gadget hdf: reduce size 1 header arrays to scalar

nose_ignores.txt

@@ -32,4 +32,5 @@
 --ignore-file=test_stream_particles\.py
 --ignore-file=test_stream_stretched\.py
 --ignore-file=test_version\.py
+--ignore-file=test_gadget_pytest\.py
 --ignore-file=test_vr_orientation\.py

tests/tests.yaml

@@ -204,6 +204,7 @@ other_tests:
      - "--ignore-file=test_stream_particles\\.py"
      - "--ignore-file=test_stream_stretched\\.py"
      - "--ignore-file=test_version\\.py"
+     - "--ignore-file=test_gadget_pytest\\.py"
      - "--ignore-file=test_vr_orientation\\.py"
      - "--exclude-test=yt.frontends.gdf.tests.test_outputs.TestGDF"
      - "--exclude-test=yt.frontends.adaptahop.tests.test_outputs"

yt/frontends/gadget/data_structures.py

@@ -629,6 +629,15 @@ def _get_hvals(self):
         if "Parameters" in handle:
             hvals.update((str(k), v) for k, v in handle["/Parameters"].attrs.items())
         handle.close()
+
+        # ensure that 1-element arrays are reduced to scalars
+        updated_hvals = {}
+        for hvalname, value in hvals.items():
+            if isinstance(value, np.ndarray) and value.size == 1:
+                mylog.info("Reducing single-element array %s to scalar.", hvalname)
+                updated_hvals[hvalname] = value.item()
+        hvals.update(updated_hvals)
+
         return hvals
 
     def _get_uvals(self):

yt/frontends/gadget/tests/test_gadget_pytest.py

@@ -0,0 +1,34 @@
+import numpy as np
+
+import yt
+from yt.testing import requires_file, requires_module
+from yt.utilities.on_demand_imports import _h5py as h5py
+
+
+@requires_file("snapshot_033/snap_033.0.hdf5")
+@requires_module("h5py")
+def test_gadget_header_array_reduction(tmp_path):
+    # first get a real header
+    ds = yt.load("snapshot_033/snap_033.0.hdf5")
+    hvals = ds._get_hvals()
+    hvals_orig = hvals.copy()
+    # wrap some of the scalar values in nested arrays
+    hvals["Redshift"] = np.array([hvals["Redshift"]])
+    hvals["Omega0"] = np.array([[hvals["Omega0"]]])
+
+    # drop those header values into a fake header-only file
+    tmp_snpshot_dir = tmp_path / "snapshot_033"
+    tmp_snpshot_dir.mkdir()
+    tmp_header_only_file = str(tmp_snpshot_dir / "fake_gadget_header.hdf5")
+    with h5py.File(tmp_header_only_file, mode="w") as f:
+        headergrp = f.create_group("Header")
+        for field, val in hvals.items():
+            headergrp.attrs[field] = val
+
+    # trick the dataset into using the header file and make sure the
+    # arrays are reduced
+    ds._input_filename = tmp_header_only_file
+    hvals = ds._get_hvals()
+    for attr in ("Redshift", "Omega0"):
+        assert hvals[attr] == hvals_orig[attr]
+        assert isinstance(hvals[attr], np.ndarray) is False