better align error (#34)

Author: benbovy

xproj/index.py

@@ -6,6 +6,10 @@
 import pyproj
 import xarray as xr
 from pyproj.exceptions import CRSError
+
+# TODO: import from xarray.errors when available
+# (https://github.com/pydata/xarray/pull/10285)
+from xarray import AlignmentError
 from xarray.indexes import Index
 
 
@@ -85,6 +89,15 @@ def equals(self, other: Index, *, exclude: frozenset[Hashable] | None = None) ->
             return False
         return True
 
+    def join(self, other: CRSIndex, how: str = "inner") -> CRSIndex:
+        # If this method is called during Xarray alignment, it means that the
+        # equality check failed. Instead of a NotImplementedError we raise a
+        # ValueError with a nice error message.
+        raise AlignmentError(
+            "Objects to align do not have the same CRS\n"
+            f"first index:\n{self!r}\n\nsecond index:\n{other!r}"
+        )
+
     def _repr_inline_(self, max_width: int) -> str:
         if max_width is None:
             max_width = xr.get_options()["display_width"]

xproj/tests/test_index.py

@@ -81,3 +81,19 @@ def test_crsindex_equals() -> None:
 
     idx4 = CRSIndex(pyproj.CRS.from_user_input("epsg:4978"))
     assert idx1.equals(idx4) is False
+
+
+def test_align() -> None:
+    ds = xr.Dataset(coords={"spatial_ref": 0})
+
+    crs1 = pyproj.CRS.from_user_input("epsg:4326")
+    crs2 = pyproj.CRS.from_user_input("epsg:4978")
+
+    ds_crs1 = ds.set_xindex("spatial_ref", CRSIndex, crs=crs1)
+    ds_crs2 = ds.set_xindex("spatial_ref", CRSIndex, crs=crs2)
+
+    with pytest.raises(xr.AlignmentError, match="do not have the same CRS"):
+        xr.align(ds_crs1, ds_crs2, join="inner")
+
+    with pytest.raises(xr.AlignmentError, match="cannot align objects with join='exact'"):
+        xr.align(ds_crs1, ds_crs2, join="exact")