BUG: Bug in interesection of non-monotonic non-unique indexes (GH8362)

Author: tom-alcorn

doc/source/v0.15.0.txt

@@ -985,3 +985,4 @@ Bug Fixes
 - Bug in OLS where running with "cluster" and "nw_lags" parameters did not work correctly, but also did not throw an error
   (:issue:`5884').
 - Bug in ``DataFrame.dropna`` that interpreted non-existent columns in the subset argument as the 'last column' (:issue:`8303`)
+- Bug in Index.intersection on non-monotonic non-unique indexes (:issue:`8362`).

pandas/core/index.py

@@ -1279,6 +1279,7 @@ def intersection(self, other):
         except:
             # duplicates
             indexer = self.get_indexer_non_unique(other.values)[0].unique()
+            indexer = indexer[indexer != -1]
 
         taken = self.take(indexer)
         if self.name != other.name:

pandas/tests/test_index.py

@@ -532,6 +532,13 @@ def test_intersection(self):
         result3 = idx1.intersection(idx3)
         self.assertTrue(tm.equalContents(result3, expected3))
         self.assertEqual(result3.name, expected3.name)
+        
+        # non-monotonic non-unique
+        idx1 = Index(['A','B','A','C'])
+        idx2 = Index(['B','D'])
+        expected = Index(['B'], dtype='object')
+        result = idx1.intersection(idx2)
+        self.assertTrue(result.equals(expected))
 
     def test_union(self):
         first = self.strIndex[5:20]