Nc load latlon fix (#4470)

* Tests, and partial fix, for latlon identification problems.

* Updated to align with tests developed against Iris 3.0.4, from the branch 'nc_load_latlon_further_RETRO'.

lib/iris/fileformats/_nc_load_rules/actions.py

@@ -184,16 +184,19 @@ def action_provides_coordinate(engine, dimcoord_fact):
 
     # Identify the "type" of a coordinate variable
     coord_type = None
-    # NOTE: must test for rotated cases *first*, as 'is_longitude' and
-    # 'is_latitude' functions also accept rotated cases.
-    if hh.is_rotated_latitude(engine, var_name):
-        coord_type = "rotated_latitude"
-    elif hh.is_rotated_longitude(engine, var_name):
-        coord_type = "rotated_longitude"
-    elif hh.is_latitude(engine, var_name):
-        coord_type = "latitude"
+
+    if hh.is_latitude(engine, var_name):
+        # N.B. result of 'is_rotated_lat/lon' checks are valid ONLY when the
+        # relevant 'is_lat/lon' is also True.
+        if hh.is_rotated_latitude(engine, var_name):
+            coord_type = "rotated_latitude"
+        else:
+            coord_type = "latitude"
     elif hh.is_longitude(engine, var_name):
-        coord_type = "longitude"
+        if hh.is_rotated_longitude(engine, var_name):
+            coord_type = "rotated_longitude"
+        else:
+            coord_type = "longitude"
     elif hh.is_time(engine, var_name):
         coord_type = "time"
     elif hh.is_time_period(engine, var_name):

lib/iris/tests/unit/fileformats/nc_load_rules/actions/__init__.py

@@ -6,9 +6,6 @@
 """
 Unit tests for the module :mod:`iris.fileformats._nc_load_rules.actions`.
 
-This module provides the engine.activate() call used in the function
-`iris.fileformats.netcdf._load_cube`.
-
 """
 from pathlib import Path
 import shutil

