Raises a ValueError for a confliction between dimension names and level names

doc/whats-new.rst

@@ -61,10 +61,16 @@ Bug fixes
   attribute being set.
   (:issue:`2201`)
   By `Thomas Voigt <https://github.com/tv3141>`_.
+
 - Tests can be run in parallel with pytest-xdist
-- Follow up the renamings in dask; from dask.ghost to dask.overlap
+  By `Tony Tung <https://github.com/ttung>`_.
+
+- Now raises a ValueError when there is a conflict between dimension names and
+  level names of MultiIndex. (:issue:`2299`)
   By `Keisuke Fujii <https://github.com/fujiisoup>`_.
 
+- Follow up the renamings in dask; from dask.ghost to dask.overlap
+  By `Keisuke Fujii <https://github.com/fujiisoup>`_.
 
 - Now :py:func:`xr.apply_ufunc` raises a ValueError when the size of
 ``input_core_dims`` is inconsistent with the number of arguments.

xarray/core/variable.py

@@ -1876,12 +1876,15 @@ def assert_unique_multiindex_level_names(variables):
     objects.
     """
     level_names = defaultdict(list)
+    all_level_names = set()
     for var_name, var in variables.items():
         if isinstance(var._data, PandasIndexAdapter):
             idx_level_names = var.to_index_variable().level_names
             if idx_level_names is not None:
                 for n in idx_level_names:
                     level_names[n].append('%r (%s)' % (n, var_name))
+            if idx_level_names:
+                all_level_names.update(idx_level_names)
 
     for k, v in level_names.items():
         if k in variables:
@@ -1892,3 +1895,9 @@ def assert_unique_multiindex_level_names(variables):
         conflict_str = '\n'.join([', '.join(v) for v in duplicate_names])
         raise ValueError('conflicting MultiIndex level name(s):\n%s'
                          % conflict_str)
+    # Check confliction between level names and dimensions GH:2299
+    for k, v in variables.items():
+        for d in v.dims:
+            if d in all_level_names:
+                raise ValueError('conflicting level / dimension names. {} '
+                                 'already exists as a level name.'.format(d))

xarray/tests/test_dataset.py

@@ -2456,6 +2456,18 @@ def test_assign_multiindex_level(self):
         with raises_regex(ValueError, 'conflicting MultiIndex'):
             data.assign(level_1=range(4))
             data.assign_coords(level_1=range(4))
+        # raise an Error when any level name is used as dimension GH:2299
+        with pytest.raises(ValueError):
+            data['y'] = ('level_1', [0, 1])
+
+    def test_merge_multiindex_level(self):
+        data = create_test_multiindex()
+        other = Dataset({'z': ('level_1', [0, 1])})  # conflict dimension
+        with pytest.raises(ValueError):
+            data.merge(other)
+        other = Dataset({'level_1': ('x', [0, 1])})  # conflict variable name
+        with pytest.raises(ValueError):
+            data.merge(other)
 
     def test_setitem_original_non_unique_index(self):
         # regression test for GH943