Improve error message

xarray/core/combine.py

@@ -327,7 +327,10 @@ def _dataarray_concat(arrays, dim, data_vars, coords, compat,
     name = result_name(arrays)
     names = [arr.name for arr in arrays]
     if compat == 'identical' and len(set(names)) != 1:
-        raise ValueError('array names not identical')
+        raise ValueError(
+            "compat='identical', but array names {!r} are not identical"
+            .format(names if len(names) <= 10 else sorted(set(names)))
+        )
     datasets = [arr.rename(name)._to_temp_dataset() for arr in arrays]
 
     if isinstance(dim, str) and len(set(names) - {None}) == len(names) \

xarray/tests/test_combine.py

@@ -306,6 +306,11 @@ def test_concat_names_and_coords(self):
         new = concat([ds.foo, ds.bar], dim='new')
         assert new.name is None
         assert (new.coords['new'] == ['foo', 'bar']).values.all()
+        # Get a useful error message for unexpectedly different names
+        with pytest.raises(ValueError) as err:
+            concat([ds.foo, ds.bar], dim='new', compat='identical')
+        assert err.value.args[0] == "compat='identical', " + \
+            "but array names ['foo', 'bar'] are not identical"
         # Concat arrays with same name, name is preserved
         # and non-unique names are not used as coords
         foobar = ds.foo.rename('bar')