Better docs and errors about expand_dims() view (#3114)

* Better docs and errors about expand_dims() view

* Add test and whatsnew note

* Remove trailing whitespace

* Whitespace for PEP8

* Py35 compatible test

* Better docs and errors about expand_dims() view

* Add test and whatsnew note

* Remove trailing whitespace

* Whitespace for PEP8

* Py35 compatible test

* Improved exception testing

Author: DavidMertz

doc/whats-new.rst

@@ -30,6 +30,9 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
+- Improved error handling and documentation for `.expand_dims()` 
+  read-only view.
+
 .. _whats-new.0.12.3:
 
 v0.12.3 (10 July 2019)

xarray/core/dataarray.py

@@ -1271,7 +1271,9 @@ def expand_dims(self, dim: Union[None, Hashable, Sequence[Hashable],
                                      Mapping[Hashable, Any]] = None,
                     axis=None, **dim_kwargs: Any) -> 'DataArray':
         """Return a new object with an additional axis (or axes) inserted at
-        the corresponding position in the array shape.
+        the corresponding position in the array shape. The new object is a
+        view into the underlying array, not a copy.
+
 
         If dim is already a scalar coordinate, it will be promoted to a 1D
         coordinate consisting of a single value.

xarray/core/dataset.py

@@ -2505,7 +2505,8 @@ def swap_dims(self, dims_dict, inplace=None):
 
     def expand_dims(self, dim=None, axis=None, **dim_kwargs):
         """Return a new object with an additional axis (or axes) inserted at
-        the corresponding position in the array shape.
+        the corresponding position in the array shape.  The new object is a
+        view into the underlying array, not a copy.
 
         If dim is already a scalar coordinate, it will be promoted to a 1D
         coordinate consisting of a single value.

xarray/core/indexing.py

@@ -1177,7 +1177,15 @@ def __getitem__(self, key):
 
     def __setitem__(self, key, value):
         array, key = self._indexing_array_and_key(key)
-        array[key] = value
+        try:
+            array[key] = value
+        except ValueError:
+            # More informative exception if read-only view
+            if not array.flags.writeable and not array.flags.owndata:
+                raise ValueError("Assignment destination is a view.  "
+                                 "Do you want to .copy() array first?")
+            else:
+                raise
 
 
 class DaskIndexingAdapter(ExplicitlyIndexedNDArrayMixin):

xarray/tests/test_indexing.py

@@ -144,6 +144,16 @@ def test_indexer(data, x, expected_pos, expected_idx=None):
                      [True, True, True, True, False, False, False, False],
                      pd.MultiIndex.from_product([[1, 2], [-1, -2]]))
 
+    def test_read_only_view(self):
+        from collections import OrderedDict
+        arr = DataArray(np.random.rand(3, 3),
+                        coords={'x': np.arange(3), 'y': np.arange(3)},
+                        dims=('x', 'y'))     # Create a 2D DataArray
+        arr = arr.expand_dims(OrderedDict([('z', 3)]), -1)  # New dimension 'z'
+        arr['z'] = np.arange(3)              # New coords to dimension 'z'
+        with pytest.raises(ValueError, match='Do you want to .copy()'):
+            arr.loc[0, 0, 0] = 999
+
 
 class TestLazyArray:
     def test_slice_slice(self):