Allowing exemption to axis guessing on coords

lib/iris/coords.py

@@ -37,6 +37,9 @@
 import iris.time
 import iris.util
 
+#: The default value for ignore_axis which controls guess_coord_axis' behaviour
+DEFAULT_IGNORE_AXIS = False
+
 
 class _DimensionalMetadata(CFVariableMixin, metaclass=ABCMeta):
     """
@@ -861,7 +864,8 @@ def xml_element(self, doc):
                 element.setAttribute(
                     "climatological", str(self.climatological)
                 )
-
+        if self.guess_coord:
+            element.setAttribute("guess_coord", str(self.guess_coord))
         if self.attributes:
             attributes_element = doc.createElement("attributes")
             for name in sorted(self.attributes.keys()):
@@ -1608,6 +1612,8 @@ def __init__(
         self.bounds = bounds
         self.climatological = climatological
 
+        self._ignore_axis = DEFAULT_IGNORE_AXIS
+
     def copy(self, points=None, bounds=None):
         """
         Returns a copy of this coordinate.
@@ -1640,6 +1646,10 @@ def copy(self, points=None, bounds=None):
             # self.
             new_coord.bounds = bounds
 
+        # The state of ignore_axis is controlled by the coordinate rather than
+        # the metadata manager
+        new_coord.ignore_axis = self.ignore_axis
+
         return new_coord
 
     @classmethod
@@ -1659,7 +1669,14 @@ def from_coord(cls, coord):
         if issubclass(cls, DimCoord):
             # DimCoord introduces an extra constructor keyword.
             kwargs["circular"] = getattr(coord, "circular", False)
-        return cls(**kwargs)
+
+        new_coord = cls(**kwargs)
+
+        # The state of ignore_axis is controlled by the coordinate rather than
+        # the metadata manager
+        new_coord.ignore_axis = coord.ignore_axis
+
+        return new_coord
 
     @property
     def points(self):
@@ -1751,6 +1768,24 @@ def climatological(self, value):
 
         self._metadata_manager.climatological = value
 
+    @property
+    def ignore_axis(self):
+        """
+        A boolean that controls whether guess_coord_axis acts on this
+        coordinate.
+
+        Defaults to False, and when set to True it will be skipped by
+        guess_coord_axis.
+        """
+        return self._ignore_axis
+
+    @ignore_axis.setter
+    def ignore_axis(self, value):
+        if not isinstance(value, bool):
+            emsg = "Ignore_axis can only be set to True or False"
+            raise ValueError(emsg)
+        self._ignore_axis = value
+
     def lazy_points(self):
         """
         Return a lazy array representing the coord points.
@@ -2697,6 +2732,8 @@ def __init__(
             Will set to True when a climatological time axis is loaded
             from NetCDF.
             Always False if no bounds exist.
+        * guess_coord (bool):
+            When True: guess_coord will guess the coord, when False it won't.
 
         """
         # Configure the metadata manager.

lib/iris/tests/unit/coords/test_Coord.py

@@ -15,6 +15,7 @@
 
 import dask.array as da
 import numpy as np
+import pytest
 
 import iris
 from iris.coords import AuxCoord, Coord, DimCoord
@@ -1150,6 +1151,27 @@ def test_change_units(self):
         self.assertFalse(coord.climatological)
 
 
+@pytest.fixture
+def coord():
+    coord = iris.coords.DimCoord(points=(1, 2, 3, 4, 5))
+    return coord
+
+
+class Test_ignore_axis:
+    def test_default(self, coord):
+        assert coord.ignore_axis is False
+
+    def test_set_true(self, coord):
+        coord.ignore_axis = True
+        assert coord.ignore_axis is True
+
+    def test_set_random_value(self, coord):
+        with pytest.raises(
+            ValueError, match=r"Ignore_axis can only be set to True or False"
+        ):
+            coord.ignore_axis = "foo"
+
+
 class Test___init____abstractmethod(tests.IrisTest):
     def test(self):
         emsg = (

lib/iris/tests/unit/util/test_guess_coord_axis.py

@@ -0,0 +1,50 @@
+# 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.
+"""Test function :func:`iris.util.guess_coord_axis"""
+
+# Import iris.tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests  # isort:skip
+
+import numpy as np
+import pytest
+
+import iris.coords
+from iris.util import guess_coord_axis
+
+
+@pytest.fixture
+def coord():
+    coord = iris.coords.DimCoord(points=(1, 2, 3, 4, 5))
+    return coord
+
+
+class TestCoords:
+    def test_longitude(self, coord):
+        coord.standard_name = "longitude"
+
+        assert guess_coord_axis(coord) == "X"
+
+    def test_latitude(self, coord):
+        coord.standard_name = "latitude"
+
+        assert guess_coord_axis(coord) == "Y"
+
+    def test_pressure_units(self, coord):
+        coord.units = "hPa"
+
+        assert guess_coord_axis(coord) == "Z"
+
+    def test_time_units(self, coord):
+        coord.units = "days since 1970-01-01 00:00:00"
+
+        assert guess_coord_axis(coord) == "T"
+
+    def test_guess_coord(self, coord):
+        coord.standard_name = "longitude"
+        coord.ignore_axis = True
+
+        assert guess_coord_axis(coord) is None

lib/iris/util.py

@@ -258,10 +258,17 @@ def guess_coord_axis(coord):
     This function maintains laziness when called; it does not realise data.
     See more at :doc:`/userguide/real_and_lazy_data`.
 
+    ``guess_coord_axis`` can be skipped by setting the coordinate property of a coord ignore_axis
+    to False.
+
     """
+
     axis = None
 
-    if coord.standard_name in (
+    if coord.ignore_axis is True:
+        return axis
+
+    elif coord.standard_name in (
         "longitude",
         "grid_longitude",
         "projection_x_coordinate",