Allow for fixture creation to be skipped (#345)

### What kind of change does this PR introduce?

* adds a check to not bother creating a particular fixture if the `-m
"not requires_netcdf"` flag is set or if the files are missing.

### Does this PR introduce a breaking change?

No.

### Other information:

`pytest.mark` doesn't work for `pytest.fixture` objects. TIL

CHANGES.rst

@@ -21,6 +21,7 @@ Internal changes
     * The OpenSSF `scorecard.yml` workflow has been added to the GitHub workflows to evaluate package security.
     * Code formatting tools (`black`, `blackdoc`, `isort`) are now hard-pinned. These need to be kept in sync with changes from `pre-commit`. (Dependabot should perform this task automatically.)
     * The versioning system has been updated to follow the Semantic Versioning 2.0.0 standard.
+* Fixed an issue with `pytest -m "not requires_netcdf"` not working as expected. (:pull:`345`).
 
 v0.8.0 (2024-01-16)
 -------------------

tests/conftest.py

@@ -26,10 +26,15 @@ def remove_data_folder():
     request.addfinalizer(remove_data_folder)
 
 
-@pytest.mark.requires_netcdf
 @pytest.fixture(scope="session")
-def samplecat():
+def samplecat(request):
     """Generate a sample catalog with the tutorial netCDFs."""
+    mark_skip = request.config.getoption("-m")
+    if "not requires_netcdf" in mark_skip or not SAMPLES_DIR.exists():
+        pytest.skip("Skipping tests that require netCDF files")
+    elif not list(SAMPLES_DIR.rglob("*.nc")):
+        pytest.skip("No netCDF files found in the tutorial samples folder")
+
     df = xs.parse_directory(
         directories=[SAMPLES_DIR],
         patterns=[