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

@@ -329,6 +329,13 @@ def _dataarray_concat(arrays, dim, data_vars, coords, compat,
         raise ValueError('array names not identical')
     datasets = [arr.rename(name)._to_temp_dataset() for arr in arrays]
 
+    if name is None and isinstance(dim, str) and dim not in arrays[0].dims:
+        # If we're concatenating uniquely named arrays along a new dimension,
+        # use the existing names as coordinates.
+        names = [arr.name for arr in arrays]
+        if len(set(names) - {None}) == len(names):
+            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 +298,19 @@ 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()
         # 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):