From e5c6a13c123a2c89806bead05217f1d9231e8135 Mon Sep 17 00:00:00 2001
From: Henri Drake <henrifdrake@ch3212.ps.uci.edu>
Date: Wed, 18 Mar 2026 11:53:33 -0700
Subject: [PATCH 1/2] Test `get_geo_corners` and its ValueError if (lon, lat)
 missing

---
 sectionate/gridutils.py        |  8 +++++--
 sectionate/tests/test_utils.py | 40 +++++++++++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/sectionate/gridutils.py b/sectionate/gridutils.py
index 3ad3e6f..035f4d3 100644
--- a/sectionate/gridutils.py
+++ b/sectionate/gridutils.py
@@ -25,7 +25,8 @@ def get_geo_corners(grid):
             vorticity coordinates at 'outer' and 'right' positions, respectively.")
 
     coords = grid._ds.coords
-    return {
+
+    geo_coord_dict = {
         axis: [
             coords[c] for c in coords
             if (
@@ -33,9 +34,12 @@ def get_geo_corners(grid):
                 (dims["X"] in coords[c].dims) and
                 (dims["Y"] in coords[c].dims)
             )
-        ][0]
+        ]
         for axis, geoc in zip(["X", "Y"], ["lon", "lat"])
     }
+    if any([len(v) == 0 for (k,v) in geo_coord_dict.items()]):
+        raise ValueError("grid._ds must contain two-dimensional coordinates, lon(X,Y) and lat(X,Y), consistent with grid.coords")
+    return {k:v[0] for (k,v) in geo_coord_dict.items()}
     
 def coord_dict(grid):
     """
diff --git a/sectionate/tests/test_utils.py b/sectionate/tests/test_utils.py
index ae3db07..6c83e36 100644
--- a/sectionate/tests/test_utils.py
+++ b/sectionate/tests/test_utils.py
@@ -1,4 +1,42 @@
+import pytest
+
 def test_load_section():
     from sectionate.utils import get_all_section_names, load_section
     section_names = get_all_section_names()
-    load_section(section_names[0])
\ No newline at end of file
+    load_section(section_names[0])
+
+def test_get_geo_corners():
+    import numpy as np
+    import xarray as xr
+    import xgcm
+    coords = {
+        "xh": np.arange(0, 10),
+        "yh": np.arange(0, 10),
+        "xq": np.arange(0, 11),
+        "yq": np.arange(0, 11),
+    }
+
+    # Without coordinates
+    ds = xr.Dataset(coords=coords)
+    grid = xgcm.Grid(
+        ds = ds,
+        coords={
+            "X":{"center":"xh", "outer":"xq"},
+            "Y":{"center":"yh", "outer":"yq"}
+        },
+        boundary={"X":"periodic", "Y":"extend"},
+        autoparse_metadata=False
+    )
+
+    from sectionate.gridutils import get_geo_corners
+
+    # Fail when (lon, lat) coordinates are missing
+    with pytest.raises(ValueError):
+        get_geo_corners(grid)
+
+    # Pass when coordinates are there
+    grid._ds = grid._ds.assign_coords({
+        "lon_c": xr.DataArray(xr.broadcast(grid._ds.xq, grid._ds.yq)[0]),
+        "lat_c": xr.DataArray(xr.broadcast(grid._ds.xq, grid._ds.yq)[1])
+    })
+    get_geo_corners(grid)
\ No newline at end of file

From 212a5ee5c1a251051f358cc78278987f8be947ab Mon Sep 17 00:00:00 2001
From: Henri Drake <henrifdrake@ch3212.ps.uci.edu>
Date: Wed, 18 Mar 2026 11:57:11 -0700
Subject: [PATCH 2/2] Clarified error message

---
 sectionate/gridutils.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sectionate/gridutils.py b/sectionate/gridutils.py
index 035f4d3..19cccfc 100644
--- a/sectionate/gridutils.py
+++ b/sectionate/gridutils.py
@@ -30,7 +30,7 @@ def get_geo_corners(grid):
         axis: [
             coords[c] for c in coords
             if (
-                (geoc in c) and
+                (geoc in c.lower()) and
                 (dims["X"] in coords[c].dims) and
                 (dims["Y"] in coords[c].dims)
             )
@@ -38,7 +38,7 @@ def get_geo_corners(grid):
         for axis, geoc in zip(["X", "Y"], ["lon", "lat"])
     }
     if any([len(v) == 0 for (k,v) in geo_coord_dict.items()]):
-        raise ValueError("grid._ds must contain two-dimensional coordinates, lon(X,Y) and lat(X,Y), consistent with grid.coords")
+        raise ValueError("""grid._ds must contain two-dimensional ("X", "Y") coordinates including the strings "lon" and "lat", consistent with grid.coords.""")
     return {k:v[0] for (k,v) in geo_coord_dict.items()}
     
 def coord_dict(grid):