check for level/var name conflicts in one place

Author: Benoit Bovy

xarray/core/coordinates.py

@@ -151,12 +151,6 @@ def to_dataset(self):
     def _update_coords(self, coords):
         from .dataset import calculate_dimensions
 
-        for key in coords:
-            if key in self._data._level_coords:
-                raise ValueError("%r is already a MultiIndex level of "
-                                 "coordinate %r"
-                                 % (key, self._data._level_coords[key]))
-
         variables = self._data._variables.copy()
         variables.update(coords)
 
@@ -202,12 +196,6 @@ def _names(self):
     def _update_coords(self, coords):
         from .dataset import calculate_dimensions
 
-        for key in coords:
-            if key in self._data._level_coords:
-                raise ValueError("%r is already a MultiIndex level of "
-                                 "coordinate %r"
-                                 % (key, self._data._level_coords[key]))
-
         dims = calculate_dimensions(coords)
         if set(dims) != set(self.dims):
             raise ValueError('cannot add coordinates with new dimensions to '

xarray/core/variable.py

@@ -1321,13 +1321,16 @@ def assert_unique_multiindex_level_names(variables):
             idx_level_names = var.to_coord().level_names
             if idx_level_names is not None:
                 for n in idx_level_names:
-                    level_names[n].append(var_name)
+                    level_names[n].append('%r (%s)' % (n, var_name))
+
+    for n in level_names:
+        if n in variables:
+            level_names[n].append('(%s)' % n)
 
     duplicate_level_names = {k: v for k, v in level_names.items()
                              if len(v) > 1}
     if duplicate_level_names:
-        duplicate_str = '\n'.join(['level %r found in %s'
-                                   % (k, ' and '.join(v))
-                                   for k, v in duplicate_level_names.items()])
-        raise ValueError('conflicting MultiIndex level names:\n%s'
+        duplicate_str = '\n'.join([', '.join(v)
+                                   for v in duplicate_level_names.values()])
+        raise ValueError('conflicting MultiIndex level name(s):\n%s'
                          % duplicate_str)