Control attrs of result in merge(), concat(), `combine_by_coords(…

…)` and `combine_nested()` (#3877)

* Optionally promote attrs from DataArray to Dataset in to_dataset

Adds option 'promote_attrs' to DataArray.to_dataset(). By default
promote_attrs=False, maintaining current behaviour. If
promote_attrs=True, the attrs of the DataArray are shallow-copied to the
Dataset returned by to_dataset().

* utils.ordered_dict_union returns the union of two compatible dicts

If the values of any shared key are not equivalent, then raises an
error.

* combine_attrs argument for merge()

Provides several options for how to combine the attributes of the passed
objects and give them to the returned Dataset.

* combine_attrs argument for concat()

Provides several options for how to combine the attributes of the passed
objects and give them to the returned DataArray or Dataset.

* combine_attrs argument for combine_by_coords() and combine_nested()

Provides several options for how to combine the attributes of the passed
objects and give them to the returned Dataset.

* Add combine_attrs changes to whats-new.rst

* Update docstrings to note default values

Apply suggestions from code review

Co-Authored-By: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

* First argument of update_safety_check and ordered_dict_union not mutable

No need for these arguments to be MutableMapping rather than just
Mapping.

* Rename ordered_dict_union -> compat_dict_union

Do not use OrderedDicts any more, so name did not make sense.

