Fix opening STAC Assets with xarray:open_kwargs engine field (#18)

Update the kwargs merging logic to have cascading priority where 
`default_kwargs` is overridden by `open_kwargs` which is overridden
by user provided `kwargs` .
Includes a regression unit test that extends the existing simple_zarr
test to ensure that the fix for duplicate keys works.

Note that the `xarray:open_kwargs` field is a part of the
[`xarray-assets`](https://github.com/stac-extensions/xarray-assets/tree/v1.0.0)
STAC extension (in Proposal stage), xref #16

Fixes #17

tests/conftest.py

@@ -49,3 +49,9 @@ def simple_zarr() -> pystac.Asset:
         catalog = pystac_client.Client.open(STAC_URLS["PLANETARY-COMPUTER"])
         collection = catalog.get_collection("daymet-daily-hi")
         return collection.assets["zarr-abfs"]
+
+
+@pytest.fixture(scope="module")
+def complex_zarr(simple_zarr) -> pystac.Asset:
+    simple_zarr.extra_fields["xarray:open_kwargs"]["engine"] = "zarr"
+    return simple_zarr

tests/test_core.py

@@ -30,4 +30,9 @@ def test_to_xarray_reference_file(simple_reference_file):
 
 def test_to_xarray_zarr(simple_zarr):
     ds = to_xarray(simple_zarr)
-    ds
+    assert ds
+
+
+def test_to_xarray_zarr_with_open_kwargs_engine(complex_zarr):
+    ds = to_xarray(complex_zarr)
+    assert ds

xpystac/core.py

@@ -64,5 +64,5 @@ def _(obj: pystac.Asset, **kwargs) -> xarray.Dataset:
     else:
         default_kwargs = {}
 
-    ds = xarray.open_dataset(obj.href, **default_kwargs, **open_kwargs, **kwargs)
+    ds = xarray.open_dataset(obj.href, **{**default_kwargs, **open_kwargs, **kwargs})
     return ds