python中的sklearn.feature_extraction dictvectorizer

Transforms lists of feature-value mappings to vectors.

This transformer turns lists of mappings (dict-like objects) of feature names to feature values into Numpy arrays or scipy.sparse matrices for use with scikit-learn estimators.

When feature values are strings, this transformer will do a binary one-hot (aka one-of-K) coding: one boolean-valued feature is constructed for each of the possible string values that the feature can take on. For instance, a feature “f” that can take on the values “ham” and “spam” will become two features in the output, one signifying “f=ham”, the other “f=spam”.

However, note that this transformer will only do a binary one-hot encoding when feature values are of type string. If categorical features are represented as numeric values such as int, the DictVectorizer can be followed by OneHotEncoder to complete binary one-hot encoding.

Features that do not occur in a sample (mapping) will have a zero value in the resulting array/matrix.

Read more in the User Guide.

See also

FeatureHasher

performs vectorization using only a hash function.

sklearn.preprocessing.OneHotEncoder

handles nominal/categorical features encoded as columns of integers.

Examples

>>> from sklearn.feature_extraction import DictVectorizer>>> v = DictVectorizer(sparse=False)>>> D = [{'foo': 1, 'bar': 2}, {'foo': 3, 'baz': 1}]>>> X = v.fit_transform(D)>>> Xarray([[ 2.,  0.,  1.],       [ 0.,  1.,  3.]])>>> v.inverse_transform(X) ==         [{'bar': 2.0, 'foo': 1.0}, {'baz': 1.0, 'foo': 3.0}]True>>> v.transform({'foo': 4, 'unseen_feature': 3})array([[ 0.,  0.,  4.]])

Methods

__init__ ( dtype=<type 'numpy.float64'>,  separator='=',  sparse=True,  sort=True ) [source]

fit ( X,  y=None ) [source]

Learn a list of feature name -> indices mappings.

Parameters:	X : Mapping or iterable over Mappings Dict(s) or Mapping(s) from feature names (arbitrary Python objects) to feature values (strings or convertible to dtype). y : (ignored)
Returns:	self :

fit_transform ( X,  y=None ) [source]

Learn a list of feature name -> indices mappings and transform X.

Like fit(X) followed by transform(X), but does not require materializing X in memory.

Parameters:	X : Mapping or iterable over Mappings Dict(s) or Mapping(s) from feature names (arbitrary Python objects) to feature values (strings or convertible to dtype). y : (ignored)
Returns:	Xa : {array, sparse matrix} Feature vectors; always 2-d.

get_feature_names ( ) [source]

Returns a list of feature names, ordered by their indices.

If one-of-K coding is applied to categorical features, this will include the constructed feature names but not the original ones.

get_params ( deep=True ) [source]

Get parameters for this estimator.

Parameters:	deep : boolean, optional If True, will return the parameters for this estimator and contained subobjects that are estimators.
Returns:	params : mapping of string to any Parameter names mapped to their values.

inverse_transform ( X,  dict_type=<type 'dict'> ) [source]

Transform array or sparse matrix X back to feature mappings.

X must have been produced by this DictVectorizer’s transform or fit_transform method; it may only have passed through transformers that preserve the number of features and their order.

In the case of one-hot/one-of-K coding, the constructed feature names and values are returned rather than the original ones.

Parameters:	X : {array-like, sparse matrix}, shape = [n_samples, n_features] Sample matrix. dict_type : callable, optional Constructor for feature mappings. Must conform to the collections.Mapping API.
Returns:	D : list of dict_type objects, length = n_samples Feature mappings for the samples in X.

restrict ( support,  indices=False ) [source]

Restrict the features to those in support using feature selection.

This function modifies the estimator in-place.

Parameters:	support : array-like Boolean mask or list of indices (as returned by the get_support member of feature selectors). indices : boolean, optional Whether support is a list of indices.
Returns:	self :

Examples

>>> from sklearn.feature_extraction import DictVectorizer>>> from sklearn.feature_selection import SelectKBest, chi2>>> v = DictVectorizer()>>> D = [{'foo': 1, 'bar': 2}, {'foo': 3, 'baz': 1}]>>> X = v.fit_transform(D)>>> support = SelectKBest(chi2, k=2).fit(X, [0, 1])>>> v.get_feature_names()['bar', 'baz', 'foo']>>> v.restrict(support.get_support())DictVectorizer(dtype=..., separator='=', sort=True,        sparse=True)>>> v.get_feature_names()['bar', 'foo']

set_params ( **params ) [source]

Set the parameters of this estimator.

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

Returns:	self :

transform ( X,  y=None ) [source]

Transform feature->value dicts to array or sparse matrix.

Named features not encountered during fit or fit_transform will be silently ignored.

Parameters:	X : Mapping or iterable over Mappings, length = n_samples Dict(s) or Mapping(s) from feature names (arbitrary Python objects) to feature values (strings or convertible to dtype). y : (ignored)
Returns:	Xa : {array, sparse matrix} Feature vectors; always 2-d. 来源： http://scikit-learn/stable/modules/generated/sklearn.feature_extraction.DictVectorizer.html