lib/iris/tests/unit/fileformats/nc_load_rules/actions/test__latlon_dimcoords.py

@@ -0,0 +1,337 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the LGPL license.
+# See COPYING and COPYING.LESSER in the root of the repository for full
+# licensing details.
+"""
+Unit tests for the engine.activate() call within the
+`iris.fileformats.netcdf._load_cube` function.
+
+Tests for rules behaviour in identifying latitude/longitude dim-coords, both
+rotated and non-rotated.
+
+"""
+import iris.tests as tests  # isort: skip
+
+from iris.coord_systems import GeogCS, RotatedGeogCS
+from iris.tests.unit.fileformats.nc_load_rules.actions import (
+    Mixin__nc_load_actions,
+)
+
+
+class Mixin_latlon_dimcoords(Mixin__nc_load_actions):
+    # Tests for the recognition and construction of latitude/longitude coords.
+
+    # Control to test either longitude or latitude coords.
+    # Set by inheritor classes, which are actual TestCases.
+    lat_1_or_lon_0 = None
+
+    def setUp(self):
+        super().setUp()
+        # Generate some useful settings : just to generalise operation over
+        # both latitude and longitude.
+        islat = self.lat_1_or_lon_0
+        assert islat in (0, 1)
+        self.unrotated_name = "latitude" if islat else "longitude"
+        self.rotated_name = "grid_latitude" if islat else "grid_longitude"
+        self.unrotated_units = "degrees_north" if islat else "degrees_east"
+        # Note: there are many alternative valid forms for the rotated units,
+        # but we are not testing that here.
+        self.rotated_units = "degrees"  # NB this one is actually constant
+        self.axis = "y" if islat else "x"
+
+    def _make_testcase_cdl(
+        self,
+        standard_name=None,
+        long_name=None,
+        var_name=None,
+        units=None,
+        axis=None,
+        grid_mapping=None,
+    ):
+        # Inner routine called by 'run_testcase' (in Mixin__nc_load_actions),
+        # to generate CDL which is then translated into a testfile and loaded.
+        if var_name is None:
+            # Can't have *no* var-name
+            # N.B. it is also the name of the dimension.
+            var_name = "dim"
+
+        def attribute_str(name, value):
+            if value is None or value == "":
+                result = ""
+            else:
+                result = f'{var_name}:{name} = "{value}" ;'
+
+            return result
+
+        standard_name_str = attribute_str("standard_name", standard_name)
+        long_name_str = attribute_str("long_name", long_name)
+        units_str = attribute_str("units", units)
+        axis_str = attribute_str("axis", axis)
+        if grid_mapping:
+            grid_mapping_str = 'phenom:grid_mapping = "crs" ;'
+        else:
+            grid_mapping_str = ""
+
+        assert grid_mapping in (None, "latlon", "rotated")
+        if grid_mapping is None:
+            crs_str = ""
+        elif grid_mapping == "latlon":
+            crs_str = """
+      int crs ;
+        crs:grid_mapping_name = "latitude_longitude" ;
+        crs:semi_major_axis = 6371000.0 ;
+        crs:inverse_flattening = 1000. ;
+"""
+        elif grid_mapping == "rotated":
+            crs_str = """
+      int crs ;
+        crs:grid_mapping_name = "rotated_latitude_longitude" ;
+        crs:grid_north_pole_latitude = 32.5 ;
+        crs:grid_north_pole_longitude = 170. ;
+"""
+
+        cdl_string = f"""
+netcdf test {{
+    dimensions:
+        {var_name} = 2 ;
+    variables:
+        double {var_name}({var_name}) ;
+            {standard_name_str}
+            {units_str}
+            {long_name_str}
+            {axis_str}
+        double phenom({var_name}) ;
+            phenom:standard_name = "air_temperature" ;
+            phenom:units = "K" ;
+            {grid_mapping_str}
+        {crs_str}
+    data:
+        {var_name} = 0., 1. ;
+}}
+"""
+        return cdl_string
+
+    def check_result(
+        self,
+        cube,
+        standard_name,
+        long_name,
+        units,
+        crs=None,
+        context_message="",
+    ):
+        # Check the existence, standard-name, long-name, units and coord-system
+        # of the resulting coord.  Also that it is always a dim-coord.
+        # NOTE: there is no "axis" arg, as this information does *not* appear
+        # as a separate property (or attribute) of the resulting coord.
+        # However, whether the file variable has an axis attribute *does*
+        # affect the results here, in some cases.
+        coords = cube.coords()
+        # There should be one and only one coord.
+        self.assertEqual(1, len(coords))
+        # It should also be a dim-coord
+        self.assertEqual(1, len(cube.coords(dim_coords=True)))
+        (coord,) = coords
+        if self.debug:
+            print("")
+            print("DEBUG : result coord =", coord)
+            print("")
+
+        coord_stdname, coord_longname, coord_units, coord_crs = [
+            getattr(coord, name)
+            for name in ("standard_name", "long_name", "units", "coord_system")
+        ]
+        self.assertEqual(standard_name, coord_stdname, context_message)
+        self.assertEqual(long_name, coord_longname, context_message)
+        self.assertEqual(units, coord_units, context_message)
+        assert crs in (None, "latlon", "rotated")
+        if crs is None:
+            self.assertEqual(None, coord_crs, context_message)
+        elif crs == "latlon":
+            self.assertIsInstance(coord_crs, GeogCS, context_message)
+        elif crs == "rotated":
+            self.assertIsInstance(coord_crs, RotatedGeogCS, context_message)
+
+    #
+    # Testcase routines
+    #
+    # NOTE: all these testcases have been verified against the older behaviour
+    # in v3.0.4, based on Pyke rules.
+    #
+
+    def test_minimal(self):
+        # Nothing but a var-name --> unrecognised dim-coord.
+        result = self.run_testcase()
+        self.check_result(result, None, None, "unknown")
+
+    def test_fullinfo_unrotated(self):
+        # Check behaviour with all normal info elements for 'unrotated' case.
+        # Includes a grid-mapping, but no axis (should not be needed).
+        result = self.run_testcase(
+            standard_name=self.unrotated_name,
+            units=self.unrotated_units,
