From 3cbd21aa8fd3a57c0dd324f2a276d83829518331 Mon Sep 17 00:00:00 2001
From: keewis <keewis@users.noreply.github.com>
Date: Sat, 3 Apr 2021 17:43:13 +0200
Subject: [PATCH] add a combine_attrs option to open_mfdataset (#4971)

* expose combine_attrs to open_mfdataset

* add tests for passing combine_attrs to open_mfdataset

* don't override the main dataset attrs

* switch the default to "override" which seems closer to current behavior

* update whats-new.rst [skip-ci]
---
 doc/whats-new.rst             |  2 ++
 xarray/backends/api.py        |  7 ++---
 xarray/tests/test_backends.py | 54 +++++++++++++++++++++++++++++++++++
 3 files changed, 59 insertions(+), 4 deletions(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index fa8fd49e80d..bfbf94e67c1 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -64,6 +64,8 @@ New Features
   :py:class:`~core.groupby.DataArrayGroupBy`, inspired by pandas'
   :py:meth:`~pandas.core.groupby.GroupBy.get_group`.
   By `Deepak Cherian <https://github.com/dcherian>`_.
+- Add a ``combine_attrs`` parameter to :py:func:`open_mfdataset` (:pull:`4971`).
+  By `Justus Magin <https://github.com/keewis>`_.
 - Disable the `cfgrib` backend if the `eccodes` library is not installed (:pull:`5083`). By `Baudouin Raoult <https://github.com/b8raoult>`_.
 - Added :py:meth:`DataArray.curvefit` and :py:meth:`Dataset.curvefit` for general curve fitting applications. (:issue:`4300`, :pull:`4849`)
   By `Sam Levang <https://github.com/slevang>`_.
diff --git a/xarray/backends/api.py b/xarray/backends/api.py
index b2bb928cb90..ab6ea3b1631 100644
--- a/xarray/backends/api.py
+++ b/xarray/backends/api.py
@@ -717,6 +717,7 @@ def open_mfdataset(
     parallel=False,
     join="outer",
     attrs_file=None,
+    combine_attrs="override",
     **kwargs,
 ):
     """Open multiple files as a single dataset.
@@ -931,7 +932,7 @@ def open_mfdataset(
                 coords=coords,
                 ids=ids,
                 join=join,
-                combine_attrs="drop",
+                combine_attrs=combine_attrs,
             )
         elif combine == "by_coords":
             # Redo ordering from coordinates, ignoring how they were ordered
@@ -942,7 +943,7 @@ def open_mfdataset(
                 data_vars=data_vars,
                 coords=coords,
                 join=join,
-                combine_attrs="drop",
+                combine_attrs=combine_attrs,
             )
         else:
             raise ValueError(
@@ -965,8 +966,6 @@ def multi_file_closer():
         if isinstance(attrs_file, Path):
             attrs_file = str(attrs_file)
         combined.attrs = datasets[paths.index(attrs_file)].attrs
-    else:
-        combined.attrs = datasets[0].attrs
 
     return combined
 
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index f6c00a2a9a9..33de0a98edb 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -3004,6 +3004,60 @@ def test_open_mfdataset_does_same_as_concat(self, combine, opt, join):
                 ds_expect = xr.concat([ds1, ds2], data_vars=opt, dim="t", join=join)
                 assert_identical(ds, ds_expect)
 
+    @pytest.mark.parametrize(
+        ["combine_attrs", "attrs", "expected", "expect_error"],
+        (
+            pytest.param("drop", [{"a": 1}, {"a": 2}], {}, False, id="drop"),
+            pytest.param(
+                "override", [{"a": 1}, {"a": 2}], {"a": 1}, False, id="override"
+            ),
+            pytest.param(
+                "no_conflicts", [{"a": 1}, {"a": 2}], None, True, id="no_conflicts"
+            ),
+            pytest.param(
+                "identical",
+                [{"a": 1, "b": 2}, {"a": 1, "c": 3}],
+                None,
+                True,
+                id="identical",
+            ),
+            pytest.param(
+                "drop_conflicts",
+                [{"a": 1, "b": 2}, {"b": -1, "c": 3}],
+                {"a": 1, "c": 3},
+                False,
+                id="drop_conflicts",
+            ),
+        ),
+    )
+    def test_open_mfdataset_dataset_combine_attrs(
+        self, combine_attrs, attrs, expected, expect_error
+    ):
+        with self.setup_files_and_datasets() as (files, [ds1, ds2]):
+            # Give the files an inconsistent attribute
+            for i, f in enumerate(files):
+                ds = open_dataset(f).load()
+                ds.attrs = attrs[i]
+                ds.close()
+                ds.to_netcdf(f)
+
+            if expect_error:
+                with pytest.raises(xr.MergeError):
+                    xr.open_mfdataset(
+                        files,
+                        combine="by_coords",
+                        concat_dim="t",
+                        combine_attrs=combine_attrs,
+                    )
+            else:
+                with xr.open_mfdataset(
+                    files,
+                    combine="by_coords",
+                    concat_dim="t",
+                    combine_attrs=combine_attrs,
+                ) as ds:
+                    assert ds.attrs == expected
+
     def test_open_mfdataset_dataset_attr_by_coords(self):
         """
         Case when an attribute differs across the multiple files