Better error message when assigning coordinates without dimensions

doc/whats-new.rst

@@ -33,6 +33,8 @@ Enhancements
   By `Guido Imperiale <https://github.com/crusaderky>`_.
 - :py:func:`~xarray.broadcast` and :py:func:`~xarray.concat` will now auto-align inputs,
   using ``join=outer``. By `Guido Imperiale <https://github.com/crusaderky>`_.
+- Better error message when assigning variables without dimensions
+  (:issue:`971`). By `Stephan Hoyer <https://github.com/shoyer>`_.
 
 Bug fixes
 ~~~~~~~~~

xarray/core/variable.py

@@ -42,7 +42,7 @@ def as_variable(obj, name=None, copy=False):
         obj = obj.copy(deep=False)
     elif hasattr(obj, 'dims') and (hasattr(obj, 'data') or
                                    hasattr(obj, 'values')):
-        obj = Variable(obj.dims, getattr(obj, 'data', obj.values),
+        obj = Variable(obj.dims, getattr(obj, 'data', getattr(obj, 'values')),
                        getattr(obj, 'attrs', None),
                        getattr(obj, 'encoding', None))
     elif isinstance(obj, tuple):
@@ -58,17 +58,24 @@ def as_variable(obj, name=None, copy=False):
     elif getattr(obj, 'name', None) is not None:
         obj = Variable(obj.name, obj)
     elif name is not None:
-        obj = Variable(name, obj)
+        data = as_compatible_data(obj)
+        if data.ndim != 1:
+            raise ValueError(
+                'cannot set variable %r with %r-dimensional data '
+                'without explicit dimension names. Pass a tuple of '
+                '(dims, data) instead.' % (name, data.ndim))
+        obj = Variable(name, obj, fastpath=True)
     else:
         raise TypeError('unable to convert object into a variable without an '
                         'explicit list of dimensions: %r' % obj)
 
     if name is not None and name in obj.dims:
         # convert the into an Index
         if obj.ndim != 1:
-            raise ValueError('the variable %r has the same name as one of its '
-                             'dimensions %r, but it is not 1-dimensional and '
-                             'thus it is not a valid index' % (name, obj.dims))
+            raise ValueError(
+                '%r has more than 1-dimension and the same name as one of its '
+                'dimensions %r. xarray disallows such variables because they '
+                'conflict with dimension coordinates.' % (name, obj.dims))
         obj = obj.to_coord()
 
     return obj

xarray/test/test_dataset.py

@@ -141,8 +141,7 @@ def test_constructor(self):
 
         with self.assertRaisesRegexp(ValueError, 'conflicting sizes'):
             Dataset({'a': x1, 'b': x2})
-        with self.assertRaisesRegexp(ValueError,
-                "variable 'x' has the same name"):
+        with self.assertRaisesRegexp(ValueError, "disallows such variables"):
             Dataset({'a': x1, 'x': z})
         with self.assertRaisesRegexp(TypeError, 'tuples to convert'):
             Dataset({'x': (1, 2, 3, 4, 5, 6, 7)})
@@ -1484,7 +1483,7 @@ def test_setitem(self):
         self.assertDatasetIdentical(data1, data2)
         # can't assign an ND array without dimensions
         with self.assertRaisesRegexp(ValueError,
-                                     'dimensions .* must have the same len'):
+                                     'without explicit dimension names'):
             data2['C'] = var.values.reshape(2, 4)
         # but can assign a 1D array
         data1['C'] = var.values