* Move combine_attrs to v0.16.0 in whats-new.rst

* Fix merge of whats-new.rst

Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

doc/whats-new.rst

@@ -24,6 +24,10 @@ Breaking changes
 
 New Features
 ~~~~~~~~~~~~
+- Control over attributes of result in :py:func:`merge`, :py:func:`concat`,
+  :py:func:`combine_by_coords` and :py:func:`combine_nested` using
+  combine_attrs keyword argument. (:issue:`3865`, :pull:`3877`)
+  By `John Omotani <https://github.com/johnomotani>`_
 
 
 Bug fixes

xarray/core/combine.py

@@ -155,6 +155,7 @@ def _combine_nd(
     compat="no_conflicts",
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="drop",
 ):
     """
     Combines an N-dimensional structure of datasets into one by applying a
@@ -202,13 +203,21 @@ def _combine_nd(
             compat=compat,
             fill_value=fill_value,
             join=join,
+            combine_attrs=combine_attrs,
         )
     (combined_ds,) = combined_ids.values()
     return combined_ds
 
 
 def _combine_all_along_first_dim(
-    combined_ids, dim, data_vars, coords, compat, fill_value=dtypes.NA, join="outer"
+    combined_ids,
+    dim,
+    data_vars,
+    coords,
+    compat,
+    fill_value=dtypes.NA,
+    join="outer",
+    combine_attrs="drop",
 ):
 
     # Group into lines of datasets which must be combined along dim
@@ -223,7 +232,7 @@ def _combine_all_along_first_dim(
         combined_ids = dict(sorted(group))
         datasets = combined_ids.values()
         new_combined_ids[new_id] = _combine_1d(
-            datasets, dim, compat, data_vars, coords, fill_value, join
+            datasets, dim, compat, data_vars, coords, fill_value, join, combine_attrs
         )
     return new_combined_ids
 
@@ -236,6 +245,7 @@ def _combine_1d(
     coords="different",
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="drop",
 ):
     """
     Applies either concat or merge to 1D list of datasets depending on value
@@ -252,6 +262,7 @@ def _combine_1d(
                 compat=compat,
                 fill_value=fill_value,
                 join=join,
+                combine_attrs=combine_attrs,
             )
         except ValueError as err:
             if "encountered unexpected variable" in str(err):
@@ -265,7 +276,13 @@ def _combine_1d(
             else:
                 raise
     else:
-        combined = merge(datasets, compat=compat, fill_value=fill_value, join=join)
+        combined = merge(
+            datasets,
+            compat=compat,
+            fill_value=fill_value,
+            join=join,
+            combine_attrs=combine_attrs,
+        )
 
     return combined
 
@@ -284,6 +301,7 @@ def _nested_combine(
     ids,
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="drop",
 ):
 
     if len(datasets) == 0:
@@ -311,6 +329,7 @@ def _nested_combine(
         coords=coords,
         fill_value=fill_value,
         join=join,
+        combine_attrs=combine_attrs,
     )
     return combined
 
@@ -323,6 +342,7 @@ def combine_nested(
     coords="different",
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="drop",
 ):
     """
     Explicitly combine an N-dimensional grid of datasets into one by using a
@@ -390,6 +410,16 @@ def combine_nested(
         - 'override': if indexes are of same size, rewrite indexes to be
           those of the first object with that dimension. Indexes for the same
           dimension must have the same size in all objects.
+    combine_attrs : {'drop', 'identical', 'no_conflicts', 'override'},
+                    default 'drop'
+        String indicating how to combine attrs of the objects being merged:
+
+        - 'drop': empty attrs on returned Dataset.
+        - 'identical': all attrs must be the same on every object.
+        - 'no_conflicts': attrs from all objects are combined, any that have
+          the same name must also have the same value.
+        - 'override': skip comparing and copy attrs from the first dataset to
+          the result.
 
     Returns
     -------
@@ -468,6 +498,7 @@ def combine_nested(
         ids=False,
         fill_value=fill_value,
         join=join,
+        combine_attrs=combine_attrs,
     )
 
 
@@ -482,6 +513,7 @@ def combine_by_coords(
     coords="different",
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="no_conflicts",
 ):
     """
     Attempt to auto-magically combine the given datasets into one by using
@@ -557,6 +589,16 @@ def combine_by_coords(
         - 'override': if indexes are of same size, rewrite indexes to be
           those of the first object with that dimension. Indexes for the same
           dimension must have the same size in all objects.
+    combine_attrs : {'drop', 'identical', 'no_conflicts', 'override'},
+                    default 'drop'
+        String indicating how to combine attrs of the objects being merged:
+
+        - 'drop': empty attrs on returned Dataset.
+        - 'identical': all attrs must be the same on every object.
+        - 'no_conflicts': attrs from all objects are combined, any that have
+          the same name must also have the same value.
+        - 'override': skip comparing and copy attrs from the first dataset to
+          the result.
 
     Returns
     -------
@@ -700,6 +742,7 @@ def combine_by_coords(
             compat=compat,
             fill_value=fill_value,