+            grid_mapping="latlon",
+        )
+        self.check_result(
+            result, self.unrotated_name, None, "degrees", "latlon"
+        )
+
+    def test_fullinfo_rotated(self):
+        # Check behaviour with all normal info elements for 'rotated' case.
+        # Includes a grid-mapping, but no axis (should not be needed).
+        result = self.run_testcase(
+            standard_name=self.rotated_name,
+            units=self.rotated_units,
+            grid_mapping="rotated",
+        )
+        self.check_result(
+            result, self.rotated_name, None, "degrees", "rotated"
+        )
+
+    def test_axis(self):
+        # A suitable axis --> unrotated lat/lon coord, but unknown units.
+        result = self.run_testcase(axis=self.axis)
+        self.check_result(result, self.unrotated_name, None, "unknown")
+
+    def test_units_unrotated(self):
+        # With a unit like 'degrees_east', we automatically identify this as a
+        # latlon coord, *and* convert units to plain 'degrees' on loading.
+        result = self.run_testcase(units=self.unrotated_units)
+        self.check_result(result, self.unrotated_name, None, "degrees")
+
+    def test_units_rotated(self):
+        # With no info except a "degrees" unit, we **don't** identify a latlon,
+        # i.e. we do not set the standard-name
+        result = self.run_testcase(units="degrees")
+        self.check_result(result, None, None, "degrees")
+
+    def test_units_unrotated_gridmapping(self):
+        # With an unrotated unit *AND* a suitable grid-mapping, we identify a
+        # rotated latlon coordinate + assign it the coord-system.
+        result = self.run_testcase(
+            units=self.unrotated_units, grid_mapping="latlon"
+        )
+        self.check_result(
+            result, self.unrotated_name, None, "degrees", "latlon"
+        )
+
+    def test_units_rotated_gridmapping_noname(self):
+        # Rotated units and grid-mapping, but *without* the expected name.
+        # Does not translate, no coord-system (i.e. grid-mapping is discarded).
+        result = self.run_testcase(
+            units="degrees",
+            grid_mapping="rotated",
+        )
+        self.check_result(result, None, None, "degrees", None)
+
+    def test_units_rotated_gridmapping_withname(self):
+        # With a "degrees" unit, a rotated grid-mapping *AND* a suitable
+        # standard-name, it recognises a rotated dimcoord.
+        result = self.run_testcase(
+            standard_name=self.rotated_name,
+            units="degrees",
+            grid_mapping="rotated",
+        )
+        self.check_result(
+            result, self.rotated_name, None, "degrees", "rotated"
+        )
+
+    def test_units_rotated_gridmapping_varname(self):
+        # Same but with var-name containing the standard-name : in this case we
+        # get NO COORDINATE-SYSTEM (which is a bit weird).
+        result = self.run_testcase(
+            var_name=self.rotated_name,
+            units="degrees",
+            grid_mapping="rotated",
+        )
+        self.check_result(result, self.rotated_name, None, "degrees", None)
+
+    def test_varname_unrotated(self):
+        # With a recognised name in the var-name, we set standard-name.
+        # But units are left undetermined.
+        result = self.run_testcase(var_name=self.unrotated_name)
+        self.check_result(result, self.unrotated_name, None, "unknown")
+
+    def test_varname_rotated(self):
+        # With a *rotated* name in the var-name, we set standard-name.
+        # But units are left undetermined.
+        result = self.run_testcase(var_name=self.rotated_name)
+        self.check_result(result, self.rotated_name, None, "unknown")
+
+    def test_varname_unrotated_units_rotated(self):
+        # With a "degrees" unit and a suitable var-name, we do identify
+        # (= set standard-name).
+        # N.B. this accepts "degrees" as a generic term, and so does *not*
+        # interpret it as a rotated coordinate.
+        result = self.run_testcase(
+            var_name=self.unrotated_name, units="degrees"
+        )
+        self.check_result(result, self.unrotated_name, None, "degrees")
+
+    def test_longname(self):
+        # A recognised form in long-name is *not* translated into standard-name.
+        result = self.run_testcase(long_name=self.unrotated_name)
+        self.check_result(result, None, self.unrotated_name, "unknown")
+
+    def test_stdname_unrotated(self):
+        # Only an (unrotated) standard name : units is not specified
+        result = self.run_testcase(standard_name=self.unrotated_name)
+        self.check_result(result, self.unrotated_name, None, None)
+
+    def test_stdname_rotated(self):
+        # Only a (rotated) standard name : units is not specified
+        result = self.run_testcase(standard_name=self.rotated_name)
+        self.check_result(result, self.rotated_name, None, None)
+
+    def test_stdname_unrotated_gridmapping(self):
+        # An unrotated standard-name and grid-mapping, translates into a
+        # coordinate system.
+        result = self.run_testcase(
+            standard_name=self.unrotated_name, grid_mapping="latlon"
+        )
+        self.check_result(
+            result, self.unrotated_name, None, "unknown", "latlon"
+        )
+
+    def test_stdname_rotated_gridmapping(self):
+        # An *rotated* standard-name and grid-mapping, translates into a
+        # coordinate system.
+        result = self.run_testcase(
+            standard_name=self.rotated_name, grid_mapping="rotated"
+        )
+        self.check_result(result, self.rotated_name, None, None, "rotated")
+
+
+class Test__longitude_coords(Mixin_latlon_dimcoords, tests.IrisTest):
+    lat_1_or_lon_0 = 0
+
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+
+    @classmethod
+    def tearDownClass(cls):
+        super().tearDownClass()
+
+    def setUp(self):
+        super().setUp()
+
+
+class Test__latitude_coords(Mixin_latlon_dimcoords, tests.IrisTest):
+    lat_1_or_lon_0 = 1
+
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+
+    @classmethod
+    def tearDownClass(cls):
+        super().tearDownClass()
+
+    def setUp(self):
+        super().setUp()
+
+
+if __name__ == "__main__":
+    tests.main()