0.12.1: `cannot import name 'MsgPack' from 'numcodecs'` in numcodecs/__init__.py

[kloczek]

Looks like on testing zarr has been exposed probably missing msgpack import (and missing install time dependency of that module as well if I'm right).

_________________________________________________________________________ ERROR collecting zarr/tests/test_core.py __________________________________________________________________________
ImportError while importing test module '/home/tkloczko/rpmbuild/BUILD/zarr-python-2.18.1/zarr/tests/test_core.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib64/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
zarr/tests/test_core.py:13: in <module>
    from numcodecs import (
E   ImportError: cannot import name 'MsgPack' from 'numcodecs' (/usr/lib64/python3.10/site-packages/numcodecs/__init__.py)

[jakirkham]

It is an optional dependency in Numcodecs

numcodecs/pyproject.toml

Lines 63 to 65 in 79a0e93

	msgpack = [
	"msgpack",
	]

Users need to install it if they need it

Edit: If Zarr needs this, then they should also add numcodecs[msgpack] where appropriate

[jakirkham]

Could you please raise this issue on the Zarr repo?

Feel free to cc me there

[kloczek]

Whatever will import numcodecs.MsgPack will fail (and zarr test suite is doing that) so this is IMO not kind of optional dependency 🤔

[dstansby]

I've been running into this recently, and it's not obvious from the error message that it can be fixed by installing numcodecs[msgpack]. I think there's two actions to take here:

• Make zarr depend on numcodecs[msgpack]
• Add a better error message here in numcodecs explaining how to fix this error

[joshmoore]

• Make zarr depend on numcodecs[msgpack]

...as a test dependency?

[dstansby]

Oh yeah, it looks like it's already a test dependency. So I guess worth just having a better error message here (if that's possible?) Help with this very welcome!