Begin merge, combine.

xarray/core/combine.py

@@ -243,11 +243,12 @@ def _combine_1d(
                 dim=concat_dim,
                 data_vars=data_vars,
                 coords=coords,
+                compat=compat,
                 fill_value=fill_value,
                 join=join,
             )
         except ValueError as err:
-            if "encountered unexpected variable" in str(err):
+            if "Encountered unexpected variable" in str(err):
                 raise ValueError(
                     "These objects cannot be combined using only "
                     "xarray.combine_nested, instead either use "
@@ -598,6 +599,7 @@ def combine_by_coords(
             concat_dims=concat_dims,
             data_vars=data_vars,
             coords=coords,
+            compat=compat,
             fill_value=fill_value,
             join=join,
         )
@@ -832,6 +834,7 @@ def _old_auto_combine(
                 dim=dim,
                 data_vars=data_vars,
                 coords=coords,
+                compat=compat,
                 fill_value=fill_value,
                 join=join,
             )
@@ -850,6 +853,7 @@ def _auto_concat(
     coords="different",
     fill_value=dtypes.NA,
     join="outer",
+    compat="no_conflicts",
 ):
     if len(datasets) == 1 and dim is None:
         # There is nothing more to combine, so kick out early.
@@ -876,5 +880,10 @@ def _auto_concat(
                 )
             dim, = concat_dims
         return concat(
-            datasets, dim=dim, data_vars=data_vars, coords=coords, fill_value=fill_value
+            datasets,
+            dim=dim,
+            data_vars=data_vars,
+            coords=coords,
+            fill_value=fill_value,
+            compat=compat,
         )

xarray/core/concat.py

@@ -70,7 +70,7 @@ def concat(
           * 'identical' means that variable attributes and global attributes
              must also be equal.
           * 'override' means that checks are skipped and values from the first dataset
-            are used.
+            are used. This cannot be used with coords='different' or data_vars='different'.
     positions : None or list of integer arrays, optional
         List of integer arrays which specifies the integer positions to which
         to assign each dataset along the concatenated dimension. If not
@@ -145,7 +145,7 @@ def concat(
             "`data_vars` and `coords` arguments"
         )
 
-    if compat not in ["equals", "identical", "override"]:
+    if compat not in ["equals", "identical", "override", "no_conflicts"]:
         raise ValueError(
             "compat=%r invalid: must be 'equals', 'identical or 'override'" % compat
         )
@@ -207,6 +207,11 @@ def _calc_concat_over(datasets, dim, data_vars, coords):
     def process_subset_opt(opt, subset):
         if isinstance(opt, str):
             if opt == "different":
+                if compat == "override":
+                    raise ValueError(
+                        "Cannot specify both %s='different' and compat='override'."
+                        % subset
+                    )
                 # all nonindexes that are not the same in each dataset
                 for k in getattr(datasets[0], subset):
                     if k not in concat_over:
@@ -395,7 +400,7 @@ def _dataarray_concat(
 
     if data_vars != "all":
         raise ValueError(
-            "data_vars is not a valid argument when " "concatenating DataArray objects"
+            "data_vars is not a valid argument when concatenating DataArray objects"
         )
 
     datasets = []

xarray/core/merge.py

@@ -44,6 +44,7 @@
         "broadcast_equals": 2,
         "minimal": 3,
         "no_conflicts": 4,
+        "override": 5,
     }
 )
 
@@ -81,7 +82,7 @@ def unique_variable(name, variables, compat="broadcast_equals"):
     variables : list of xarray.Variable
         List of Variable objects, all of which go by the same name in different
         inputs.
-    compat : {'identical', 'equals', 'broadcast_equals', 'no_conflicts'}, optional
+    compat : {'identical', 'equals', 'broadcast_equals', 'no_conflicts', 'override'}, optional
         Type of equality check to use.
 
     Returns
@@ -93,7 +94,7 @@ def unique_variable(name, variables, compat="broadcast_equals"):
     MergeError: if any of the variables are not equal.
     """  # noqa
     out = variables[0]
-    if len(variables) > 1:
+    if len(variables) > 1 and compat != "override":
         combine_method = None
 
         if compat == "minimal":
@@ -152,7 +153,7 @@ def merge_variables(
     priority_vars : mapping with Variable or None values, optional
         If provided, variables are always taken from this dict in preference to
         the input variable dictionaries, without checking for conflicts.
-    compat : {'identical', 'equals', 'broadcast_equals', 'minimal', 'no_conflicts'}, optional
+    compat : {'identical', 'equals', 'broadcast_equals', 'minimal', 'no_conflicts', 'override'}, optional
         Type of equality check to use when checking for conflicts.
 
     Returns
@@ -449,7 +450,7 @@ def merge_core(
     ----------
     objs : list of mappings
         All values must be convertable to labeled arrays.
-    compat : {'identical', 'equals', 'broadcast_equals', 'no_conflicts'}, optional
+    compat : {'identical', 'equals', 'broadcast_equals', 'no_conflicts', 'override'}, optional
         Compatibility checks to use when merging variables.
     join : {'outer', 'inner', 'left', 'right'}, optional
         How to combine objects with different indexes.

xarray/tests/test_concat.py

@@ -17,6 +17,30 @@
 from .test_dataset import create_test_data
 
 
+def test_concat_compat():
+    ds1 = Dataset(
+        {
+            "has_x_y": (("y", "x"), [[1, 2]]),
+            "has_x": ("x", [1, 2]),
+            "no_x_y": ("z", [1, 2]),
+        },
+        coords={"x": [0, 1], "y": [0], "z": [-1, -2]},
+    )
+    ds2 = Dataset(
+        {
+            "has_x_y": (("y", "x"), [[3, 4]]),
+            "has_x": ("x", [1, 2]),
+            "no_x_y": (("q", "z"), [[1, 2]]),
+        },
+        coords={"x": [0, 1], "y": [1], "z": [-1, -2], "q": [0]},
+    )
+
+    result = concat([ds1, ds2], dim="y", compat="equals")
+
+    for var in ["has_x", "no_x_y", "const1"]:
+        assert "y" not in result[var]
+
+
 class TestConcatDataset:
     @pytest.fixture
     def data(self):
@@ -404,6 +428,10 @@ def test_compat_override(self, data_vars, coords):
         for var in equal_to_first_ds:
             assert_equal(actual[var], self.dsets[0][var])
 
+    def test_compat_override_different_error(self):
+        with raises_regex(ValueError, "Cannot specify both .*='different'"):
+            concat(self.dsets, data_vars="different", compat="override")
+
 
 class TestConcatDataArray:
     def test_concat(self):

xarray/tests/test_merge.py

@@ -196,6 +196,8 @@ def test_merge_compat(self):
         with raises_regex(ValueError, "compat=.* invalid"):
             ds1.merge(ds2, compat="foobar")
 
+        assert ds1.identical(ds1.merge(ds2, compat="override"))
+
     def test_merge_auto_align(self):
         ds1 = xr.Dataset({"a": ("x", [1, 2]), "x": [0, 1]})
         ds2 = xr.Dataset({"b": ("x", [3, 4]), "x": [1, 2]})