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User-provided attributes dictionary is mutated by the public API #4059
Description
Zarr version
v3.2.1
Numcodecs version
0.16.5
Python Version
3.12
Operating System
Linux
Installation
using pip into virtual environment
Description
When creating a Zarr group or array with a user-provided attributes dictionary, the original dictionary gets mutated when the user subsequently modifies attributes via the public API (attrs.setitem, attrs.put, or del attrs[key]).
Steps to reproduce
# /// script
# requires-python = ">=3.12"
# 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
init_dict = {"foo": "bar"}
user_attrs = dict(init_dict)
print("User's dict before passing to Zarr:", user_attrs)
g = zarr.group(attributes=user_attrs)
g.attrs["new_key"] = "new_value"
print("User's dict after passing to Zarr:", user_attrs)
print("Expected: ", init_dict)
assert user_attrs == init_dict, "FAIL: Original dict mutated by"
zarr.print_debug_info()Additional output
There is a key called cache_attrs under zarr.group() which is not yet implemented. Is there a reason to pursue that over just copying the attributes which user provides?
Activity
d-v-b commentedon Jun 11, 2026
that's an unpleasant bug. We should be copying the user input.