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class sklearn.preprocessing.OneHotEncoder(n_values='auto', categorical_features='all', dtype=<type 'float'>, sparse=True, handle_unknown='error')[source]

Encode categorical integer features using a one-hot aka one-of-K scheme.

The input to this transformer should be a matrix of integers, denoting the values taken on by categorical (discrete) features. The output will be a sparse matrix where each column corresponds to one possible value of one feature. It is assumed that input features take on values in the range [0, n_values).

This encoding is needed for feeding categorical data to many scikit-learn estimators, notably linear models and SVMs with the standard kernels.


n_values : ‘auto’, int or array of ints

Number of values per feature.

  • ‘auto’ : determine value range from training data.
  • int : maximum value for all features.
  • array : maximum value per feature.

categorical_features: “all” or array of indices or mask :

Specify what features are treated as categorical.

  • ‘all’ (default): All features are treated as categorical.
  • array of indices: Array of categorical feature indices.
  • mask: Array of length n_features and with dtype=bool.

Non-categorical features are always stacked to the right of the matrix.

dtype : number type, default=np.float

Desired dtype of output.

sparse : boolean, default=True

Will return sparse matrix if set True else will return an array.

handle_unknown : str, ‘error’ or ‘ignore’

Whether to raise an error or ignore if a unknown categorical feature is present during transform.


active_features_ : array

Indices for active features, meaning values that actually occur in the training set. Only available when n_values is 'auto'.

feature_indices_ : array of shape (n_features,)

Indices to feature ranges. Feature i in the original data is mapped to features from feature_indices_[i] to feature_indices_[i+1] (and then potentially masked by active_features_ afterwards)

n_values_ : array of shape (n_features,)

Maximum number of values per feature.

See also

performs a one-hot encoding of dictionary items (also handles string-valued features).
performs an approximate one-hot encoding of dictionary items or strings.


Given a dataset with three features and two samples, we let the encoder find the maximum value per feature and transform the data to a binary one-hot encoding.

>>> from sklearn.preprocessing import OneHotEncoder
>>> enc = OneHotEncoder()
>>>[[0, 0, 3], [1, 1, 0], [0, 2, 1], [1, 0, 2]])  
OneHotEncoder(categorical_features='all', dtype=<... 'float'>,
       handle_unknown='error', n_values='auto', sparse=True)
>>> enc.n_values_
array([2, 3, 4])
>>> enc.feature_indices_
array([0, 2, 5, 9])
>>> enc.transform([[0, 1, 1]]).toarray()
array([[ 1.,  0.,  0.,  1.,  0.,  0.,  1.,  0.,  0.]])


fit(X[, y]) Fit OneHotEncoder to X.
fit_transform(X[, y]) Fit OneHotEncoder to X, then transform X.
get_params([deep]) Get parameters for this estimator.
set_params(**params) Set the parameters of this estimator.
transform(X) Transform X using one-hot encoding.
__init__(n_values='auto', categorical_features='all', dtype=<type 'float'>, sparse=True, handle_unknown='error')[source]
fit(X, y=None)[source]

Fit OneHotEncoder to X.


X : array-like, shape=(n_samples, n_feature)

Input array of type int.


self :

fit_transform(X, y=None)[source]

Fit OneHotEncoder to X, then transform X.

Equivalent to, but more convenient and more efficient. See fit for the parameters, transform for the return value.


Get parameters for this estimator.


deep: boolean, optional :

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


params : mapping of string to any

Parameter names mapped to their values.


Set the parameters of this estimator.

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

Returns:self :

Transform X using one-hot encoding.


X : array-like, shape=(n_samples, n_features)

Input array of type int.


X_out : sparse matrix if sparse=True else a 2-d array, dtype=int

Transformed input.