BUG: raise when wrong level name is passed to "unstack"

closes pandas-dev#18303

Author: toobaz

pandas/core/indexes/base.py

@@ -1524,7 +1524,9 @@ def _validate_index_level(self, level):
                     "Too many levels:" " Index has only 1 level, not %d" % (level + 1)
                 )
         elif level != self.name:
-            raise KeyError("Level %s must be same as name (%s)" % (level, self.name))
+            raise KeyError(
+                'Level ("%s") is different from index name ("%s")' % (level, self.name)
+            )
 
     def _get_level_number(self, level):
         self._validate_index_level(level)

pandas/core/reshape/reshape.py

@@ -402,6 +402,8 @@ def unstack(obj, level, fill_value=None):
         else:
             level = level[0]
 
+    level = obj.index._get_level_number(level)
+
     if isinstance(obj, DataFrame):
         if isinstance(obj.index, MultiIndex):
             return _unstack_frame(obj, level, fill_value=fill_value)

pandas/tests/test_multilevel.py

@@ -524,6 +524,16 @@ def test_stack_unstack_preserve_names(self):
         restacked = unstacked.stack()
         assert restacked.index.names == self.frame.index.names
 
+    @pytest.mark.parametrize("method", ['stack', 'unstack'])
+    def test_stack_unstack_wrong_level_name(self, method):
+        # GH 18303 - wrong level name should raise
+
+        # A frame with flat axes:
+        df = self.frame.loc['foo']
+
+        with pytest.raises(KeyError, match="is different from index name"):
+            getattr(df, method)('mistake')
+
     def test_unstack_level_name(self):
         result = self.frame.unstack("second")
         expected = self.frame.unstack(level=1)