pizzarr

A Zarr implementation for R.

Installation

Installation requires R 4.1.0 or greater.

install.packages("devtools")
devtools::install_github("zarr-developers/pizzarr")

Usage

library(pizzarr)

# Open a sample BCSD climate dataset (Zarr V3)
v3_root <- pizzarr_sample("bcsd_v3")
v3 <- zarr_open(v3_root)

# Print the group summary
v3
#> <ZarrGroup> /
#>   Store type  : DirectoryStore
#>   Zarr format : 3
#>   Read-only   : FALSE
#>   No. members : 5

# View the hierarchy
v3$tree()
#> / 
#> ├── latitude (33) <f4
#> ├── longitude (81) <f4
#> ├── pr (12, 33, 81) <f4
#> ├── tas (12, 33, 81) <f4
#> └── time (12) <f8

# Inspect an array
v3$get_item("pr")
#> <ZarrArray> /pr
#>   Shape       : (12, 33, 81)
#>   Chunks      : (12, 33, 81)
#>   Data type   : <f4
#>   Fill value  : 1.00000002004088e+20
#>   Order       : C
#>   Read-only   : FALSE
#>   Compressor  : ZstdCodec
#>   Store type  : DirectoryStore
#>   Zarr format : 3

# Read a slice: first 3 time steps, first 3 latitudes, first longitude
v3$get_item("pr")$get_item(list(slice(1, 3), slice(1, 3), 1))$data
#> , , 1
#> 
#>        [,1]   [,2]   [,3]
#> [1,] 133.97 144.51 149.92
#> [2,]  75.40  72.38  68.62
#> [3,]  93.14  91.24  89.37

Create an array from scratch:

a <- array(data = 1:20, dim = c(2, 10))
z <- zarr_create(shape = dim(a), dtype = "<f4", fill_value = NA)
z$set_item("...", a)

z
#> <ZarrArray> /
#>   Shape       : (2, 10)
#>   Chunks      : (2, 10)
#>   Data type   : <f4
#>   Fill value  : 0
#>   Order       : F
#>   Read-only   : FALSE
#>   Compressor  : ZstdCodec
#>   Store type  : MemoryStore
#>   Zarr format : 2
z$get_item(list(slice(1, 2), slice(1, 5)))$data
#>      [,1] [,2] [,3] [,4] [,5]
#> [1,]    1    3    5    7    9
#> [2,]    2    4    6    8   10

Features

• Zarr V2 and V3 read and write (format auto-detected on open)
• Stores: MemoryStore, DirectoryStore (read/write); HttpStore (read-only)
• Data types: boolean, int8–int64, uint8–uint64, float16/32/64, string, Unicode, VLenUTF8
• Compression: zlib/gzip, bzip2, blosc, LZMA, LZ4, Zstd
• Blosc requires the optional blosc package (install.packages("blosc"))

How It Works

pizzarr uses R6 classes mirroring the zarr-python object model:

• Store — backend storage (DirectoryStore for local files, MemoryStore for in-memory, HttpStore for remote read-only)
• ZarrGroup — hierarchical container holding arrays and sub-groups (like a directory)
• ZarrArray — chunked, compressed N-dimensional array (like a file)
• Codec — compression/decompression (zlib, zstd, blosc, lz4, etc.)
• Dtype — data-type mapping between R and Zarr

Data flows through the stack: a Store holds raw chunk bytes, a Codec pipeline compresses and decompresses them, and ZarrArray presents typed N-dimensional data to R. Groups and arrays are addressed by path within a store, just like files in a directory tree.

See vignette("pizzarr") for a full walkthrough.

Ecosystem

pizzarr implements the Zarr specification (V2 and V3) for R. Related projects:

• zarr-python — the reference Python implementation
• zarr.js — JavaScript implementation
• zarr — native R V3 implementation (CRAN)
• Rarr — Bioconductor package for reading and writing individual Zarr arrays (V2, limited write support)
• zarr-conformance-tests — cross-implementation validation

Validation with zarr-python

A standalone integration test cross-validates that pizzarr and zarr-python produce equivalent Zarr stores. Both implementations write the same arrays (V2 and V3 formats, multiple dtypes, codecs, chunk layouts, and groups with attributes), then each reads the other’s output and verifies the data matches.

Prerequisites: Python 3.10+ with zarr>=3 and numpy installed.

Rscript inst/extdata/cross-validate.R

The script skips gracefully (exit 0) if Python is not available. On success all checks pass and exit code is 0; any mismatch is reported and exits 1.

Zarr Conformance Tests

pizzarr participates in the zarr-conformance-tests framework, which validates that Zarr implementations can correctly read standard test arrays (V2 and V3 formats, multiple dtypes). These tests run automatically in CI on every push and pull request to main.

Contributing

See CONTRIBUTING.md for development setup, testing, and documentation build instructions.

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