Proposal for better error message about in-place operation (#3976)

* Improve error message: automatic alignment during in-place operation.

* Sorted imports.

* Fix tests.

* Add suggestions from S. Hoyer.

xarray/core/dataarray.py

@@ -53,7 +53,7 @@
 from .formatting import format_item
 from .indexes import Indexes, default_indexes, propagate_indexes
 from .indexing import is_fancy_indexer
-from .merge import PANDAS_TYPES, _extract_indexes_from_coords
+from .merge import PANDAS_TYPES, MergeError, _extract_indexes_from_coords
 from .options import OPTIONS
 from .utils import Default, ReprObject, _check_inplace, _default, either_dict_or_kwargs
 from .variable import (
@@ -2713,8 +2713,15 @@ def func(self, other):
             # don't support automatic alignment with in-place arithmetic.
             other_coords = getattr(other, "coords", None)
             other_variable = getattr(other, "variable", other)
-            with self.coords._merge_inplace(other_coords):
-                f(self.variable, other_variable)
+            try:
+                with self.coords._merge_inplace(other_coords):
+                    f(self.variable, other_variable)
+            except MergeError as exc:
+                raise MergeError(
+                    "Automatic alignment is not supported for in-place operations.\n"
+                    "Consider aligning the indices manually or using a not-in-place operation.\n"
+                    "See https://github.com/pydata/xarray/issues/3910 for more explanations."
+                ) from exc
             return self
 
         return func

xarray/tests/test_dataarray.py

@@ -1930,9 +1930,9 @@ def test_inplace_math_basics(self):
     def test_inplace_math_automatic_alignment(self):
         a = DataArray(range(5), [("x", range(5))])
         b = DataArray(range(1, 6), [("x", range(1, 6))])
-        with pytest.raises(xr.MergeError):
+        with pytest.raises(xr.MergeError, match="Automatic alignment is not supported"):
             a += b
-        with pytest.raises(xr.MergeError):
+        with pytest.raises(xr.MergeError, match="Automatic alignment is not supported"):
             b += a
 
     def test_math_name(self):