Able to delete group even when opened as readonly

[TomNicholas]

Zarr version

v3.0.8

Numcodecs version

0.16.1

Python Version

3.12.4

Operating System

linux

Installation

pip

Description

I was not expecting to be able to delete a group when opening from the store using mode='r'.

Steps to reproduce

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "zarr@git+https://github.com/zarr-developers/zarr-python.git@main",
# ]
# ///

import zarr

store = zarr.storage.MemoryStore()
root = zarr.create_group(store=store)
foo = root.create_group('foo')
bar = root.create_group('bar')
root.members()
# (('foo', <Group memory://139839476802624/foo>),
# ('bar', <Group memory://139839476802624/bar>))

# re-open root group in a supposedly readonly state
root_readonly = zarr.open_group(store, path='/', mode='r')

# delete a child group
del root_readonly['foo']

root_readonly.members()
# (('bar', <Group memory://139839476802624/bar>),)

---

zarr.print_debug_info()

platform: Linux-5.10.220-209.869.amzn2.x86_64-x86_64-with-glibc2.35
python: 3.12.4
zarr: 3.0.8

**Required dependencies:**
packaging: 24.1
numpy: 1.26.4
numcodecs: 0.16.1
typing_extensions: 4.12.2
donfig: 0.8.1.post1

**Optional dependencies:**
botocore: 1.34.106
fsspec: 2024.6.0
numcodecs: 0.16.1
s3fs: 2024.6.0
gcsfs: 2024.6.0
rich: 13.7.1

**Not Installed:**
cupy-cuda12x
universal-pathlib
obstore

Additional output

No response

[d-v-b]

at the moment mode=r for arrays and groups is a placebo. This is related to #3068 (comment)

[maxrjones]

Since #3068 (currently unreleased), zarr.open_group(store=writeable_store, read_only=True) errors:

Store is not read-only but mode is 'r'
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/src/zarr/storage/_common.py", line 90, in open
    raise ValueError(f"Store is not read-only but mode is '{mode}'")
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/src/zarr/storage/_common.py", line 337, in make_store_path
    result = await StorePath.open(store, path=path_normalized, mode=mode)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/src/zarr/api/asynchronous.py", line 837, in open_group
    store_path = await make_store_path(store, mode=mode, storage_options=storage_options, path=path)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/src/zarr/core/sync.py", line 119, in _runner
    return await coro
           ^^^^^^^^^^
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/src/zarr/core/sync.py", line 163, in sync
    raise return_result
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/src/zarr/api/synchronous.py", line 535, in open_group
            async_api.open_group(
    ...<14 lines>...
    
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/src/zarr/_compat.py", line 43, in inner_f
    return f(*args, **kwargs)
  File "/Users/max/Documents/Code/zarr-developers/zarr-python/.vscode/del-group.py", line 12, in <module>
    root_readonly = zarr.open_group(store, path='/', mode='r')
  File "/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/runpy.py", line 88, in _run_code
    exec(code, run_globals)
  File "/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/runpy.py", line 198, in _run_module_as_main (Current frame)
    return _run_code(code, main_globals, None,
ValueError: Store is not read-only but mode is 'r'

with the proposal in #3068 (comment) you would instead get an error on del root_readonly['foo'].

[TomNicholas]

with the proposal in #3068 (comment) you would instead get an error on del root_readonly['foo'].

I don't have much context, but that sounds more intuitive to me.