Where does "Filters" come from in "array.info"?

[leuchthelp]

Zarr version used: v3.0.1

Wondering where the filters attribute comes from as the documentation makes no mention of this existing when using .info. Also shows up when using the exact example from the docs provided here .

Type               : Array
Zarr format        : 3
Data type          : DataType.float32
Shape              : (10,)
Chunk shape        : (2,)
Order              : C
Read-only          : False
Store type         : MemoryStore
Filters            : ()
Serializer         : BytesCodec(endian=<Endian.little: 'little'>)
Compressors        : (ZstdCodec(level=0, checksum=False),)
No. bytes          : 40

For Context I specifically want to create a v2 zarr file as I want to open it in netcdf4 via NCZarr. Though this fails with undefined filter encountered which I believe is caused by this. It's an issue specific to NCZarr though, so I just need some information on why I'm seeing it pop up in the first place and how I can tell zarr to not export it or set it correctly like in the examples on the wiki.

Output:

Type               : Array
Zarr format        : 2
Data type          : float64
Shape              : (134217728,)
Chunk shape        : (262144,)
Order              : C
Read-only          : False
Store type         : LocalStore
Filters            : ()
Compressors        : (Blosc(cname='lz4', clevel=5, shuffle=SHUFFLE, blocksize=0),)
No. bytes          : 1073741824 (1.0G)

Code:

import zarr
import numpy as np

root = zarr.create_group(store="c-stuff/data/datasets/test_dataset.zarr", zarr_format=2, overwrite=True)

x = root.create_array(name="X", shape=(134217728), dtype="f8")
x[:] = np.random.random_sample(134217728)

root["X"].info

[d-v-b]

filters is one of the fields of the zarr v2 array metadata document. Our implementation of that field is here. It's possible that nczarr cannot handle a length-0 tuple of filters and would prefer that you use the literal None, maybe give that a shot?

[d-v-b]

(and filters is a keyword argument of create_array)

[leuchthelp]

Uploaded an image of trying to simply set filters=None for completeness.

[d-v-b]

this might reveal a bug in our v2 implementation -- I think the spec actually states that filters is either a list of 1 or more objects, or None: https://github.com/zarr-developers/zarr-specs/blob/214ba9b6326aadcf406a69a1a7268a7055cea185/docs/v2/v2.0.rst?plain=1#L236-L241

[leuchthelp]

Right that is what I read too, which made me wonder, though I haven't dug into the implementation. Also apologies for the delayed response.

I have since tried to simply used filters set to None which had the same outcome (that is the field filters showing up as an empty tuple again). Additionally I wanted to figure out how to populated that field with the matching filter as NCZarr is supposed to handle all filters used by zarr, but couldn't find an example of how to do it properly.

Could you advise me on how to populate the field with a matching filter by using .create_array( filters = "something" ) ? I believe that approach might suffices for now.

[leuchthelp]

Also additionally this might also be an issue in v3 if I understand correctly.

When setting zarr_format=3 and filters=None manually it still shows as an empty tuple while it should just be None, correct?