HashingVectorizer#

class sklearn.feature_extraction.text.HashingVectorizer(*, input='content', encoding='utf-8', decode_error='strict', strip_accents=None, lowercase=True, preprocessor=None, tokenizer=None, stop_words=None, token_pattern='(?u)\\b\\w\\w+\\b', ngram_range=(1, 1), analyzer='word', n_features=1048576, binary=False, norm='l2', alternate_sign=True, dtype=<class 'numpy.float64'>)[source]#

Convert a collection of text documents to a matrix of token occurrences.

It turns a collection of text documents into a scipy.sparse matrix holding token occurrence counts (or binary occurrence information), possibly normalized as token frequencies if norm=’l1’ or projected on the euclidean unit sphere if norm=’l2’.

This text vectorizer implementation uses the hashing trick to find the token string name to feature integer index mapping.

This strategy has several advantages:

  • it is very low memory scalable to large datasets as there is no need to store a vocabulary dictionary in memory.

  • it is fast to pickle and un-pickle as it holds no state besides the constructor parameters.

  • it can be used in a streaming (partial fit) or parallel pipeline as there is no state computed during fit.

There are also a couple of cons (vs using a CountVectorizer with an in-memory vocabulary):

  • there is no way to compute the inverse transform (from feature indices to string feature names) which can be a problem when trying to introspect which features are most important to a model.

  • there can be collisions: distinct tokens can be mapped to the same feature index. However in practice this is rarely an issue if n_features is large enough (e.g. 2 ** 18 for text classification problems).

  • no IDF weighting as this would render the transformer stateful.

The hash function employed is the signed 32-bit version of Murmurhash3.

For an efficiency comparison of the different feature extractors, see FeatureHasher and DictVectorizer Comparison.

For an example of document clustering and comparison with TfidfVectorizer, see Clustering text documents using k-means.

Read more in the User Guide.

Parameters:
input{‘filename’, ‘file’, ‘content’}, default=’content’

  • If 'filename', the sequence passed as an argument to fit is expected to be a list of filenames that need reading to fetch the raw content to analyze.

  • If 'file', the sequence items must have a ‘read’ method (file-like object) that is called to fetch the bytes in memory.

  • If 'content', the input is expected to be a sequence of items that can be of type string or byte.

encodingstr, default=’utf-8’

If bytes or files are given to analyze, this encoding is used to decode.

decode_error{‘strict’, ‘ignore’, ‘replace’}, default=’strict’

Instruction on what to do if a byte sequence is given to analyze that contains characters not of the given encoding. By default, it is ‘strict’, meaning that a UnicodeDecodeError will be raised. Other values are ‘ignore’ and ‘replace’.

strip_accents{‘ascii’, ‘unicode’} or callable, default=None

Remove accents and perform other character normalization during the preprocessing step. ‘ascii’ is a fast method that only works on characters that have a direct ASCII mapping. ‘unicode’ is a slightly slower method that works on any character. None (default) means no character normalization is performed.

Both ‘ascii’ and ‘unicode’ use NFKD normalization from unicodedata.normalize.

lowercasebool, default=True

Convert all characters to lowercase before tokenizing.

preprocessorcallable, default=None

Override the preprocessing (string transformation) stage while preserving the tokenizing and n-grams generation steps. Only applies if analyzer is not callable.

tokenizercallable, default=None

Override the string tokenization step while preserving the preprocessing and n-grams generation steps. Only applies if analyzer == 'word'.

stop_words{‘english’}, list, default=None

If ‘english’, a built-in stop word list for English is used. There are several known issues with ‘english’ and you should consider an alternative (see Using stop words).

If a list, that list is assumed to contain stop words, all of which will be removed from the resulting tokens. Only applies if analyzer == 'word'.

token_patternstr or None, default=r”(?u)\b\w\w+\b”

Regular expression denoting what constitutes a “token”, only used if analyzer == 'word'. The default regexp selects tokens of 2 or more alphanumeric characters (punctuation is completely ignored and always treated as a token separator).

If there is a capturing group in token_pattern then the captured group content, not the entire match, becomes the token. At most one capturing group is permitted.

ngram_rangetuple (min_n, max_n), default=(1, 1)

The lower and upper boundary of the range of n-values for different n-grams to be extracted. All values of n such that min_n <= n <= max_n will be used. For example an ngram_range of (1, 1) means only unigrams, (1, 2) means unigrams and bigrams, and (2, 2) means only bigrams. Only applies if analyzer is not callable.

analyzer{‘word’, ‘char’, ‘char_wb’} or callable, default=’word’

Whether the feature should be made of word or character n-grams. Option ‘char_wb’ creates character n-grams only from text inside word boundaries; n-grams at the edges of words are padded with space.

