Fix _get_default_engine_netcdf to check for h5netcdf (#10557)

Co-authored-by: Deepak Cherian <[email protected]>

Author: scottstanie

doc/whats-new.rst

@@ -32,7 +32,8 @@ Bug fixes
 
 - Fix transpose of boolean arrays read from disk. (:issue:`10536`)
   By `Deepak Cherian <https://github.com/dcherian>`_.
-
+- Fix detection of the ``h5netcdf`` backend. Xarray now selects ``h5netcdf`` if the default ``netCDF4`` engine is not available (:issue:`10401`, :pull:`10557`).
+  By `Scott Staniewicz <https://github.com/scottstanie>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/backends/api.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import importlib.util
 import os
 from collections.abc import (
     Callable,
@@ -122,23 +123,21 @@ def _get_default_engine_gz() -> Literal["scipy"]:
     return engine
 
 
-def _get_default_engine_netcdf() -> Literal["netcdf4", "scipy"]:
-    engine: Literal["netcdf4", "scipy"]
-    try:
-        import netCDF4  # noqa: F401
+def _get_default_engine_netcdf() -> Literal["netcdf4", "h5netcdf", "scipy"]:
+    candidates: list[tuple[str, str]] = [
+        ("netcdf4", "netCDF4"),
+        ("h5netcdf", "h5netcdf"),
+        ("scipy", "scipy.io.netcdf"),
+    ]
 
-        engine = "netcdf4"
-    except ImportError:  # pragma: no cover
-        try:
-            import scipy.io.netcdf  # noqa: F401
+    for engine, module_name in candidates:
+        if importlib.util.find_spec(module_name) is not None:
+            return cast(Literal["netcdf4", "h5netcdf", "scipy"], engine)
 
-            engine = "scipy"
-        except ImportError as err:
-            raise ValueError(
-                "cannot read or write netCDF files without "
-                "netCDF4-python or scipy installed"
-            ) from err
-    return engine
+    raise ValueError(
+        "cannot read or write NetCDF files because none of "
+        "'netCDF4-python', 'h5netcdf', or 'scipy' are installed"
+    )
 
 
 def _get_default_engine(path: str, allow_remote: bool = False) -> T_NetcdfEngine:

xarray/tests/test_backends_api.py

@@ -1,16 +1,18 @@
 from __future__ import annotations
 
+import sys
 from numbers import Number
 
 import numpy as np
 import pytest
 
 import xarray as xr
-from xarray.backends.api import _get_default_engine
+from xarray.backends.api import _get_default_engine, _get_default_engine_netcdf
 from xarray.tests import (
     assert_identical,
     assert_no_warnings,
     requires_dask,
+    requires_h5netcdf,
     requires_netCDF4,
     requires_scipy,
 )
@@ -29,6 +31,17 @@ def test__get_default_engine() -> None:
     assert engine_default == "netcdf4"
 
 
+@requires_h5netcdf
+def test_default_engine_h5netcdf(monkeypatch):
+    """Test the default netcdf engine when h5netcdf is the only importable module."""
+
+    monkeypatch.delitem(sys.modules, "netCDF4", raising=False)
+    monkeypatch.delitem(sys.modules, "scipy", raising=False)
+    monkeypatch.setattr(sys, "meta_path", [])
+
+    assert _get_default_engine_netcdf() == "h5netcdf"
+
+
 def test_custom_engine() -> None:
     expected = xr.Dataset(
         dict(a=2 * np.arange(5)), coords=dict(x=("x", np.arange(5), dict(units="s")))