VerboseModule cannot be pickled

[vladidobro]

Zarr version

3.2.1

Numcodecs version

0.16.5

Python Version

3.12.12

Operating System

Mac

Installation

using uv run

Description

Hi,

I was trying to send zarr.storage.ObjectStore-backed Dask arrays to Dask, and I hit this error.
Apparently zarr.storage, the module, cannot be pickled, which in my opinion should be considered a bug, because modules are in general always picklable.
The problem goes away when I change import zarr.storage; zarr.storage.ObjectStore to from zarr.storage import ObjectStore; ObjectStore, because then there is no attempt to serialize zarr.storage.

It is also very confusing that "zarr" does not come up anywhere in the traceback, which makes it hard to find the culprit.

The problem is that zarr.storage is doing some import magic in

zarr-python/src/zarr/storage/__init__.py

Line 31 in fe22910

class VerboseModule(ModuleType):

Steps to reproduce

# /// script
# requires-python = ">=3.12"
# dependencies = [
#   "zarr@git+https://github.com/zarr-developers/zarr-python.git@main",
#   "dask",
#   "distributed",
# ]
# ///
#
# This script automatically imports the development branch of zarr to check for issues

import zarr
zarr.print_debug_info()

import zarr.storage
import dask.distributed

def do():
    print(zarr.storage.ObjectStore)

if __name__ == '__main__':
    with dask.distributed.LocalCluster(n_workers=1, processes=True) as cluster:
        client = cluster.get_client()
        client.submit(do).result()

Additional output

Installed 25 packages in 62ms
platform: macOS-26.4.1-arm64-arm-64bit
python: 3.12.12
zarr: 3.2.2.dev40+gfe229107

Required dependencies:
packaging: 26.2
numpy: 2.4.6
numcodecs: 0.16.5
typing_extensions: 4.15.0
donfig: 0.8.1.post1

Optional dependencies:
fsspec: 2026.4.0
numcodecs: 0.16.5

Not Installed:
botocore
cupy-cuda12x
s3fs
gcsfs
universal-pathlib
obstore
platform: macOS-26.4.1-arm64-arm-64bit
python: 3.12.12
zarr: 3.2.2.dev40+gfe229107

Required dependencies:
packaging: 26.2
numpy: 2.4.6
numcodecs: 0.16.5
typing_extensions: 4.15.0
donfig: 0.8.1.post1

Optional dependencies:
fsspec: 2026.4.0
numcodecs: 0.16.5

Not Installed:
botocore
cupy-cuda12x
s3fs
gcsfs
universal-pathlib
obstore
2026-06-03 22:35:23,265 - distributed.protocol.pickle - ERROR - Failed to serialize LLGExpr(dsk={'do-8b75dbdc4db9fb9776a06e901a3f5600': <Task 'do-8b75dbdc4db9fb9776a06e901a3f5600' do()>}).
Traceback (most recent call last):
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/pickle.py", line 76, in dumps
result = cloudpickle.dumps(x, **dump_kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1544, in dumps
cp.dump(obj)
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1313, in dump
return super().dump(obj)
^^^^^^^^^^^^^^^^^
TypeError: cannot pickle 'VerboseModule' object

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/pickle.py", line 80, in dumps
result = cloudpickle.dumps(x, **dump_kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1544, in dumps
cp.dump(obj)
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1313, in dump
return super().dump(obj)
^^^^^^^^^^^^^^^^^
TypeError: cannot pickle 'VerboseModule' object
Traceback (most recent call last):
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/pickle.py", line 76, in dumps
result = cloudpickle.dumps(x, **dump_kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1544, in dumps
cp.dump(obj)
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1313, in dump
return super().dump(obj)
^^^^^^^^^^^^^^^^^
TypeError: cannot pickle 'VerboseModule' object

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/serialize.py", line 366, in serialize
header, frames = dumps(x, context=context) if wants_context else dumps(x)
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/serialize.py", line 78, in pickle_dumps
frames[0] = pickle.dumps(
^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/pickle.py", line 80, in dumps
result = cloudpickle.dumps(x, **dump_kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1544, in dumps
cp.dump(obj)
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 1313, in dump
return super().dump(obj)
^^^^^^^^^^^^^^^^^
TypeError: cannot pickle 'VerboseModule' object

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/Users/vladislavwohlrath/dskzr.py", line 24, in
client.submit(do).result()
^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/client.py", line 2180, in submit
futures = self._graph_to_futures(
^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/client.py", line 3392, in _graph_to_futures
expr_ser = Serialized(*serialize(to_serialize(expr), on_error="raise"))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/serialize.py", line 284, in serialize
return serialize(
^^^^^^^^^^
File "/Users/vladislavwohlrath/.cache/uv/environments-v2/dskzr-2f5da7d8eb1debb7/lib/python3.12/site-packages/distributed/protocol/serialize.py", line 392, in serialize
raise TypeError(msg, str_x) from exc
TypeError: ('Could not serialize object of type LLGExpr', "LLGExpr(dsk={'do-8b75dbdc4db9fb9776a06e901a3f5600': <Task 'do-8b75dbdc4db9fb9776a06e901a3f5600' do()>})")

[d-v-b]

sorry about this! I feel like we can remove that verbosemodule thing -- it only existed to emit a warning for long-deprecated behavior