If a callable is passed it is used to extract the sequence of features out of the raw, unprocessed input.

Changed in version 0.21: Since v0.21, if input is 'filename' or 'file', the data is first read from the file and then passed to the given callable analyzer.

n_featuresint, 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.

binarybool, default=False

If True, all non zero counts are set to 1. This is useful for discrete probabilistic models that model binary events rather than integer counts.

norm{‘l1’, ‘l2’}, default=’l2’

Norm used to normalize term vectors. None for no normalization.

alternate_signbool, 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.

Added in version 0.19.

dtypetype, default=np.float64

Type of the matrix returned by fit_transform() or transform().

See also

CountVectorizer

Convert a collection of text documents to a matrix of token counts.

TfidfVectorizer

Convert a collection of raw documents to a matrix of TF-IDF features.

Notes

This estimator is stateless and does not need to be fitted. However, we recommend to call fit_transform instead of transform, as parameter validation is only performed in fit.

Examples

>>> from sklearn.feature_extraction.text import HashingVectorizer
>>> corpus = [
...     'This is the first document.',
...     'This document is the second document.',
...     'And this is the third one.',
...     'Is this the first document?',
... ]
>>> vectorizer = HashingVectorizer(n_features=2**4)
>>> X = vectorizer.fit_transform(corpus)
>>> print(X.shape)
(4, 16)
build_analyzer()[source]#

Return a callable to process input data.

The callable handles preprocessing, tokenization, and n-grams generation.

Returns:
analyzer: callable

A function to handle preprocessing, tokenization and n-grams generation.

build_preprocessor()[source]#

Return a function to preprocess the text before tokenization.

Returns:
preprocessor: callable

A function to preprocess the text before tokenization.

build_tokenizer()[source]#

Return a function that splits a string into a sequence of tokens.

Returns:
tokenizer: callable

A function to split a string into a sequence of tokens.

decode(doc)[source]#

Decode the input into a string of unicode symbols.

The decoding strategy depends on the vectorizer parameters.

Parameters:
docbytes or str

The string to decode.

Returns:
doc: str

A string of unicode symbols.

fit(X, y=None)[source]#

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:
Xndarray of shape [n_samples, n_features]

Training data.

yIgnored

Not used, present for API consistency by convention.

Returns:
selfobject

HashingVectorizer instance.

fit_transform(X, y=None)[source]#

Transform a sequence of documents to a document-term matrix.

Parameters:
Xiterable over raw text documents, length = n_samples

Samples. Each sample must be a text document (either bytes or unicode strings, file name or file object depending on the constructor argument) which will be tokenized and hashed.

yany

Ignored. This parameter exists only for compatibility with sklearn.pipeline.Pipeline.

Returns:
Xsparse matrix of shape (n_samples, n_features)

Document-term matrix.

get_metadata_routing()[source]#

Get metadata routing of this object.

Please check User Guide on how the routing mechanism works.

Returns:
routingMetadataRequest

A MetadataRequest encapsulating routing information.

get_params(deep=True)[source]#

Get parameters for this estimator.

Parameters:
deepbool, default=True

If True, will return the parameters for this estimator and contained subobjects that are estimators.

Returns:
paramsdict

Parameter names mapped to their values.

get_stop_words()[source]#

Build or fetch the effective stop words list.

Returns:
stop_words: list or None

A list of stop words.

partial_fit(X, y=None)[source]#

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:
Xndarray of shape [n_samples, n_features]

Training data.

yIgnored

Not used, present for API consistency by convention.

Returns:
selfobject

HashingVectorizer instance.

set_output(*, transform=None)[source]#

Set output container.

See Introducing the set_output API for an example on how to use the API.

Parameters:
transform{“default”, “pandas”, “polars”}, default=None

Configure output of transform and fit_transform.

  • "default": Default output format of a transformer

  • "pandas": DataFrame output

  • "polars": Polars output

  • None: Transform configuration is unchanged

Added in version 1.4: "polars" option was added.

Returns:
selfestimator instance

Estimator instance.

set_params(**params)[source]#

Set the parameters of this estimator.

The method works on simple estimators as well as on nested objects (such as Pipeline). The latter have parameters of the form <component>__<parameter> so that it’s possible to update each component of a nested object.

Parameters:
**paramsdict

Estimator parameters.

Returns:
selfestimator instance

Estimator instance.

transform(X)[source]#

Transform a sequence of documents to a document-term matrix.

Parameters:
Xiterable over raw text documents, length = n_samples

Samples. Each sample must be a text document (either bytes or unicode strings, file name or file object depending on the constructor argument) which will be tokenized and hashed.

Returns:
Xsparse matrix of shape (n_samples, n_features)

Document-term matrix.

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