Fix missing coordinate attributes after apply_ufunc operations

doc/whats-new.rst

@@ -57,7 +57,8 @@ Bug fixes
   Haacker <https://github.com/j-haacker>`_.
 - Fix ``isel`` for multi-coordinate Xarray indexes (:issue:`10063`, :pull:`10066`).
   By `Benoit Bovy <https://github.com/benbovy>`_.
-
+- Always merge coordinates with "no_conflicts" to fix missing attributes after apply_ufunc
+  operations (:issue:`9317`). By `Jasper de Jong <https://github.com/JdeJong96>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/computation.py

@@ -306,7 +306,7 @@ def apply_dataarray_vfunc(
         first_obj = _first_of_type(args, DataArray)
         name = first_obj.name
     result_coords, result_indexes = build_output_coords_and_indexes(
-        args, signature, exclude_dims, combine_attrs=keep_attrs
+        args, signature, exclude_dims, combine_attrs="no_conflicts"
     )
 
     data_vars = [getattr(a, "variable", a) for a in args]
@@ -522,7 +522,7 @@ def apply_dataset_vfunc(
         )
 
     list_of_coords, list_of_indexes = build_output_coords_and_indexes(
-        args, signature, exclude_dims, combine_attrs=keep_attrs
+        args, signature, exclude_dims, combine_attrs="no_conflicts"
     )
     args = tuple(getattr(arg, "data_vars", arg) for arg in args)
 

xarray/tests/test_computation.py

@@ -691,6 +691,12 @@
     )
 
 
+def test_keep_coordattrs() -> None:
+    a = xr.DataArray([0, 1], [("x", [0, 1], {"a": "b"})])
+    actual = apply_ufunc(operator.abs, a)
+    assert actual.x.attrs == a.x.attrs
+
+
 def test_keep_attrs() -> None:
     def add(a, b, keep_attrs):
         if keep_attrs:
@@ -955,7 +961,7 @@
             data=[0, 3],
             coords={"x": ("x", [0, 1], dim_attrs), "u": ("x", [0, 1], coord_attrs)},
         )
         actual = apply_ufunc(lambda *args: sum(args), a, b, c, keep_attrs=strategy)
 
         assert_identical(actual, expected)
 
@@ -2211,7 +2217,7 @@
     y["a"].attrs = {"attr": "y_coord"}
 
     # 3 DataArrays, takes attrs from x
     actual = xr.where(cond, x, y, keep_attrs=True)
     expected = xr.DataArray([1, 0], coords={"a": [0, 1]}, attrs={"attr": "x_da"})
     expected["a"].attrs = {"attr": "x_coord"}
     assert_identical(expected, actual)