Quieter datum warning (SciTools#6050)

* Only warn about datum handling if a datum is present
.

* Partial conversion to PyTest.

* Add test to confirm warnings not raised if no datum is present.

* What's New entry.

* Rewrite duplicate warning testing.

* What's New typo.

docs/src/whatsnew/latest.rst

@@ -59,6 +59,10 @@ This document explains the changes made to Iris for this release
 #. `@pp-mo`_ corrected the use of mesh dimensions when saving with multiple
    meshes.  (:issue:`5908`, :pull:`6004`)
 
+#. `@trexfeathers`_ fixed the datum :class:`python:FutureWarning` to only be raised if
+   the ``datum_support`` :class:`~iris.Future` flag is disabled AND a datum is
+   present on the loaded NetCDF grid mapping. (:issue:`5749`, :pull:`6050`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/fileformats/_nc_load_rules/helpers.py

@@ -522,7 +522,7 @@ def _get_ellipsoid(cf_grid_var):
     if datum == "unknown":
         datum = None
 
-    if not iris.FUTURE.datum_support:
+    if datum is not None and not iris.FUTURE.datum_support:
         wmsg = (
             "Ignoring a datum in netCDF load for consistency with existing "
             "behaviour. In a future version of Iris, this datum will be "

lib/iris/tests/integration/netcdf/test_coord_systems.py

@@ -11,6 +11,9 @@
 from os.path import join as path_join
 import shutil
 import tempfile
+import warnings
+
+import pytest
 
 import iris
 from iris.coords import DimCoord
@@ -135,15 +138,25 @@ def test_load_datum_wkt(self):
             cube = iris.load_cube(nc_path)
         test_crs = cube.coord("projection_y_coordinate").coord_system
         actual = str(test_crs.as_cartopy_crs().datum)
-        self.assertMultiLineEqual(expected, actual)
+        assert actual == expected
 
     def test_no_load_datum_wkt(self):
         nc_path = tlc.cdl_to_nc(self.datum_wkt_cdl)
-        with self.assertWarnsRegex(FutureWarning, "iris.FUTURE.datum_support"):
+        with pytest.warns(FutureWarning, match="iris.FUTURE.datum_support"):
             cube = iris.load_cube(nc_path)
         test_crs = cube.coord("projection_y_coordinate").coord_system
         actual = str(test_crs.as_cartopy_crs().datum)
-        self.assertMultiLineEqual(actual, "unknown")
+        assert actual == "unknown"
+
+    def test_no_datum_no_warn(self):
+        new_cdl = self.datum_wkt_cdl.splitlines()
+        new_cdl = [line for line in new_cdl if "DATUM" not in line]
+        new_cdl = "\n".join(new_cdl)
+        nc_path = tlc.cdl_to_nc(new_cdl)
+        with warnings.catch_warnings():
+            # pytest's recommended way to assert for no warnings.
+            warnings.simplefilter("error", FutureWarning)
+            _ = iris.load_cube(nc_path)
 
     def test_load_datum_cf_var(self):
         expected = "OSGB 1936"
@@ -152,15 +165,15 @@ def test_load_datum_cf_var(self):
             cube = iris.load_cube(nc_path)
         test_crs = cube.coord("projection_y_coordinate").coord_system
         actual = str(test_crs.as_cartopy_crs().datum)
-        self.assertMultiLineEqual(expected, actual)
+        assert actual == expected
 
     def test_no_load_datum_cf_var(self):
         nc_path = tlc.cdl_to_nc(self.datum_cf_var_cdl)
-        with self.assertWarnsRegex(FutureWarning, "iris.FUTURE.datum_support"):
+        with pytest.warns(FutureWarning, match="iris.FUTURE.datum_support"):
             cube = iris.load_cube(nc_path)
         test_crs = cube.coord("projection_y_coordinate").coord_system
         actual = str(test_crs.as_cartopy_crs().datum)
-        self.assertMultiLineEqual(actual, "unknown")
+        assert actual == "unknown"
 
     def test_save_datum(self):
         expected = "OSGB 1936"
@@ -199,7 +212,7 @@ def test_save_datum(self):
 
         test_crs = cube.coord("projection_y_coordinate").coord_system
         actual = str(test_crs.as_cartopy_crs().datum)
-        self.assertMultiLineEqual(expected, actual)
+        assert actual == expected
 
 
 class TestLoadMinimalGeostationary(tests.IrisTest):
@@ -270,9 +283,9 @@ def test_geostationary_no_false_offsets(self):
         cube = iris.load_cube(self.path_test_nc)
         # Check the coordinate system properties has the correct default properties.
         cs = cube.coord_system()
-        self.assertIsInstance(cs, iris.coord_systems.Geostationary)
-        self.assertEqual(cs.false_easting, 0.0)
-        self.assertEqual(cs.false_northing, 0.0)
+        assert isinstance(cs, iris.coord_systems.Geostationary)
+        assert cs.false_easting == 0.0
+        assert cs.false_northing == 0.0
 
 
 if __name__ == "__main__":

lib/iris/tests/integration/netcdf/test_general.py

@@ -488,37 +488,38 @@ def test_path_string_save_same(self):
 
 @tests.skip_data
 class TestWarningRepeats(tests.IrisTest):
-    def test_datum_once(self):
-        """Tests for warnings being duplicated.
+    def test_warning_repeats(self):
+        """Confirm Iris load does not break Python duplicate warning handling."""
+        # units.nc is designed for testing Iris' 'ignoring invalid units'
+        #  warning; it contains two variables with invalid units, producing two
+        #  unique warnings (due to two different messages).
+        file_path = tests.get_data_path(("NetCDF", "testing", "units.nc"))
 
-        Notes
-        -----
-        This test relies on `iris.load` throwing a warning. This warning might
-        be removed in the future, in which case `assert len(record) == 2 should`
-        be change to `assert len(record) == 1`.
-
-        toa_brightness_temperature.nc has an AuxCoord with lazy data, and triggers a
-        specific part of dask which contains a `catch_warnings()` call which
-        causes warnings to be repeated, and so has been removed from the
-        `fnames` list until a solution is found for such a file.
-
-        """
-        #
-        fnames = [
-            "false_east_north_merc.nc",
-            "non_unit_scale_factor_merc.nc",
-            # toa_brightness_temperature.nc,
-        ]
-        fpaths = [
-            tests.get_data_path(("NetCDF", "mercator", fname)) for fname in fnames
-        ]
+        def _raise_warning() -> None:
+            # Contain in function so warning always has identical line number.
+            warnings.warn("Dummy warning", category=iris.warnings.IrisUserWarning)
 
         with warnings.catch_warnings(record=True) as record:
             warnings.simplefilter("default")
-            for fpath in fpaths:
-                iris.load(fpath)
-                warnings.warn("Dummy warning", category=iris.warnings.IrisUserWarning)
-        assert len(record) == 2
+
+            # Warn before Iris has been invoked.
+            _raise_warning()
+            assert len(record) == 1
+
+            # This Iris call should raise 2 warnings and should NOT affect
+            #  Python's duplicate warning handling.
+            _ = iris.load(file_path)
+            assert len(record) == 3
+            # Raise a duplicate warning.
+            _raise_warning()
+            assert len(record) == 3
+
+            # Repeated identical calls should only raise duplicate warnings
+            #  and therefore not affect the record.
+            for i in range(2):
+                _ = iris.load(file_path)
+                _raise_warning()
+            assert len(record) == 3
 
 
 if __name__ == "__main__":