Assign default group name in groupby if name=None (#158) (#4098)

* Assign default group name in groupby (#158)

* When groupby receives a DataArray with name=None assign name='group'

* Previously when name=None a ValueError: `group` must have a name was raised

* Closes #158

* Add test

* Update whats-new.rst

* black

* Add assert statement to test group name was added to DataArray

Co-authored-by: phillipbutcher <phillip.butcher@noaa.gov>

doc/whats-new.rst

@@ -100,6 +100,8 @@ New Features
 
 Bug fixes
 ~~~~~~~~~
+- If groupby receives a ``DataArray`` with name=None, assign a default name (:issue:`158`)
+  By `Phil Butcher <https://github.com/pjbutcher>`_.
 - Support dark mode in VS code (:issue:`4024`)
   By `Keisuke Fujii <https://github.com/fujiisoup>`_.
 - Fix bug when converting multiindexed Pandas objects to sparse xarray objects. (:issue:`4019`)

xarray/core/groupby.py

@@ -321,7 +321,7 @@ def __init__(
                 group = _DummyGroup(obj, group.name, group.coords)
 
         if getattr(group, "name", None) is None:
-            raise ValueError("`group` must have a name")
+            group.name = "group"
 
         group, obj, stacked_dim, inserted_dims = _ensure_1d(group, obj)
         (group_dim,) = group.dims

xarray/tests/test_groupby.py

@@ -538,4 +538,16 @@ def test_groupby_bins_timeseries():
     assert_identical(actual, expected)
 
 
+def test_groupby_none_group_name():
+    # GH158
+    # xarray should not fail if a DataArray's name attribute is None
+
+    data = np.arange(10) + 10
+    da = xr.DataArray(data)  # da.name = None
+    key = xr.DataArray(np.floor_divide(data, 2))
+
+    mean = da.groupby(key).mean()
+    assert "group" in mean.dims
+
+
 # TODO: move other groupby tests from test_dataset and test_dataarray over here