Infer coord for array concat

This is really nice to have when using concat to produce faceted plots of various kinds, and harmless when it's useless.

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.

xarray/core/combine.py

@@ -325,10 +325,23 @@ def _dataarray_concat(arrays, dim, data_vars, coords, compat,
                          'concatenating DataArray objects')
 
     name = result_name(arrays)
-    if name is None and compat == 'identical':
-        raise ValueError('array names not identical')
+    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)
     return arrays[0]._from_temp_dataset(ds, name)

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:
@@ -297,15 +298,24 @@ def test_concat_lazy(self):
         assert combined.shape == (2, 3, 3)
         assert combined.dims == ('z', 'x', 'y')
 
-    def test_concat_names(self):
+    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):