Improved default behavior when concatenating DataArrays

doc/whats-new.rst

@@ -40,6 +40,9 @@ Enhancements
   `Spencer Clark <https://github.com/spencerkclark>`_.
 - Add ``data=False`` option to ``to_dict()`` methods. (:issue:`2656`)
   By `Ryan Abernathey <https://github.com/rabernat>`_
+- Use new dimension name and unique array names to create a new coordinate
+  when concatenating arrays, if no coordinates are given.
+  (:issue:`2775`). By `Zac Hatfield-Dodds <https://github.com/Zac-HD>`_.
 - :py:meth:`~xarray.DataArray.coarsen` and
   :py:meth:`~xarray.Dataset.coarsen` are newly added.
   See :ref:`comput.coarsen` for details.
@@ -82,6 +85,10 @@ Bug fixes
 
 - Silenced warnings that appear when using pandas 0.24.
   By `Stephan Hoyer <https://github.com/shoyer>`_
+- Concatenating a sequence of :py:class:`~xarray.DataArray` with varying names
+  sets the name of the output array to ``None``, instead of the name of the
+  first input array.
+  (:issue:`2775`). By `Zac Hatfield-Dodds <https://github.com/Zac-HD>`_.
 - Interpolating via resample now internally specifies ``bounds_error=False``
   as an argument to ``scipy.interpolate.interp1d``, allowing for interpolation
   from higher frequencies to lower frequencies.  Datapoints outside the bounds

xarray/core/combine.py

@@ -6,6 +6,7 @@
 
 from . import utils
 from .alignment import align
+from .computation import result_name
 from .merge import merge
 from .variable import IndexVariable, Variable, as_variable
 from .variable import concat as concat_vars
@@ -323,16 +324,23 @@ def _dataarray_concat(arrays, dim, data_vars, coords, compat,
         raise ValueError('data_vars is not a valid argument when '
                          'concatenating DataArray objects')
 
-    datasets = []
-    for n, arr in enumerate(arrays):
-        if n == 0:
-            name = arr.name
-        elif name != arr.name:
-            if compat == 'identical':
-                raise ValueError('array names not identical')
-            else:
-                arr = arr.rename(name)
-        datasets.append(arr._to_temp_dataset())
+    name = result_name(arrays)
+    names = [arr.name for arr in arrays]
+    if compat == 'identical' and len(set(names)) != 1:
+        raise ValueError(
+            "compat='identical', but array names {!r} are not identical"
+            .format(names if len(names) <= 10 else sorted(set(names)))
+        )
+    datasets = [arr.rename(name)._to_temp_dataset() for arr in arrays]
+
+    if (
+        isinstance(dim, str)
+        and len(set(names) - {None}) == len(names)
+        and not any(dim in a.dims or dim in a.coords for a in arrays)
+    ):
+        # We're concatenating arrays with unique non-None names along
+        # a new dimension, so we use the existing names as coordinates.
+        dim = pd.Index(names, name=dim)
 
     ds = _dataset_concat(datasets, dim, data_vars, coords, compat,
                          positions)

xarray/tests/test_combine.py

@@ -248,7 +248,8 @@ def test_concat(self):
 
         # from dataset array:
         expected = DataArray(np.array([foo.values, bar.values]),
-                             dims=['w', 'x', 'y'], coords={'x': [0, 1]})
+                             dims=['w', 'x', 'y'],
+                             coords={'x': [0, 1], 'w': ['foo', 'bar']})
         actual = concat([foo, bar], 'w')
         assert_equal(expected, actual)
         # from iteration:
@@ -297,6 +298,25 @@ def test_concat_lazy(self):
         assert combined.shape == (2, 3, 3)
         assert combined.dims == ('z', 'x', 'y')
 
+    def test_concat_names_and_coords(self):
+        ds = Dataset({'foo': (['x', 'y'], np.random.random((2, 2))),
+                      'bar': (['x', 'y'], np.random.random((2, 2)))})
+        # Concat arrays with different names, new name is None
+        # and unique array names are used as coordinates
+        new = concat([ds.foo, ds.bar], dim='new')
+        assert new.name is None
+        assert (new.coords['new'] == ['foo', 'bar']).values.all()
+        # Get a useful error message for unexpectedly different names
+        with pytest.raises(ValueError) as err:
+            concat([ds.foo, ds.bar], dim='new', compat='identical')
+        assert err.value.args[0] == "compat='identical', " + \
+            "but array names ['foo', 'bar'] are not identical"
+        # Concat arrays with same name, name is preserved
+        # and non-unique names are not used as coords
+        foobar = ds.foo.rename('bar')
+        assert concat([foobar, ds.bar], dim='new').name == 'bar'
+        assert 'new' not in concat([foobar, ds.bar], dim='new').coords
+
 
 class TestAutoCombine(object):
 

xarray/tutorial.py

@@ -91,16 +91,17 @@ def open_dataset(name, cache=True, cache_dir=_default_cache_dir,
 
 def load_dataset(*args, **kwargs):
     """
-    `load_dataset` will be removed in version 0.12. The current behavior of
-    this function can be achived by using `tutorial.open_dataset(...).load()`.
+    `load_dataset` is deprecated and will be removed in a future version.
+    The current behavior of this function can be achived by using
+    `tutorial.open_dataset(...).load()`.
 
     See Also
     --------
     open_dataset
     """
     warnings.warn(
-        "load_dataset` will be removed in xarray version 0.12. The current "
-        "behavior of this function can be achived by using "
+        "load_dataset` will be removed in a future version of Xarray. "
+        "The current behavior of this function can be achived by using "
         "`tutorial.open_dataset(...).load()`.",
         DeprecationWarning, stacklevel=2)
     return open_dataset(*args, **kwargs).load()