X.shape[1]:
raise ValueError("X has a different shape than during fitting.")
if X.ndim == 1:
X = X[None, :]
Xt = np.zeros((X.shape[0], support.size), dtype=X.dtype)
Xt[:, support] = X
return Xt
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