is it OK if I transfer this issue to numcodecs?

Probably need to use cloudpickle. Did you already try?

Or is this coming from some other Zarr code path?

Can't comment on Numcodecs Zarr 3 code as I haven't been involved in this

@jakirkham the cause is this function:

def _make_bytes_bytes_codec(codec_name: str, cls_name: str) -> type[_NumcodecsBytesBytesCodec]:
    # rename for class scope
    _codec_name = CODEC_PREFIX + codec_name


    class _Codec(_NumcodecsBytesBytesCodec):
        codec_name = _codec_name


        def __init__(self, **codec_config: JSON) -> None:
            super().__init__(**codec_config)


    _Codec.__name__ = cls_name
    return _Codec

It defines a class inside function scope, but this approach is not needed to solve the underlying problem. As this function gets called fewer than 10 times, we could instead have fewer than 10 separate normal class definitions. That would be my recommended short-term fix.

we could also use __init_subclass__ and make the codec name a parameter of the class definition.

Yes I understand. Read the same code. This is the kind of thing cloudpickle handles routinely

That said, Idk why we went with that design choice. This is what I mean by I cannot comment on why that was added that way

@TomNicholas i transferred this to numcodecs, because it requires changes to numcodecs code.

As this function gets called fewer than 10 times, we could instead have fewer than 10 separate normal class definitions. That would be my recommended short-term fix.
...
we could also use init_subclass and make the codec name a parameter of the class definition.

I'm happy to submit a PR to do this - I had to make a janky branch to get what I'm working on to work anyway - I might as well turn it into a proper PR.

that would be great @TomNicholas, feel free to reach out if you want any pointers.