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Large Margin Nearest Neighbor implementation in python

Project description

PyLMNN is an implementation of the Large Margin Nearest Neighbor algorithm for metric learning in pure python.

This implementation follows closely the original MATLAB code by Kilian Weinberger found at This version solves the unconstrained optimisation problem and finds a linear transformation using L-BFGS as the backend optimizer.

This package can also find optimal hyper-parameters for LMNN via Bayesian Optimization using the excellent GPyOpt package.


The code was developed in python 3.5 under Ubuntu 16.04 and was also tested under Ubuntu 18.04 and python 3.6. You can clone the repo with:

git clone

or install it via pip:

pip3 install pylmnn


  • numpy>=1.11.2
  • scipy>=0.18.1
  • scikit_learn>=0.18.1

Optional dependencies

In case you want to use the hyperparameter optimization module, you should also install:

  • GPy>=1.5.6
  • GPyOpt>=1.0.3


Here is a minimal use case:

from sklearn.neighbors import KNeighborsClassifier
from sklearn.model_selection import train_test_split
from sklearn.datasets import load_iris

from pylmnn import LargeMarginNearestNeighbor as LMNN

# Load a data set
X, y = load_iris(return_X_y=True)

# Split in training and testing set
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.7, stratify=y, random_state=42)

# Set up the hyperparameters
k_train, k_test, n_components, max_iter = 3, 3, X.shape[1], 180

# Instantiate the metric learner
lmnn = LMNN(n_neighbors=k_train, max_iter=max_iter, n_components=n_components)

# Train the metric learner, y_train)

# Fit the nearest neighbors classifier
knn = KNeighborsClassifier(n_neighbors=k_test), y_train)

# Compute the k-nearest neighbor test accuracy after applying the learned transformation
lmnn_acc = knn.score(lmnn.transform(X_test), y_test)
print('LMNN accuracy on test set of {} points: {:.4f}'.format(X_test.shape[0], lmnn_acc))

You can check the examples directory for a demonstration of how to use the code with different datasets and how to estimate good hyperparameters with Bayesian Optimisation.

Documentation can also be found at .


If you use this code in your work, please cite the following publication.

    title={Distance metric learning for large margin nearest neighbor classification},
    author={Weinberger, K.Q. and Saul, L.K.},
    journal={The Journal of Machine Learning Research},
    publisher={MIT Press}

License and Contact

This work is released under the 3-Clause BSD License.

Contact John Chiotellis :envelope: for questions, comments and reporting bugs.

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