             join=join,
+            combine_attrs=combine_attrs,
         )
 
         # Check the overall coordinates are monotonically increasing
@@ -717,6 +760,7 @@ def combine_by_coords(
         compat=compat,
         fill_value=fill_value,
         join=join,
+        combine_attrs=combine_attrs,
     )
 
 

xarray/core/concat.py

@@ -3,7 +3,7 @@
 from . import dtypes, utils
 from .alignment import align
 from .duck_array_ops import lazy_array_equiv
-from .merge import _VALID_COMPAT, unique_variable
+from .merge import _VALID_COMPAT, merge_attrs, unique_variable
 from .variable import IndexVariable, Variable, as_variable
 from .variable import concat as concat_vars
 
@@ -17,6 +17,7 @@ def concat(
     positions=None,
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="override",
 ):
     """Concatenate xarray objects along a new or existing dimension.
 
@@ -92,15 +93,21 @@ def concat(
         - 'override': if indexes are of same size, rewrite indexes to be
           those of the first object with that dimension. Indexes for the same
           dimension must have the same size in all objects.
+    combine_attrs : {'drop', 'identical', 'no_conflicts', 'override'},
+                    default 'override
+        String indicating how to combine attrs of the objects being merged:
+
+        - 'drop': empty attrs on returned Dataset.
+        - 'identical': all attrs must be the same on every object.
+        - 'no_conflicts': attrs from all objects are combined, any that have
+          the same name must also have the same value.
+        - 'override': skip comparing and copy attrs from the first dataset to
+          the result.
 
     Returns
     -------
     concatenated : type of objs
 
-    Notes
-    -----
-    Each concatenated Variable preserves corresponding ``attrs`` from the first element of ``objs``.
-
     See also
     --------
     merge
@@ -132,7 +139,9 @@ def concat(
             "can only concatenate xarray Dataset and DataArray "
             "objects, got %s" % type(first_obj)
         )
-    return f(objs, dim, data_vars, coords, compat, positions, fill_value, join)
+    return f(
+        objs, dim, data_vars, coords, compat, positions, fill_value, join, combine_attrs
+    )
 
 
 def _calc_concat_dim_coord(dim):
@@ -306,6 +315,7 @@ def _dataset_concat(
     positions,
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="override",
 ):
     """
     Concatenate a sequence of datasets along a new or existing dimension
@@ -362,7 +372,7 @@ def _dataset_concat(
     result_vars.update(dim_coords)
 
     # assign attrs and encoding from first dataset
-    result_attrs = datasets[0].attrs
+    result_attrs = merge_attrs([ds.attrs for ds in datasets], combine_attrs)
     result_encoding = datasets[0].encoding
 
     # check that global attributes are fixed across all datasets if necessary
@@ -425,6 +435,7 @@ def _dataarray_concat(
     positions,
     fill_value=dtypes.NA,
     join="outer",
+    combine_attrs="override",
 ):
     arrays = list(arrays)
 
@@ -453,5 +464,12 @@ def _dataarray_concat(
         positions,
         fill_value=fill_value,
         join=join,
+        combine_attrs="drop",
     )
-    return arrays[0]._from_temp_dataset(ds, name)
+
+    merged_attrs = merge_attrs([da.attrs for da in arrays], combine_attrs)
+
+    result = arrays[0]._from_temp_dataset(ds, name)
+    result.attrs = merged_attrs
+
+    return result

xarray/core/dataarray.py

@@ -475,7 +475,13 @@ def _to_dataset_whole(
         dataset = Dataset._construct_direct(variables, coord_names, indexes=indexes)
         return dataset
 
-    def to_dataset(self, dim: Hashable = None, *, name: Hashable = None) -> Dataset:
+    def to_dataset(
+        self,
+        dim: Hashable = None,
+        *,
+        name: Hashable = None,
+        promote_attrs: bool = False,
+    ) -> Dataset:
         """Convert a DataArray to a Dataset.
 
         Parameters
@@ -487,6 +493,8 @@ def to_dataset(self, dim: Hashable = None, *, name: Hashable = None) -> Dataset:
         name : hashable, optional
             Name to substitute for this array's name. Only valid if ``dim`` is
             not provided.
+        promote_attrs : bool, default False
+            Set to True to shallow copy attrs of DataArray to returned Dataset.
 
         Returns
         -------
@@ -500,9 +508,14 @@ def to_dataset(self, dim: Hashable = None, *, name: Hashable = None) -> Dataset:
         if dim is not None:
             if name is not None:
                 raise TypeError("cannot supply both dim and name arguments")
-            return self._to_dataset_split(dim)
+            result = self._to_dataset_split(dim)
         else:
-            return self._to_dataset_whole(name)
+            result = self._to_dataset_whole(name)
+
+        if promote_attrs:
+            result.attrs = dict(self.attrs)
+
+        return result
 
     @property
     def name(self) -> Optional[Hashable]:

xarray/core/dataset.py

@@ -536,7 +536,7 @@ def __init__(
         if isinstance(coords, Dataset):
             coords = coords.variables
 
-        variables, coord_names, dims, indexes = merge_data_and_coords(
+        variables, coord_names, dims, indexes, _ = merge_data_and_coords(
             data_vars, coords, compat="broadcast_equals"
         )