Background

Unsupervised clustering is ubiquitously used for the interpretation of ‘omics datasets [1,2,3,4,5,6,7]. Clustering is a particularly central challenge in the analysis of single-cell measurement data (e.g. single cell RNA-seq) due to its high dimensionality [8,9,10]. Clustering is also increasingly being used for disease subtype classification and risk stratification [11,12,13,14,15,16,17,18,19]. It is therefore essential that optimal clustering results are easily and robustly obtainable, without user-selected hyperparameters introducing bias and impeding rapid analysis.

Clustering is inherently under-defined [20,21,22]. The definition of “cluster” differs from problem to problem and the desired goal of the analysis [14], and therefore it is not possible to make a single algorithm or metric that can universally identify the “best” clusters [23]. Researchers therefore often compare results from multiple algorithms and hyperparameters [7, 24,25,26,27,28]. Typically, the effect of hyperparameter choice on the quality of clustering results cannot be described with a convex function, meaning that hyperparameters should be chosen through exhaustive grid search [29], a slow and cumbersome process. Software packages for automatic hyperparameter tuning and model selection for regression and classification exist, notably auto-sklearn from AutoML [30], and some groups have made excellent tools for distributing a single clustering calculation for huge datasets [31, 32], but to the best of our knowledge, there is no package for comparing several clustering algorithms and hyperparameters.

Here we present hypercluster, a python package and SnakeMake pipeline for rigorous, reproducible and parallelized clustering calculation and evaluation. This package allows users to compare multiple hyperparameters and algorithms, then easily visualize evaluation metrics for each result [33]. The SnakeMake pipeline allows parallelization, greatly reducing wall-clock time for users [34]. Hypercluster provides researchers with a flexible, parallelized, distributed and user-friendly method for clustering algorithm selection and hyper-parameter tuning.

Implementation

Requirements

The hypercluster package uses scikit-learn [35], python-igraph [36], leidenalg [37] and louvain-igraph [38] to assign cluster labels and uses scikit-learn and custom metrics to compare clustering algorithms and hyperparameters to find optimal clusters for any given input data (Fig. 1). Hypercluster requires python3, pandas [39], numpy [40], scipy [41], matplotlib [42], seaborn [43], scikit-learn [35], python-igraph [36], leidenalg [37], louvain-igraph [38] and SnakeMake [34].

Fig. 1
figure 1

Hypercluster workflow schematic. a Clustering algorithms and their respective hyperparameters are user-specified. Hypercluster then uses those combinations to create exhaustive configurations, and if selected a random subset is chosen. b Snakemake is then used to distribute each clustering calculation into different jobs. c Each set of clustering labels is then evaluated in a separate job by a user-specified list of metrics. d All clustering results and evaluation results are aggregated into tables. Best labels can also be chosen by a user-specified metric.

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General workflow and examples

Hypercluster can be run independently of SnakeMake, as a standalone python package. Input and output structure, as well as example workflows on a breast cancer RNA-seq data set [43] and scRNA-seq [45] can be found at https://github.com/ruggleslab/hypercluster/tree/master/examples. Briefly, the workflow starts with instantiating an AutoClusterer (for a single algorithm) or MultiAutoClusterer (for multiple algorithms) object with default or user-defined hyperparameters (Fig. 1a). To run through hyperparameters for a dataset, users simply provide a pandas DataFrame to the “fit'' method on either object (Fig. 1b). Users evaluate the labeling results with a variety of metrics by running the “evaluate” method (Fig. 1c). Clustering labels and evaluations are then aggregated into convenient tables (Fig. 1d), which can be visualized with built in functions (e.g. Additional file 1: Fig. S1, Additional file 2: Fig. S2).

Configuring the SnakeMake pipeline

The SnakeMake pipeline allows users to parallelize clustering calculations on multiple threads on a single computer, multiple compute nodes on a high performance cluster or in a cloud cluster [34]. The pipeline is configured through a config.yml file (Table 1), which contains user-specified input and output directories and files (Table 1, lines 1–3, 5–7) and the hyperparameter search space (Fig. 1a, Table 1, line 18). This file contains predefined defaults for the search space that allow the pipeline to be used “out of the box.” Further, users can specify whether to use exhaustive grid search or random search; if random search is selected, probability weights for each hyperparameter can be chosen (Table 1, line 9). The pipeline then schedules each clustering calculation and evaluation as a separate job (Fig. 1b). Users can specify which evaluation metrics to apply (Fig. 1c, Table 1, line 10) and add keyword arguments to tune several steps in the process (Table 1, lines 4, 8–9, 11–16). Clustering and evaluation results are then aggregated into final tables (Fig. 1d). Users can reference the documentation and examples for more information.

Table 1 Parameters in SnakeMake configuration file
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As input, users provide a data table with samples to be clustered as rows and features as columns. Users can then simply run “snakemake -s hypercluster.smk -configfile config.yml” in the command line, with any additional SnakeMake flags appropriate for their system. Applying the same configuration to new files or testing new algorithms on old data simply requires editing the inputs in the config.yml file and rerunning the SnakeMake command.

Extending hypercluster

Currently, hypercluster can perform any clustering algorithm and calculate any evaluation available in scikit-learn [35, 46], as well as non-negative matrix factorization (NMF) [47], Louvain [38] and Leiden [37] clustering. Additional clustering classes and evaluation metric functions can be added by users in the additional_clusterer.py and additional_metrics.py files, respectively, if written to accommodate the same input, outputs and methods (see additional_clusterers.py and additional_metrics.py for examples).

Outputs

For each set of labels, hypercluster generates a file with sample labels and a file containing evaluations of those labels. It also outputs aggregated tables of all labels and evaluations. Hypercluster can also generate several helpful visualizations, including a heatmap showing the evaluation metrics for each set of hyperparameters (Fig. 1c) and a table and heatmap of pairwise comparisons of labeling similarities with a user-specified metric (Additional file 1: Fig. S1). This visualization is particularly useful for finding labels that are robust to differences in hyperparameters. It can also optionally output a table and heatmap showing how often each pair of samples were assigned the same cluster (Additional file 2: Fig. S2). Other useful custom visualizations that are simple for users to create due to the aggregated clustering results are available in our examples (https://github.com/ruggleslab/hypercluster/tree/dev/examples).

Conclusions

Hypercluster allows comprehensive evaluation of multiple hyperparameters and clustering algorithms simultaneously, reducing the allure of biased or arbitrary parameter selection. It also aids computational biologists who are testing and benchmarking new clustering algorithms, evaluation metrics and pre- or post-processing steps [10]. Future iterations of hypercluster could include further cutting-edge clustering techniques, including those designed for larger data sets [31, 32] or account for multiple types of data [48]. Hypercluster streamlines comparative unsupervised clustering, allowing the prioritization of both convenience and rigor.

Availability and requirements

  • Project Name: Hypercluster.

  • Project homepage: https://github.com/ruggleslab/hypercluster/.

  • Operating system: Platform independent.

  • Programming Language: Python.

  • Other requirements: Hypercluster runs with the following versions or higher: python 3.7, pandas 0.24.2, numpy 1.16.4, scipy 1.2.1, matplotlib 3.1.0, seaborn 0.9.0, scikit-learn 0.22.0, hdbscan 0.8.24, snakemake 5.8.2, python-igraph 0.7.1, leidenalg 0.7.0, louvain 0.6.1

  • License: MIT license, open for use by academic and non-academic users.

  • Any restrictions to use by non-academics: Not applicable.