combine_by_coords: error on differing calendars (#4503)

* mend

* add test and doc

* remove newline

Author: mathause

doc/whats-new.rst

@@ -45,6 +45,8 @@ Bug fixes
   is now ignored when not applicable, i.e. when ``skipna=False`` or when ``skipna=None``
   and the dtype does not have a missing value (:issue:`4352`).
   By `Mathias Hauser <https://github.com/mathause>`_.
+- :py:func:`combine_by_coords` now raises an informative error when passing coordinates
+  with differing calendars (:issue:`4495`). By `Mathias Hauser <https://github.com/mathause>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/combine.py

@@ -93,7 +93,9 @@ def _infer_concat_order_from_coords(datasets):
                 # position indices - they should be concatenated along another
                 # dimension, not along this one
                 series = first_items.to_series()
-                rank = series.rank(method="dense", ascending=ascending)
+                rank = series.rank(
+                    method="dense", ascending=ascending, numeric_only=False
+                )
                 order = rank.astype(int).values - 1
 
                 # Append positions along extra dimension to structure which

xarray/tests/test_combine.py

@@ -839,3 +839,20 @@ def test_combine_by_coords_distant_cftime_dates():
         [0, 1, 2], dims=["time"], coords=[expected_time], name="a"
     ).to_dataset()
     assert_identical(result, expected)
+
+
+@requires_cftime
+def test_combine_by_coords_raises_for_differing_calendars():
+    # previously failed with uninformative StopIteration instead of TypeError
+    # https://github.com/pydata/xarray/issues/4495
+
+    import cftime
+
+    time_1 = [cftime.DatetimeGregorian(2000, 1, 1)]
+    time_2 = [cftime.DatetimeProlepticGregorian(2001, 1, 1)]
+
+    da_1 = DataArray([0], dims=["time"], coords=[time_1], name="a").to_dataset()
+    da_2 = DataArray([1], dims=["time"], coords=[time_2], name="a").to_dataset()
+
+    with raises_regex(TypeError, r"cannot compare .* \(different calendars\)"):
+        combine_by_coords([da_1, da_2])