# PyNNDescent for fast Approximate Nearest Neighbors¶

PyNNDescent is a Python nearest neighbor descent for approximate nearest neighbors. It provides a python implementation of Nearest Neighbor Descent for k-neighbor-graph construction and approximate nearest neighbor search, as per the paper:

Dong, Wei, Charikar Moses, and Kai Li. “Efficient k-nearest neighbor graph construction for generic similarity measures.” Proceedings of the 20th international conference on World wide web. ACM, 2011.

This library supplements that approach with the use of random projection trees for initialisation. This can be particularly useful for the metrics that are amenable to such approaches (euclidean, minkowski, angular, cosine, etc.). Graph diversification is also performed, pruning the longest edges of any triangles in the graph.

Currently this library targets relatively high accuracy (80%-100% accuracy rate) approximate nearest neighbor searches.

## Why use PyNNDescent?¶

PyNNDescent provides fast approximate nearest neighbor queries. The ann-benchmarks system puts it solidly in the mix of top performing ANN libraries:

SIFT-128 Euclidean

NYTimes-256 Angular

While PyNNDescent is among fastest ANN library, it is also both easy to install (pip and conda installable) with no platform or compilation issues, and is very flexible, supporting a wide variety of distance metrics by default:

Minkowski style metrics

• euclidean
• manhattan
• chebyshev
• minkowski

Miscellaneous spatial metrics

• canberra
• braycurtis
• haversine

Normalized spatial metrics

• mahalanobis
• wminkowski
• seuclidean

Angular and correlation metrics

• cosine
• dot
• correlation
• spearmanr
• tsss
• true_angular

Probability metrics

• hellinger
• wasserstein

Metrics for binary data

• hamming
• jaccard
• dice
• russelrao
• kulsinski
• rogerstanimoto
• sokalmichener
• sokalsneath
• yule

and also custom user defined distance metrics while still retaining performance.

PyNNDescent also integrates well with Scikit-learn, including providing support for the KNeighborTransformer as a drop in replacement for algorithms that make use of nearest neighbor computations.

## Installing¶

PyNNDescent is designed to be easy to install being a pure python module with relatively light requirements:

• numpy
• scipy
• scikit-learn >= 0.22
• numba >= 0.51

all of which should be pip or conda installable. The easiest way to install should be via conda:

conda install -c conda-forge pynndescent


or via pip:

pip install pynndescent


API Reference: