173 lines
6.7 KiB
Python
173 lines
6.7 KiB
Python
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# Author: Lars Buitinck
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# License: BSD 3 clause
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import numbers
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import warnings
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import numpy as np
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import scipy.sparse as sp
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from . import _hashing
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from ..base import BaseEstimator, TransformerMixin
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def _iteritems(d):
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"""Like d.iteritems, but accepts any collections.Mapping."""
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return d.iteritems() if hasattr(d, "iteritems") else d.items()
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class FeatureHasher(BaseEstimator, TransformerMixin):
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"""Implements feature hashing, aka the hashing trick.
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This class turns sequences of symbolic feature names (strings) into
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scipy.sparse matrices, using a hash function to compute the matrix column
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corresponding to a name. The hash function employed is the signed 32-bit
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version of Murmurhash3.
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Feature names of type byte string are used as-is. Unicode strings are
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converted to UTF-8 first, but no Unicode normalization is done.
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Feature values must be (finite) numbers.
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This class is a low-memory alternative to DictVectorizer and
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CountVectorizer, intended for large-scale (online) learning and situations
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where memory is tight, e.g. when running prediction code on embedded
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devices.
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Read more in the :ref:`User Guide <feature_hashing>`.
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Parameters
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----------
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n_features : integer, optional
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The number of features (columns) in the output matrices. Small numbers
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of features are likely to cause hash collisions, but large numbers
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will cause larger coefficient dimensions in linear learners.
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input_type : string, optional, default "dict"
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Either "dict" (the default) to accept dictionaries over
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(feature_name, value); "pair" to accept pairs of (feature_name, value);
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or "string" to accept single strings.
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feature_name should be a string, while value should be a number.
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In the case of "string", a value of 1 is implied.
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The feature_name is hashed to find the appropriate column for the
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feature. The value's sign might be flipped in the output (but see
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non_negative, below).
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dtype : numpy type, optional, default np.float64
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The type of feature values. Passed to scipy.sparse matrix constructors
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as the dtype argument. Do not set this to bool, np.boolean or any
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unsigned integer type.
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alternate_sign : boolean, optional, default True
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When True, an alternating sign is added to the features as to
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approximately conserve the inner product in the hashed space even for
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small n_features. This approach is similar to sparse random projection.
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non_negative : boolean, optional, default False
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When True, an absolute value is applied to the features matrix prior to
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returning it. When used in conjunction with alternate_sign=True, this
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significantly reduces the inner product preservation property.
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.. deprecated:: 0.19
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This option will be removed in 0.21.
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Examples
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--------
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>>> from sklearn.feature_extraction import FeatureHasher
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>>> h = FeatureHasher(n_features=10)
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>>> D = [{'dog': 1, 'cat':2, 'elephant':4},{'dog': 2, 'run': 5}]
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>>> f = h.transform(D)
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>>> f.toarray()
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array([[ 0., 0., -4., -1., 0., 0., 0., 0., 0., 2.],
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[ 0., 0., 0., -2., -5., 0., 0., 0., 0., 0.]])
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See also
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--------
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DictVectorizer : vectorizes string-valued features using a hash table.
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sklearn.preprocessing.OneHotEncoder : handles nominal/categorical features
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encoded as columns of integers.
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"""
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def __init__(self, n_features=(2 ** 20), input_type="dict",
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dtype=np.float64, alternate_sign=True, non_negative=False):
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self._validate_params(n_features, input_type)
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if non_negative:
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warnings.warn("the option non_negative=True has been deprecated"
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" in 0.19 and will be removed"
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" in version 0.21.", DeprecationWarning)
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self.dtype = dtype
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self.input_type = input_type
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self.n_features = n_features
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self.alternate_sign = alternate_sign
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self.non_negative = non_negative
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@staticmethod
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def _validate_params(n_features, input_type):
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# strangely, np.int16 instances are not instances of Integral,
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# while np.int64 instances are...
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if not isinstance(n_features, (numbers.Integral, np.integer)):
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raise TypeError("n_features must be integral, got %r (%s)."
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% (n_features, type(n_features)))
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elif n_features < 1 or n_features >= 2 ** 31:
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raise ValueError("Invalid number of features (%d)." % n_features)
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if input_type not in ("dict", "pair", "string"):
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raise ValueError("input_type must be 'dict', 'pair' or 'string',"
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" got %r." % input_type)
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def fit(self, X=None, y=None):
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"""No-op.
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This method doesn't do anything. It exists purely for compatibility
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with the scikit-learn transformer API.
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Parameters
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----------
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X : array-like
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Returns
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-------
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self : FeatureHasher
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"""
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# repeat input validation for grid search (which calls set_params)
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self._validate_params(self.n_features, self.input_type)
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return self
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def transform(self, raw_X):
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"""Transform a sequence of instances to a scipy.sparse matrix.
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Parameters
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----------
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raw_X : iterable over iterable over raw features, length = n_samples
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Samples. Each sample must be iterable an (e.g., a list or tuple)
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containing/generating feature names (and optionally values, see
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the input_type constructor argument) which will be hashed.
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raw_X need not support the len function, so it can be the result
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of a generator; n_samples is determined on the fly.
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Returns
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-------
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X : scipy.sparse matrix, shape = (n_samples, self.n_features)
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Feature matrix, for use with estimators or further transformers.
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"""
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raw_X = iter(raw_X)
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if self.input_type == "dict":
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raw_X = (_iteritems(d) for d in raw_X)
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elif self.input_type == "string":
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raw_X = (((f, 1) for f in x) for x in raw_X)
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indices, indptr, values = \
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_hashing.transform(raw_X, self.n_features, self.dtype,
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self.alternate_sign)
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n_samples = indptr.shape[0] - 1
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if n_samples == 0:
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raise ValueError("Cannot vectorize empty sequence.")
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X = sp.csr_matrix((values, indices, indptr), dtype=self.dtype,
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shape=(n_samples, self.n_features))
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X.sum_duplicates() # also sorts the indices
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if self.non_negative:
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np.abs(X.data, X.data)
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return X
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