`intake_xarray` does not lazy read metadata from files

[kadykov]

The entries powered by intake_xarray driver does not lazy read metadata from the files.

# %%
import intake
import xarray as xr

ds = xr.Dataset(
    {
        "test_var": [0],
    },
    attrs={"xarray_metadata": "The metadata in the xarray file"},
)
ds.to_netcdf("test_metadata.nc")
ds.to_zarr("test_metadata.zarr", mode="w")
# %%
catalog_content = """sources:
  netcdf:
    driver: netcdf
    args:
      urlpath: '{{ CATALOG_DIR }}/test_metadata.nc'
      metadata:
        catalog_metadata: The metadata in the catalog entry
  zarr_intake_xarray:
    description: zarr archive read by intake_xarray
    driver: zarr
    args:
      urlpath: '{{ CATALOG_DIR }}/test_metadata.zarr'
      metadata:
        catalog_metadata: The metadata in the catalog entry
  zarr_intake:
    description: zarr archive read by intake
    driver: zarr_cat
    args:
      urlpath: '{{ CATALOG_DIR }}/test_metadata.zarr'
      metadata:
        catalog_metadata: The metadata in the catalog entry
"""

with open("catalog.yml", "w") as f:
    f.write(catalog_content)

cat = intake.open_catalog("catalog.yml")
print(f"{cat.netcdf.metadata = }")
print(f"{cat.zarr_intake_xarray.metadata = }")
print(f"{cat.zarr_intake.metadata = }")

As you see from the output, the metadata from the entry powered by intake driver has the field from the zarr file:

cat.netcdf.metadata = {'catalog_metadata': 'The metadata in the catalog entry'}
cat.zarr_intake_xarray.metadata = {'catalog_metadata': 'The metadata in the catalog entry'}
cat.zarr_intake.metadata = {'catalog_metadata': 'The metadata in the catalog entry', 'xarray_metadata': 'The metadata in the xarray file'}

However, after reading the files, the metadata is complete:

cat.netcdf.read()
cat.zarr_intake_xarray.read()

print(f"Netcdf metadata after reading: {cat.netcdf.metadata}")
print(f"Zarr metadata after reading: {cat.zarr_intake_xarray.metadata}")

Output:

Netcdf metadata after reading: {'catalog_metadata': 'The metadata in the catalog entry', 'dims': {'test_var': 1}, 'data_vars': {}, 'coords': ('test_var',), 'xarray_metadata': 'The metadata in the xarray file'}
Zarr metadata after reading: {'catalog_metadata': 'The metadata in the catalog entry', 'dims': {'test_var': 1}, 'data_vars': {}, 'coords': ('test_var',), 'xarray_metadata': 'The metadata in the xarray file'}

OS: Windows 10
python 3.11.5
intake 0.7.0
intake_xarray 0.7.0
xarray 2023.8.0
zarr 2.16.1

[martindurant]

What do you think the right behaviour should be? Catalog entries are special in Intake (<2.0) in that they get their subentries eagerly, so they have access to the file metadata immediately, is this what you are getting at?

[kadykov]

I expected that cat.netcdf.metadata includes also the metadata from the file like this: {'catalog_metadata': 'The metadata in the catalog entry', 'xarray_metadata': 'The metadata in the xarray file'}.
But now, the xarray_metadata key appears only after reading the whole file by executing cat.netcdf.read().

I think it would be better to have "lazy" metadata reading from files because there also could be some useful information... What do you think?

[martindurant]

The .discover() method is meant exactly for this purpose, to get information from the file with a minimum of reads. It's usefulness varies by file type.

Actually, xarray is lazy by default, so even if you do a .read(), you do no load all the data into memory, only enough for xarray to be able to understand the file's layout (typically the attributes and coordinate arrays).