Add join='override' (#3175)

* Add join='override'

* Add coords='skip_nondim'

* Revert "Add coords='skip_nondim'"

This reverts commit 8263d38.

* black

* black2

* join='override' concat tests.

* Add whats-new.rst

* Improve error message.

* da error message.

* Refactor + fix edge cases.

* Add da test.

* more darray tests.

* Update docstrings.

* Address review comments.

doc/whats-new.rst

@@ -55,6 +55,8 @@ New functions/methods
 Enhancements
 ~~~~~~~~~~~~
 
+- Added ``join='override'``. This only checks that index sizes are equal among objects and skips
+  checking indexes for equality. By `Deepak Cherian <https://github.com/dcherian>`_.
 - :py:func:`~xarray.concat` and :py:func:`~xarray.open_mfdataset` now support the ``join`` kwarg.
   It is passed down to :py:func:`~xarray.align`. By `Deepak Cherian <https://github.com/dcherian>`_.
 - In :py:meth:`~xarray.Dataset.to_zarr`, passing ``mode`` is not mandatory if

xarray/backends/api.py

@@ -813,7 +813,7 @@ def open_mfdataset(
     parallel : bool, optional
         If True, the open and preprocess steps of this function will be
         performed in parallel using ``dask.delayed``. Default is False.
-    join : {'outer', 'inner', 'left', 'right', 'exact'}, optional
+    join : {'outer', 'inner', 'left', 'right', 'exact, 'override'}, optional
         String indicating how to combine differing indexes
         (excluding concat_dim) in objects
 
@@ -823,6 +823,9 @@ def open_mfdataset(
         - 'right': use indexes from the last object with each dimension
         - 'exact': instead of aligning, raise `ValueError` when indexes to be
           aligned are not equal
+        - '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.
     **kwargs : optional
         Additional arguments passed on to :py:func:`xarray.open_dataset`.
 

xarray/core/alignment.py

@@ -31,10 +31,34 @@ def _get_joiner(join):
         # We cannot return a function to "align" in this case, because it needs
         # access to the dimension name to give a good error message.
         return None
+    elif join == "override":
+        # We rewrite all indexes and then use join='left'
+        return operator.itemgetter(0)
     else:
         raise ValueError("invalid value for join: %s" % join)
 
 
+def _override_indexes(objects, all_indexes, exclude):
+    for dim, dim_indexes in all_indexes.items():
+        if dim not in exclude:
+            lengths = {index.size for index in dim_indexes}
+            if len(lengths) != 1:
+                raise ValueError(
+                    "Indexes along dimension %r don't have the same length."
+                    " Cannot use join='override'." % dim
+                )
+
+    objects = list(objects)
+    for idx, obj in enumerate(objects[1:]):
+        new_indexes = dict()
+        for dim in obj.dims:
+            if dim not in exclude:
+                new_indexes[dim] = all_indexes[dim][0]
+        objects[idx + 1] = obj._overwrite_indexes(new_indexes)
+
+    return objects
+
+
 def align(
     *objects,
     join="inner",
@@ -57,7 +81,7 @@ def align(
     ----------
     *objects : Dataset or DataArray
         Objects to align.
-    join : {'outer', 'inner', 'left', 'right', 'exact'}, optional
+    join : {'outer', 'inner', 'left', 'right', 'exact', 'override'}, optional
         Method for joining the indexes of the passed objects along each
         dimension:
 
@@ -67,6 +91,9 @@ def align(
         - 'right': use indexes from the last object with each dimension
         - 'exact': instead of aligning, raise `ValueError` when indexes to be
           aligned are not equal
+        - '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.
     copy : bool, optional
         If ``copy=True``, data in the return values is always copied. If
         ``copy=False`` and reindexing is unnecessary, or can be performed with
@@ -111,6 +138,9 @@ def align(
                 else:
                     all_indexes[dim].append(index)
 
+    if join == "override":
+        objects = _override_indexes(list(objects), all_indexes, exclude)
+
     # We don't reindex over dimensions with all equal indexes for two reasons:
     # - It's faster for the usual case (already aligned objects).
     # - It ensures it's possible to do operations that don't require alignment

xarray/core/combine.py

@@ -379,6 +379,9 @@ def combine_nested(
         - 'right': use indexes from the last object with each dimension
         - 'exact': instead of aligning, raise `ValueError` when indexes to be
           aligned are not equal
+        - '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.
 
     Returns
     -------
@@ -529,6 +532,9 @@ def combine_by_coords(
         - 'right': use indexes from the last object with each dimension
         - 'exact': instead of aligning, raise `ValueError` when indexes to be
           aligned are not equal
+        - '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.
 
     Returns
     -------
@@ -688,6 +694,9 @@ def auto_combine(
         - 'right': use indexes from the last object with each dimension
         - 'exact': instead of aligning, raise `ValueError` when indexes to be
           aligned are not equal
+        - '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.
 
     Returns
     -------

xarray/core/concat.py

@@ -85,6 +85,9 @@ def concat(
         - 'right': use indexes from the last object with each dimension
