Implement capturing for cell methods.

trexfeathers · trexfeathers · commit 8887c695b164 · 2025-05-28T14:21:13.000+01:00
diff --git a/lib/iris/fileformats/_nc_load_rules/actions.py b/lib/iris/fileformats/_nc_load_rules/actions.py
@@ -109,6 +109,7 @@ def action_default(engine):
     """Perform standard operations for every cube."""
     # Future pattern (iris#6319).
     hh.build_and_add_names(engine)
+    hh.build_and_add_cell_methods(engine)
 
     # Legacy pattern.
     hh.build_cube_metadata(engine)
diff --git a/lib/iris/fileformats/_nc_load_rules/helpers.py b/lib/iris/fileformats/_nc_load_rules/helpers.py
@@ -650,6 +650,29 @@ def setter(attr_name):
     )
 
 
+################################################################################
+def _build_cell_methods(cf_var: cf.CFDataVariable) -> List[iris.coords.CellMethod]:
+    nc_att_cell_methods = getattr(cf_var, CF_ATTR_CELL_METHODS, None)
+    return parse_cell_methods(nc_att_cell_methods, cf_var.cf_name)
+
+
+def build_and_add_cell_methods(engine: Engine):
+    """Add cell methods to the cube."""
+    assert engine.cf_var is not None
+    assert engine.cube is not None
+
+    _add_or_capture(
+        build_func=partial(_build_cell_methods, engine.cf_var),
+        add_method=partial(setattr, engine.cube, "cell_methods"),
+        cf_var=engine.cf_var,
+        attr_key=CF_ATTR_CELL_METHODS,
+        destination=LoadProblems.Problem.Destination(
+            iris_class=Cube,
+            identifier=engine.cf_var.cf_name,
+        ),
+    )
+
+
 ################################################################################
 def build_cube_metadata(engine):
     """Add the standard meta data to the cube."""
@@ -664,10 +687,6 @@ def build_cube_metadata(engine):
     attr_units = get_attr_units(cf_var, cube.attributes)
     cube.units = attr_units
 
-    # Incorporate cell methods
-    nc_att_cell_methods = getattr(cf_var, CF_ATTR_CELL_METHODS, None)
-    cube.cell_methods = parse_cell_methods(nc_att_cell_methods, cf_var.cf_name)
-
     # Set the cube global attributes.
     for attr_name, attr_value in cf_var.cf_group.global_attributes.items():
         try:
diff --git a/lib/iris/tests/unit/fileformats/nc_load_rules/helpers/test_build_and_add_cell_methods.py b/lib/iris/tests/unit/fileformats/nc_load_rules/helpers/test_build_and_add_cell_methods.py
@@ -0,0 +1,96 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the BSD license.
+# See LICENSE in the root of the repository for full licensing details.
+"""Test function :func:`iris.fileformats._nc_load_rules.helpers.build_and_add_cell_methods`."""
+
+from unittest import mock
+
+import pytest
+
+from iris.coords import CellMethod
+from iris.cube import Cube
+from iris.fileformats._nc_load_rules import helpers
+from iris.fileformats.netcdf import _thread_safe_nc as threadsafe_nc
+from iris.loading import LOAD_PROBLEMS
+
+
+@pytest.fixture
+def mock_cf_data_var():
+    yield mock.Mock(
+        spec=threadsafe_nc.VariableWrapper,
+        cell_methods="time: mean",
+        cf_name="wibble",
+        filename="DUMMY",
+    )
+
+
+@pytest.fixture
+def mock_engine(mock_cf_data_var):
+    yield mock.Mock(
+        cube=mock.Mock(),
+        cf_var=mock_cf_data_var,
+        filename=mock_cf_data_var.filename,
+    )
+
+
+def test_construction(mock_engine):
+    expected_method = CellMethod("mean", coords=["time"])
+    helpers.build_and_add_cell_methods(mock_engine)
+    assert mock_engine.cube.cell_methods == (expected_method,)
+
+
+def test_not_built(monkeypatch, mock_engine, mock_cf_data_var):
+    cm_original = mock_engine.cube.cell_methods
+
+    def mock_parse_cell_methods(nc_cell_methods, cf_name=None):
+        raise RuntimeError("Not built")
+
+    with monkeypatch.context() as m:
+        m.setattr(helpers, "parse_cell_methods", mock_parse_cell_methods)
+        helpers.build_and_add_cell_methods(mock_engine)
+
+    load_problem = LOAD_PROBLEMS.problems[-1]
+    assert "Not built" in "".join(load_problem.stack_trace.format())
+    assert mock_engine.cube.cell_methods == cm_original
+
+
+def test_not_added(monkeypatch, mock_engine, mock_cf_data_var):
+    cm_original = mock_engine.cube.cell_methods
+
+    class NoCellMethods(mock.Mock):
+        def __setattr__(self, key, value):
+            if key == "cell_methods":
+                raise RuntimeError("Not added")
+            super().__setattr__(key, value)
+
+    with monkeypatch.context() as m:
+        m.setattr(mock_engine, "cube", NoCellMethods())
+        helpers.build_and_add_cell_methods(mock_engine)
+
+    load_problem = LOAD_PROBLEMS.problems[-1]
+    assert "Not added" in "".join(load_problem.stack_trace.format())
+    assert mock_engine.cube.cell_methods == cm_original
+
+
+def test_unhandlable_error(monkeypatch, mock_engine):
+    # Confirm that the code can redirect an error to LOAD_PROBLEMS even
+    #  when there is no specific handling code for it.
+    n_problems = len(LOAD_PROBLEMS.problems)
+
+    with monkeypatch.context() as m:
+        m.setattr(mock_engine, "cube", "foo")
+        helpers.build_and_add_cell_methods(mock_engine)
+
+    assert len(LOAD_PROBLEMS.problems) > n_problems
+
+
+def test_problem_destination(monkeypatch, mock_engine):
+    # Confirm that the destination of the problem is set correctly.
+    with monkeypatch.context() as m:
+        m.setattr(mock_engine, "cube", "foo")
+        helpers.build_and_add_cell_methods(mock_engine)
+
+    destination = LOAD_PROBLEMS.problems[-1].destination
+    assert destination.iris_class is Cube
+    assert destination.identifier == mock_engine.cf_var.cf_name
