combine keep_attrs and combine_attrs in apply_ufunc

doc/whats-new.rst

@@ -67,6 +67,9 @@ New Features
 - apply ``combine_attrs`` on data variables and coordinate variables when concatenating
   and merging datasets and dataarrays (:pull:`4902`).
   By `Justus Magin <https://github.com/keewis>`_.
+- allow passing ``combine_attrs`` strategy names to the ``keep_attrs`` parameter of
+  :py:func:`apply_ufunc` (:pull:`5041`)
+  By `Justus Magin <https://github.com/keewis>`_.
 - Add :py:meth:`Dataset.to_pandas` (:pull:`5247`)
   By `Giacomo Caria <https://github.com/gcaria>`_.
 - Add :py:meth:`DataArray.plot.surface` which wraps matplotlib's `plot_surface` to make

xarray/core/computation.py

@@ -27,8 +27,8 @@
 
 from . import dtypes, duck_array_ops, utils
 from .alignment import align, deep_align
-from .merge import merge_coordinates_without_align
-from .options import OPTIONS
+from .merge import merge_attrs, merge_coordinates_without_align
+from .options import OPTIONS, _get_keep_attrs
 from .pycompat import is_duck_dask_array
 from .utils import is_dict_like
 from .variable import Variable
@@ -50,6 +50,11 @@ def _first_of_type(args, kind):
     raise ValueError("This should be unreachable.")
 
 
+def _all_of_type(args, kind):
+    """Return all objects of type 'kind'"""
+    return [arg for arg in args if isinstance(arg, kind)]
+
+
 class _UFuncSignature:
     """Core dimensions signature for a given function.
 
@@ -202,7 +207,10 @@ def _get_coords_list(args) -> List["Coordinates"]:
 
 
 def build_output_coords(
-    args: list, signature: _UFuncSignature, exclude_dims: AbstractSet = frozenset()
+    args: list,
+    signature: _UFuncSignature,
+    exclude_dims: AbstractSet = frozenset(),
+    combine_attrs: str = "override",
 ) -> "List[Dict[Any, Variable]]":
     """Build output coordinates for an operation.
 
@@ -230,7 +238,7 @@ def build_output_coords(
     else:
         # TODO: save these merged indexes, instead of re-computing them later
         merged_vars, unused_indexes = merge_coordinates_without_align(
-            coords_list, exclude_dims=exclude_dims
+            coords_list, exclude_dims=exclude_dims, combine_attrs=combine_attrs
         )
 
     output_coords = []
@@ -248,7 +256,12 @@ def build_output_coords(
 
 
 def apply_dataarray_vfunc(
-    func, *args, signature, join="inner", exclude_dims=frozenset(), keep_attrs=False
+    func,
+    *args,
+    signature,
+    join="inner",
+    exclude_dims=frozenset(),
+    keep_attrs="override",
 ):
     """Apply a variable level function over DataArray, Variable and/or ndarray
     objects.
@@ -260,12 +273,16 @@ def apply_dataarray_vfunc(
             args, join=join, copy=False, exclude=exclude_dims, raise_on_invalid=False
         )
 
-    if keep_attrs:
+    objs = _all_of_type(args, DataArray)
+
+    if keep_attrs == "drop":
+        name = result_name(args)
+    else:
         first_obj = _first_of_type(args, DataArray)
         name = first_obj.name
-    else:
-        name = result_name(args)
-    result_coords = build_output_coords(args, signature, exclude_dims)
+    result_coords = build_output_coords(
+        args, signature, exclude_dims, combine_attrs=keep_attrs
+    )
 
     data_vars = [getattr(a, "variable", a) for a in args]
     result_var = func(*data_vars)
@@ -279,13 +296,12 @@ def apply_dataarray_vfunc(
         (coords,) = result_coords
         out = DataArray(result_var, coords, name=name, fastpath=True)
 
-    if keep_attrs:
-        if isinstance(out, tuple):
-            for da in out:
-                # This is adding attrs in place
-                da._copy_attrs_from(first_obj)
-        else:
-            out._copy_attrs_from(first_obj)
+    attrs = merge_attrs([x.attrs for x in objs], combine_attrs=keep_attrs)
+    if isinstance(out, tuple):
+        for da in out:
+            da.attrs = attrs
+    else:
+        out.attrs = attrs
 
     return out
 
@@ -400,7 +416,7 @@ def apply_dataset_vfunc(
     dataset_join="exact",
     fill_value=_NO_FILL_VALUE,
     exclude_dims=frozenset(),
-    keep_attrs=False,
+    keep_attrs="override",
 ):
     """Apply a variable level function over Dataset, dict of DataArray,
     DataArray, Variable and/or ndarray objects.
@@ -414,15 +430,16 @@ def apply_dataset_vfunc(
             "dataset_fill_value argument."
         )
 
