# Author: Lars Buitinck # License: BSD 3 clause from numbers import Integral from itertools import chain import numpy as np import scipy.sparse as sp from ..base import BaseEstimator, TransformerMixin from ._hashing_fast import transform as _hashing_transform from ..utils._param_validation import Interval, StrOptions def _iteritems(d): """Like d.iteritems, but accepts any collections.Mapping.""" return d.iteritems() if hasattr(d, "iteritems") else d.items() class FeatureHasher(TransformerMixin, BaseEstimator): """Implements feature hashing, aka the hashing trick. This class turns sequences of symbolic feature names (strings) into scipy.sparse matrices, using a hash function to compute the matrix column corresponding to a name. The hash function employed is the signed 32-bit version of Murmurhash3. Feature names of type byte string are used as-is. Unicode strings are converted to UTF-8 first, but no Unicode normalization is done. Feature values must be (finite) numbers. This class is a low-memory alternative to DictVectorizer and CountVectorizer, intended for large-scale (online) learning and situations where memory is tight, e.g. when running prediction code on embedded devices. Read more in the :ref:`User Guide `. .. versionadded:: 0.13 Parameters ---------- n_features : int, default=2**20 The number of features (columns) in the output matrices. Small numbers of features are likely to cause hash collisions, but large numbers will cause larger coefficient dimensions in linear learners. input_type : str, default='dict' Choose a string from {'dict', 'pair', 'string'}. Either "dict" (the default) to accept dictionaries over (feature_name, value); "pair" to accept pairs of (feature_name, value); or "string" to accept single strings. feature_name should be a string, while value should be a number. In the case of "string", a value of 1 is implied. The feature_name is hashed to find the appropriate column for the feature. The value's sign might be flipped in the output (but see non_negative, below). dtype : numpy dtype, default=np.float64 The type of feature values. Passed to scipy.sparse matrix constructors as the dtype argument. Do not set this to bool, np.boolean or any unsigned integer type. alternate_sign : bool, default=True When True, an alternating sign is added to the features as to approximately conserve the inner product in the hashed space even for small n_features. This approach is similar to sparse random projection. .. versionchanged:: 0.19 ``alternate_sign`` replaces the now deprecated ``non_negative`` parameter. See Also -------- DictVectorizer : Vectorizes string-valued features using a hash table. sklearn.preprocessing.OneHotEncoder : Handles nominal/categorical features. Notes ----- This estimator is :term:`stateless` and does not need to be fitted. However, we recommend to call :meth:`fit_transform` instead of :meth:`transform`, as parameter validation is only performed in :meth:`fit`. Examples -------- >>> from sklearn.feature_extraction import FeatureHasher >>> h = FeatureHasher(n_features=10) >>> D = [{'dog': 1, 'cat':2, 'elephant':4},{'dog': 2, 'run': 5}] >>> f = h.transform(D) >>> f.toarray() array([[ 0., 0., -4., -1., 0., 0., 0., 0., 0., 2.], [ 0., 0., 0., -2., -5., 0., 0., 0., 0., 0.]]) With `input_type="string"`, the input must be an iterable over iterables of strings: >>> h = FeatureHasher(n_features=8, input_type="string") >>> raw_X = [["dog", "cat", "snake"], ["snake", "dog"], ["cat", "bird"]] >>> f = h.transform(raw_X) >>> f.toarray() array([[ 0., 0., 0., -1., 0., -1., 0., 1.], [ 0., 0., 0., -1., 0., -1., 0., 0.], [ 0., -1., 0., 0., 0., 0., 0., 1.]]) """ _parameter_constraints: dict = { "n_features": [Interval(Integral, 1, np.iinfo(np.int32).max, closed="both")], "input_type": [StrOptions({"dict", "pair", "string"})], "dtype": "no_validation", # delegate to numpy "alternate_sign": ["boolean"], } def __init__( self, n_features=(2**20), *, input_type="dict", dtype=np.float64, alternate_sign=True, ): self.dtype = dtype self.input_type = input_type self.n_features = n_features self.alternate_sign = alternate_sign def fit(self, X=None, y=None): """Only validates estimator's parameters. This method allows to: (i) validate the estimator's parameters and (ii) be consistent with the scikit-learn transformer API. Parameters ---------- X : Ignored Not used, present here for API consistency by convention. y : Ignored Not used, present here for API consistency by convention. Returns ------- self : object FeatureHasher class instance. """ # repeat input validation for grid search (which calls set_params) self._validate_params() return self def transform(self, raw_X): """Transform a sequence of instances to a scipy.sparse matrix. Parameters ---------- raw_X : iterable over iterable over raw features, length = n_samples Samples. Each sample must be iterable an (e.g., a list or tuple) containing/generating feature names (and optionally values, see the input_type constructor argument) which will be hashed. raw_X need not support the len function, so it can be the result of a generator; n_samples is determined on the fly. Returns ------- X : sparse matrix of shape (n_samples, n_features) Feature matrix, for use with estimators or further transformers. """ raw_X = iter(raw_X) if self.input_type == "dict": raw_X = (_iteritems(d) for d in raw_X) elif self.input_type == "string": first_raw_X = next(raw_X) if isinstance(first_raw_X, str): raise ValueError( "Samples can not be a single string. The input must be an iterable" " over iterables of strings." ) raw_X_ = chain([first_raw_X], raw_X) raw_X = (((f, 1) for f in x) for x in raw_X_) indices, indptr, values = _hashing_transform( raw_X, self.n_features, self.dtype, self.alternate_sign, seed=0 ) n_samples = indptr.shape[0] - 1 if n_samples == 0: raise ValueError("Cannot vectorize empty sequence.") X = sp.csr_matrix( (values, indices, indptr), dtype=self.dtype, shape=(n_samples, self.n_features), ) X.sum_duplicates() # also sorts the indices return X def _more_tags(self): return {"X_types": [self.input_type]}