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Using r+ mode with zarr.open creates an empty directory #3295
Description
Zarr version
v3.1.0
Numcodecs version
0.16.1
Python Version
3.13
Operating System
Windows
Installation
Using pip into a virtual environment
Description
Opening a non-existent location via zarr.open with r+ mode (see 'steps to reproduce') creates an empty directory at that location. Given the description in the zarr.open docstring: 'r+ means read/write (must exist)', I would expect no files to be changed before the GroupNotFoundError is thrown.
Steps to reproduce
# /// script
# requires-python = ">=3.11"
# dependencies = [
# "zarr@git+https://github.com/zarr-developers/zarr-python.git@main",
# ]
# ///
#
# This script automatically imports the development branch of zarr to check for issues
import zarr
# creates the new directory data/images/test_r_plus.zarr, then throws GroupNotFoundError
zarr_object = zarr.open(store="data/images/test_r_plus.zarr", mode="r+")
# Using mode r, doesn't create a new directory before it throw GroupNotFoundError
zarr_object = zarr.open(store="data/images/test_r.zarr", mode="r")Additional output
No response
Reacted by David Stansby
Activity
K-Meech commentedon Jul 28, 2025
The issue seems to be inside
make_store_pathinsrc/zarr/storage/_common.py:_read_onlyis only set formode == 'r'await LocalStore.open(root=Path(store_like), read_only=_read_only)(where__read_onlyis False forr+mode)LocalStoreinsrc/zarr/storage/_local.py- a new directory is made by_openin every case where_read_only = False