BUG: don't lose names in MultiIndex.droplevel, fix #394

RELEASE.rst

@@ -135,8 +135,8 @@ pandas 0.5.1
   - Handle min_periods=0 case in moving window functions (GH #365)
   - Fixed corner cases in DataFrame.apply/pivot with empty DataFrame (GH #378)
   - Fixed repr exception when Series name is a tuple
-  - Always return DateRange from asfreq (GH #390)
-  -
+  - Always return DateRange from `asfreq` (GH #390)
+  - Pass level names to `swaplavel` (GH #379)
 
 Thanks
 ------

pandas/core/index.py

@@ -73,6 +73,11 @@ def dtype(self):
     def nlevels(self):
         return 1
 
+    @property
+    def names(self):
+        # for compat with multindex code
+        return [self.name]
+
     @property
     def _constructor(self):
         return Index
@@ -864,6 +869,9 @@ class MultiIndex(Index):
     labels : list or tuple of arrays
         Integers for each level designating which label at each location
     """
+    # shadow property
+    names = None
+
     def __new__(cls, levels=None, labels=None, sortorder=None, names=None):
         assert(len(levels) == len(labels))
         if len(levels) == 0:
@@ -1178,11 +1186,16 @@ def droplevel(self, level=0):
         new_levels.pop(level)
         new_labels = list(self.labels)
         new_labels.pop(level)
+        new_names = list(self.names)
+        new_names.pop(level)
 
         if len(new_levels) == 1:
-            return new_levels[0].take(new_labels[0])
+            result = new_levels[0].take(new_labels[0])
+            result.name = new_names[0]
+            return result
         else:
-            return MultiIndex(levels=new_levels, labels=new_labels)
+            return MultiIndex(levels=new_levels, labels=new_labels,
+                              names=new_names)
 
     def swaplevel(self, i, j):
         """

pandas/tests/test_index.py

@@ -1074,6 +1074,16 @@ def test_droplevel_with_names(self):
         dropped = index.droplevel(0)
         self.assertEqual(dropped.name, 'second')
 
+        index = MultiIndex(levels=[Index(range(4)),
+                                   Index(range(4)),
+                                   Index(range(4))],
+                           labels=[np.array([0, 0, 1, 2, 2, 2, 3, 3]),
+                                   np.array([0, 1, 0, 0, 0, 1, 0, 1]),
+                                   np.array([1, 0, 1, 1, 0, 0, 1, 0])],
+                           names=['one', 'two', 'three'])
+        dropped = index.droplevel(0)
+        self.assertEqual(dropped.names, ['two', 'three'])
+
     def test_insert(self):
         # key contained in all levels
         new_index = self.index.insert(0, ('bar', 'two'))

pandas/tests/test_multilevel.py

@@ -652,6 +652,17 @@ def test_multilevel_consolidate(self):
         df['Totals', ''] = df.sum(1)
         df = df.consolidate()
 
+    def test_ix_preserve_names(self):
+        result = self.ymd.ix[2000]
+        result2 = self.ymd['A'].ix[2000]
+        self.assertEquals(result.index.names, self.ymd.index.names[1:])
+        self.assertEquals(result2.index.names, self.ymd.index.names[1:])
+
+        result = self.ymd.ix[2000, 2]
+        result2 = self.ymd['A'].ix[2000, 2]
+        self.assertEquals(result.index.name, self.ymd.index.names[2])
+        self.assertEquals(result2.index.name, self.ymd.index.names[2])
+
 if __name__ == '__main__':
 
     # unittest.main()