Alterlab Zarr

---
name: alterlab-zarr
description: Chunked, compressed N-dimensional arrays for cloud storage with Zarr — parallel I/O, S3/GCS integration, and NumPy/Dask/Xarray compatibility. Use when storing or reading large N-D scientific arrays, streaming chunked data to/from cloud object stores, or building large-scale scientific computing pipelines. Part of the AlterLab Academic Skills suite.
license: MIT
allowed-tools: Read Write Edit Bash(python:*) Bash(uv:*)
compatibility: No API key required. Runs locally via `uv run python`; requires the zarr Python package (cloud credentials only needed for S3/GCS object stores).
metadata:
    skill-author: AlterLab
    version: "1.0.0"
---

# Zarr Python

## Overview

Zarr is a Python library for storing large N-dimensional arrays with chunking and compression. Apply this skill for efficient parallel I/O, cloud-native workflows, and seamless integration with NumPy, Dask, and Xarray.

## Quick Start

### Installation

```bash
uv pip install zarr
```

Requires Python 3.11+ and Zarr v3 (`zarr>=3`). For cloud storage support, install the matching
fsspec backend:
```bash
uv pip install s3fs   # For S3
uv pip install gcsfs  # For Google Cloud Storage
```

### Basic Array Creation

```python
import zarr
import numpy as np

# Create a 2D array with chunking and compression
z = zarr.create_array(
    store="data/my_array.zarr",
    shape=(10000, 10000),
    chunks=(1000, 1000),
    dtype="f4"
)

# Write data using NumPy-style indexing
z[:, :] = np.random.random((10000, 10000))

# Read data
data = z[0:100, 0:100]  # Returns NumPy array
```

## Core Workflow

1. **Create or open** an array/group, picking a store appropriate to the environment (local, in-memory, ZIP, S3/GCS).
2. **Choose chunking** aligned to your access pattern (aim for 1-10 MB chunks; rows-first → chunks span columns, and vice versa). This is the single biggest performance lever.
3. **Pick compression** via `compressors=` based on workload — Zstandard (the default), Blosc+LZ4 (fast), Gzip (max ratio); `compressors=None` to disable.
4. **Read/write** with NumPy-style indexing; resize/append as data grows.
5. **Scale out** with Dask (lazy, out-of-core, parallel) or label with Xarray for climate/geospatial data.
6. **For cloud and many-array stores**, consolidate metadata and consider sharding to cut object/file count.

```python
# Minimal end-to-end
import zarr, numpy as np
z = zarr.create_array(store="data/my_array.zarr", shape=(10000, 10000),
                      chunks=(1000, 1000), dtype="f4")
z[:, :] = np.random.random((10000, 10000))
sub = z[0:100, 0:100]            # returns a NumPy array
```

## Routing — where to look

| You need… | Go to |
|-----------|-------|
| Array create/open, read/write, resize/append, attributes, groups & hierarchies, consolidated metadata | `references/array_operations.md` |
| Chunk-size guidelines, aligning chunks to access patterns, sharding, compression codecs & tips | `references/chunking_compression.md` |
| Local / in-memory / ZIP / S3 / GCS stores and cloud best practices | `references/storage_backends.md` |
| NumPy / Dask / Xarray integration, thread- and process-safe parallel writes | `references/integration.md` |
| Performance checklist, profiling, common patterns (time series, large matrices, cloud-native, format conversion), troubleshooting | `references/patterns_performance.md` |
| Full API surface | `references/api_reference.md` |

## Additional Resources

- **Official Documentation**: https://zarr.readthedocs.io/
- **Zarr Specifications**: https://zarr-specs.readthedocs.io/
- **GitHub Repository**: https://github.com/zarr-developers/zarr-python
- **Related**: Xarray (labeled arrays), Dask (parallel computing), NumCodecs (compression codecs)