class imblearn.under_sampling.AllKNN(*, sampling_strategy='auto', n_neighbors=3, kind_sel='all', allow_minority=False, n_jobs=None)[source]#

Undersample based on the AllKNN method.

This method will apply EditedNearestNeighbours several times varying the number of nearest neighbours at each round. It begins by examining 1 closest neighbour, and it incrases the neighbourhood by 1 at each round.

The algorithm stops when the maximum number of neighbours are examined or when the majority class becomes the minority class, whichever comes first.

Read more in the User Guide.

sampling_strategystr, list or callable

Sampling information to sample the data set.

  • When str, specify the class targeted by the resampling. Note the the number of samples will not be equal in each. Possible choices are:

    'majority': resample only the majority class;

    'not minority': resample all classes but the minority class;

    'not majority': resample all classes but the majority class;

    'all': resample all classes;

    'auto': equivalent to 'not minority'.

  • When list, the list contains the classes targeted by the resampling.

  • When callable, function taking y and returns a dict. The keys correspond to the targeted classes. The values correspond to the desired number of samples for each class.

n_neighborsint or estimator object, default=3

If int, size of the maximum neighbourhood to examine for the undersampling. If n_neighbors=3, in the first iteration the algorithm will examine 1 closest neigbhour, in the second round 2, and in the final round 3. If object, an estimator that inherits from KNeighborsMixin that will be used to find the nearest-neighbors. Note that if you want to examine the 3 closest neighbours of a sample, you need to pass a 4-KNN.

kind_sel{‘all’, ‘mode’}, default=’all’

Strategy to use to exclude samples.

  • If 'all', all neighbours should be of the same class of the examined sample for it not be excluded.

  • If 'mode', most neighbours should be of the same class of the examined sample for it not be excluded.

The strategy "all" will be less conservative than 'mode'. Thus, more samples will be removed when kind_sel="all", generally.

allow_minoritybool, default=False

If True, it allows the majority classes to become the minority class without early stopping.

New in version 0.3.

n_jobsint, default=None

Number of CPU cores used during the cross-validation loop. None means 1 unless in a joblib.parallel_backend context. -1 means using all processors. See Glossary for more details.


Dictionary containing the information to sample the dataset. The keys correspond to the class labels from which to sample and the values are the number of samples to sample.

nn_estimator object

Validated K-nearest Neighbours estimator linked to the parameter n_neighbors.

enn_sampler object

The validated EditedNearestNeighbours instance.

sample_indices_ndarray of shape (n_new_samples,)

Indices of the samples selected.

New in version 0.4.


Number of features in the input dataset.

New in version 0.9.

feature_names_in_ndarray of shape (n_features_in_,)

Names of features seen during fit. Defined only when X has feature names that are all strings.

New in version 0.10.

See also


Under-sampling by condensing samples.


Under-sampling by editing samples.


Under-sampling by repeating ENN.


The method is based on [1].

Supports multi-class resampling. A one-vs.-rest scheme is used when sampling a class as proposed in [1].


[1] (1,2)

I. Tomek, “An Experiment with the Edited Nearest-Neighbor Rule,” IEEE Transactions on Systems, Man, and Cybernetics, vol. 6(6), pp. 448-452, June 1976.


>>> from collections import Counter
>>> from sklearn.datasets import make_classification
>>> from imblearn.under_sampling import AllKNN
>>> X, y = make_classification(n_classes=2, class_sep=2,
... weights=[0.1, 0.9], n_informative=3, n_redundant=1, flip_y=0,
... n_features=20, n_clusters_per_class=1, n_samples=1000, random_state=10)
>>> print('Original dataset shape %s' % Counter(y))
Original dataset shape Counter({1: 900, 0: 100})
>>> allknn = AllKNN()
>>> X_res, y_res = allknn.fit_resample(X, y)
>>> print('Resampled dataset shape %s' % Counter(y_res))
Resampled dataset shape Counter({1: 887, 0: 100})


fit(X, y)

Check inputs and statistics of the sampler.

fit_resample(X, y)

Resample the dataset.


Get output feature names for transformation.


Get metadata routing of this object.


Get parameters for this estimator.


Set the parameters of this estimator.

fit(X, y)[source]#

Check inputs and statistics of the sampler.

You should use fit_resample in all cases.

X{array-like, dataframe, sparse matrix} of shape (n_samples, n_features)

Data array.

yarray-like of shape (n_samples,)

Target array.


Return the instance itself.

fit_resample(X, y)[source]#

Resample the dataset.

X{array-like, dataframe, sparse matrix} of shape (n_samples, n_features)

Matrix containing the data which have to be sampled.

yarray-like of shape (n_samples,)

Corresponding label for each sample in X.

X_resampled{array-like, dataframe, sparse matrix} of shape (n_samples_new, n_features)

The array containing the resampled data.

y_resampledarray-like of shape (n_samples_new,)

The corresponding label of X_resampled.


Get output feature names for transformation.

input_featuresarray-like of str or None, default=None

Input features.

  • If input_features is None, then feature_names_in_ is used as feature names in. If feature_names_in_ is not defined, then the following input feature names are generated: ["x0", "x1", ..., "x(n_features_in_ - 1)"].

  • If input_features is an array-like, then input_features must match feature_names_in_ if feature_names_in_ is defined.

feature_names_outndarray of str objects

Same as input features.


Get metadata routing of this object.

Please check User Guide on how the routing mechanism works.


A MetadataRequest encapsulating routing information.


Get parameters for this estimator.

deepbool, default=True

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


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 Pipeline). The latter have parameters of the form <component>__<parameter> so that it’s possible to update each component of a nested object.


Estimator parameters.

selfestimator instance

Estimator instance.

Examples using imblearn.under_sampling.AllKNN#

Compare under-sampling samplers

Compare under-sampling samplers