ENH: Make coordinate detection more robust (Fixes #3416)

This works around an issue where some ERA5 data had lat/lon with a
"positive" attribute that, according to the CF specification, should
indicate a vertical coordinate. We can fix this up by making sure each
coordinate variable is only associated with a single option, and making
use of Python's key-ordered dictionaries to prioritize the matching
order. Having lat be y and lon be x is already handled by a later block.

src/metpy/xarray.py

@@ -338,15 +338,16 @@ def assign_coordinates(self, coordinates):
     def _generate_coordinate_map(self):
         """Generate a coordinate map via CF conventions and other methods."""
         coords = self._data_array.coords.values()
-        # Parse all the coordinates, attempting to identify x, longitude, y, latitude,
-        # vertical, time
-        coord_lists = {'time': [], 'vertical': [], 'y': [], 'latitude': [], 'x': [],
-                       'longitude': []}
+        # Parse all the coordinates, attempting to identify longitude, latitude, x, y,
+        # time, vertical, in that order.
+        coord_lists = {'longitude': [], 'latitude': [], 'x': [], 'y': [], 'time': [],
+                       'vertical': []}
         for coord_var in coords:
             # Identify the coordinate type using check_axis helper
             for axis in coord_lists:
                 if check_axis(coord_var, axis):
                     coord_lists[axis].append(coord_var)
+                    break  # Ensure a coordinate variable only goes to one axis
 
         # Fill in x/y with longitude/latitude if x/y not otherwise present
         for geometric, graticule in (('y', 'latitude'), ('x', 'longitude')):

tests/test_xarray.py

@@ -273,6 +273,14 @@ def test_missing_grid_mapping_invalid(test_var_multidim_no_xy):
     assert 'metpy_crs' not in data_var.coords
 
 
+def test_xy_not_vertical(test_ds):
+    """Test not detecting x/y as a vertical coordinate based on metadata."""
+    test_ds.x.attrs['positive'] = 'up'
+    test_ds.y.attrs['positive'] = 'up'
+    data_var = test_ds.metpy.parse_cf('Temperature')
+    assert data_var.metpy.vertical.identical(data_var.coords['isobaric'])
+
+
 def test_missing_grid_mapping_var(caplog):
     """Test behavior when we can't find the variable pointed to by grid_mapping."""
     x = xr.DataArray(np.arange(3),