_automatically_determine_filetype netCDF3 vs netCDF4

pyproject.toml

@@ -32,6 +32,7 @@ test = [
     "pre-commit",
     "pytest-mypy",
     "pytest",
+    "netCDF4"
 ]
 
 

virtualizarr/kerchunk.py

@@ -1,3 +1,4 @@
+import importlib.util
 from pathlib import Path
 from typing import List, NewType, Optional, Tuple, Union, cast
 
@@ -61,10 +62,26 @@ def _automatically_determine_filetype(filepath: str) -> str:
     file_extension = Path(filepath).suffix
 
     if file_extension == ".nc":
-        # TODO how can we automatically distinguish netCDF3 and 4?
-        raise NotImplementedError(
-            "Cannot unambiguously automatically determine which kerchunk file format reader to use"
-        )
+        # checks if netCDF library is installed.
+        # It currently is not a requirement in the pyproj.toml.
+
+        if importlib.util.find_spec("netCDF4") is None:
+            raise ImportError(
+                "netCDF4 library is required to determine NetCDF file type."
+            )
+
+        import netCDF4
+
+        with netCDF4.Dataset(filepath, "r") as dataset:
+            if dataset.data_model == "NETCDF4":
+                filetype = "netCDF4"
+            elif dataset.data_model == "NETCDF3_CLASSIC":
+                filetype = "netCDF3"
+            else:
+                raise NotImplementedError(
+                    ".nc file does not appear to be NETCDF3 OR NETCDF4"
+                )
+
     elif file_extension == ".zarr":
         # TODO we could imagine opening an existing zarr store, concatenating it, and writing a new virtual one...
         raise NotImplementedError()

virtualizarr/tests/test_kerchunk.py

@@ -3,6 +3,7 @@
 import xarray as xr
 import xarray.testing as xrt
 
+from virtualizarr.kerchunk import _automatically_determine_filetype
 from virtualizarr.manifests import ChunkEntry, ChunkManifest, ManifestArray
 from virtualizarr.xarray import dataset_from_kerchunk_refs
 
@@ -130,3 +131,17 @@ def test_kerchunk_roundtrip_in_memory_no_concat():
 
     # Assert equal to original dataset
     xrt.assert_equal(roundtrip, ds)
+
+
+def test_automatically_determine_filetype_netcdf3_netcdf4():
+    # test the NetCDF3 vs NetCDF4 automatic file type selection
+
+    ds = xr.Dataset({"a": (["x"], [0, 1])})
+    netcdf3_file_path = "/tmp/netcdf3.nc"
+    netcdf4_file_path = "/tmp/netcdf4.nc"
+
+    # write two version of NetCDF
+    ds.to_netcdf(netcdf3_file_path, engine="scipy", format="NETCDF3_CLASSIC")
+    ds.to_netcdf(netcdf4_file_path)
+    assert "netCDF3" == _automatically_determine_filetype(netcdf3_file_path)
+    assert "netCDF4" == _automatically_determine_filetype(netcdf4_file_path)