-    if keep_attrs:
-        first_obj = _first_of_type(args, Dataset)
+    objs = _all_of_type(args, Dataset)
 
     if len(args) > 1:
         args = deep_align(
             args, join=join, copy=False, exclude=exclude_dims, raise_on_invalid=False
         )
 
-    list_of_coords = build_output_coords(args, signature, exclude_dims)
+    list_of_coords = build_output_coords(
+        args, signature, exclude_dims, combine_attrs=keep_attrs
+    )
     args = [getattr(arg, "data_vars", arg) for arg in args]
 
     result_vars = apply_dict_of_variables_vfunc(
@@ -435,13 +452,13 @@ def apply_dataset_vfunc(
         (coord_vars,) = list_of_coords
         out = _fast_dataset(result_vars, coord_vars)
 
-    if keep_attrs:
-        if isinstance(out, tuple):
-            for ds in out:
-                # This is adding attrs in place
-                ds._copy_attrs_from(first_obj)
-        else:
-            out._copy_attrs_from(first_obj)
+    attrs = merge_attrs([x.attrs for x in objs], combine_attrs=keep_attrs)
+    if isinstance(out, tuple):
+        for ds in out:
+            ds.attrs = attrs
+    else:
+        out.attrs = attrs
+
     return out
 
 
@@ -609,14 +626,12 @@ def apply_variable_ufunc(
     dask="forbidden",
     output_dtypes=None,
     vectorize=False,
-    keep_attrs=False,
+    keep_attrs="override",
     dask_gufunc_kwargs=None,
 ):
     """Apply a ndarray level function over Variable and/or ndarray objects."""
     from .variable import Variable, as_compatible_data
 
-    first_obj = _first_of_type(args, Variable)
-
     dim_sizes = unified_dim_sizes(
         (a for a in args if hasattr(a, "dims")), exclude_dims=exclude_dims
     )
@@ -736,6 +751,12 @@ def func(*arrays):
             )
         )
 
+    objs = _all_of_type(args, Variable)
+    attrs = merge_attrs(
+        [obj.attrs for obj in objs],
+        combine_attrs=keep_attrs,
+    )
+
     output = []
     for dims, data in zip(output_dims, result_data):
         data = as_compatible_data(data)
@@ -758,8 +779,7 @@ def func(*arrays):
                     )
                 )
 
-        if keep_attrs:
-            var.attrs.update(first_obj.attrs)
+        var.attrs = attrs
         output.append(var)
 
     if signature.num_outputs == 1:
@@ -801,7 +821,7 @@ def apply_ufunc(
     join: str = "exact",
     dataset_join: str = "exact",
     dataset_fill_value: object = _NO_FILL_VALUE,
-    keep_attrs: bool = False,
+    keep_attrs: Union[bool, str] = None,
     kwargs: Mapping = None,
     dask: str = "forbidden",
     output_dtypes: Sequence = None,
@@ -1091,6 +1111,12 @@ def earth_mover_distance(first_samples,
     if kwargs:
         func = functools.partial(func, **kwargs)
 
+    if keep_attrs is None:
+        keep_attrs = _get_keep_attrs(default=False)
+
+    if isinstance(keep_attrs, bool):
+        keep_attrs = "override" if keep_attrs else "drop"
+
     variables_vfunc = functools.partial(
         apply_variable_ufunc,
         func,

xarray/core/merge.py

@@ -315,6 +315,7 @@ def merge_coordinates_without_align(
     objects: "List[Coordinates]",
     prioritized: Mapping[Hashable, MergeElement] = None,
     exclude_dims: AbstractSet = frozenset(),
+    combine_attrs: str = "override",
 ) -> Tuple[Dict[Hashable, Variable], Dict[Hashable, pd.Index]]:
     """Merge variables/indexes from coordinates without automatic alignments.
 
@@ -336,7 +337,7 @@ def merge_coordinates_without_align(
     else:
         filtered = collected
 
-    return merge_collected(filtered, prioritized)
+    return merge_collected(filtered, prioritized, combine_attrs=combine_attrs)
 
 
 def determine_coords(