Replaces Exceptions with ValueError in MultiIndex, closes #21770 (#21780)

Author: glyg

doc/source/whatsnew/v0.24.0.txt

@@ -190,6 +190,8 @@ Other API Changes
 
 .. _whatsnew_0240.api.incompatibilities:
 
+- Trying to reindex a ``DataFrame`` with a non unique ``MultiIndex`` now raises a ``ValueError`` instead of an ``Exception`` (:issue:`21770`)
+
 Series and Index Data-Dtype Incompatibilities
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

pandas/core/indexes/multi.py

@@ -1958,8 +1958,8 @@ def get_indexer(self, target, method=None, limit=None, tolerance=None):
                                                           tolerance=tolerance)
 
         if not self.is_unique:
-            raise Exception('Reindexing only valid with uniquely valued Index '
-                            'objects')
+            raise ValueError('Reindexing only valid with uniquely valued '
+                             'Index objects')
 
         if method == 'pad' or method == 'backfill':
             if tolerance is not None:
@@ -2023,7 +2023,7 @@ def reindex(self, target, method=None, level=None, limit=None,
                                                limit=limit,
                                                tolerance=tolerance)
                 else:
-                    raise Exception("cannot handle a non-unique multi-index!")
+                    raise ValueError("cannot handle a non-unique multi-index!")
 
         if not isinstance(target, MultiIndex):
             if indexer is None:

pandas/tests/indexes/multi/test_reindex.py

@@ -97,3 +97,12 @@ def test_reindex_base(idx):
 
     with tm.assert_raises_regex(ValueError, 'Invalid fill method'):
         idx.get_indexer(idx, method='invalid')
+
+
+def test_reindex_non_unique():
+    idx = pd.MultiIndex.from_tuples([(0, 0), (1, 1), (1, 1), (2, 2)])
+    a = pd.Series(np.arange(4), index=idx)
+    new_idx = pd.MultiIndex.from_tuples([(0, 0), (1, 1), (2, 2)])
+    with tm.assert_raises_regex(ValueError,
+                                'cannot handle a non-unique multi-index!'):
+        a.reindex(new_idx)