         - 'exact': instead of aligning, raise `ValueError` when indexes to be
           aligned are not equal
+        - '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.
 
     indexers, mode, concat_over : deprecated
 

xarray/core/dataarray.py

@@ -375,7 +375,7 @@ def _replace_maybe_drop_dims(
             )
         return self._replace(variable, coords, name)
 
-    def _replace_indexes(self, indexes: Mapping[Hashable, Any]) -> "DataArray":
+    def _overwrite_indexes(self, indexes: Mapping[Hashable, Any]) -> "DataArray":
         if not len(indexes):
             return self
         coords = self._coords.copy()

xarray/core/merge.py

@@ -540,6 +540,9 @@ def merge(objects, compat="no_conflicts", join="outer", fill_value=dtypes.NA):
         - 'right': use indexes from the last object with each dimension
         - 'exact': instead of aligning, raise `ValueError` when indexes to be
           aligned are not equal
+        - '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.
     fill_value : scalar, optional
         Value to use for newly missing values
 

xarray/tests/test_concat.py

@@ -187,6 +187,10 @@ def test_concat_join_kwarg(self):
             {"a": (("x", "y"), np.array([np.nan, 0], ndmin=2).T)},
             coords={"x": [0, 1], "y": [0.0001]},
         )
+        expected["override"] = Dataset(
+            {"a": (("x", "y"), np.array([0, 0], ndmin=2).T)},
+            coords={"x": [0, 1], "y": [0]},
+        )
 
         with raises_regex(ValueError, "indexes along dimension 'y'"):
             actual = concat([ds1, ds2], join="exact", dim="x")
@@ -396,6 +400,10 @@ def test_concat_join_kwarg(self):
             {"a": (("x", "y"), np.array([np.nan, 0], ndmin=2).T)},
             coords={"x": [0, 1], "y": [0.0001]},
         )
+        expected["override"] = Dataset(
+            {"a": (("x", "y"), np.array([0, 0], ndmin=2).T)},
+            coords={"x": [0, 1], "y": [0]},
+        )
 
         with raises_regex(ValueError, "indexes along dimension 'y'"):
             actual = concat([ds1, ds2], join="exact", dim="x")

xarray/tests/test_dataarray.py

@@ -3144,6 +3144,56 @@ def test_align_copy(self):
         assert_identical(x, x2)
         assert source_ndarray(x2.data) is not source_ndarray(x.data)
 
+    def test_align_override(self):
+        left = DataArray([1, 2, 3], dims="x", coords={"x": [0, 1, 2]})
+        right = DataArray(
+            np.arange(9).reshape((3, 3)),
+            dims=["x", "y"],
+            coords={"x": [0.1, 1.1, 2.1], "y": [1, 2, 3]},
+        )
+
+        expected_right = DataArray(
+            np.arange(9).reshape(3, 3),
+            dims=["x", "y"],
+            coords={"x": [0, 1, 2], "y": [1, 2, 3]},
+        )
+
+        new_left, new_right = align(left, right, join="override")
+        assert_identical(left, new_left)
+        assert_identical(new_right, expected_right)
+
+        new_left, new_right = align(left, right, exclude="x", join="override")
+        assert_identical(left, new_left)
+        assert_identical(right, new_right)
+
+        new_left, new_right = xr.align(
+            left.isel(x=0, drop=True), right, exclude="x", join="override"
+        )
+        assert_identical(left.isel(x=0, drop=True), new_left)
+        assert_identical(right, new_right)
+
+        with raises_regex(ValueError, "Indexes along dimension 'x' don't have"):
+            align(left.isel(x=0).expand_dims("x"), right, join="override")
+
+    @pytest.mark.parametrize(
+        "darrays",
+        [
+            [
+                DataArray(0),
+                DataArray([1], [("x", [1])]),
+                DataArray([2, 3], [("x", [2, 3])]),
+            ],
+            [
+                DataArray([2, 3], [("x", [2, 3])]),
+                DataArray([1], [("x", [1])]),
+                DataArray(0),
+            ],
+        ],
+    )
+    def test_align_override_error(self, darrays):
+        with raises_regex(ValueError, "Indexes along dimension 'x' don't have"):
+            xr.align(*darrays, join="override")
+
     def test_align_exclude(self):
         x = DataArray([[1, 2], [3, 4]], coords=[("a", [-1, -2]), ("b", [3, 4])])
         y = DataArray([[1, 2], [3, 4]], coords=[("a", [-1, 20]), ("b", [5, 6])])

xarray/tests/test_dataset.py

@@ -1921,6 +1921,28 @@ def test_align_exact(self):
         with raises_regex(ValueError, "indexes .* not equal"):
             xr.align(left, right, join="exact")
 
+    def test_align_override(self):
+        left = xr.Dataset(coords={"x": [0, 1, 2]})
+        right = xr.Dataset(coords={"x": [0.1, 1.1, 2.1], "y": [1, 2, 3]})
+        expected_right = xr.Dataset(coords={"x": [0, 1, 2], "y": [1, 2, 3]})
+
+        new_left, new_right = xr.align(left, right, join="override")
+        assert_identical(left, new_left)
+        assert_identical(new_right, expected_right)
+
+        new_left, new_right = xr.align(left, right, exclude="x", join="override")
+        assert_identical(left, new_left)
+        assert_identical(right, new_right)
+
+        new_left, new_right = xr.align(
+            left.isel(x=0, drop=True), right, exclude="x", join="override"
+        )
+        assert_identical(left.isel(x=0, drop=True), new_left)
+        assert_identical(right, new_right)
+
+        with raises_regex(ValueError, "Indexes along dimension 'x' don't have"):
+            xr.align(left.isel(x=0).expand_dims("x"), right, join="override")
+
     def test_align_exclude(self):
         x = Dataset(
             {