GriSPy: A Python package for fixed-radius nearest neighbors search

We present a new regular grid search algorithm for quick fixed-radius nearest-neighbor lookup developed in Python. This module indexes a set of k-dimensional points in a regular grid, with optional periodic conditions, providing a fast approach for nearest neighbors queries. In this first installment, we provide three types of queries: bubble, shell and the nth-nearest. For these queries we include three different metrics of interest in astronomy, namely, the euclidean, the haversine and the Vincenty, the last two in spherical coordinates. We also provide the possibility of using a custom distance function. This package results particularly useful for large datasets where a brute-force search turns impractical.