add fix for bug pandas-dev#19029

As of version 0.23.0 MultiIndex throws an exception in case it contains
duplicated level names. This can happen as a result of various groupby
operations (pandas-dev#21075). This commit changes the behavior of groupby slightly: In
case there are duplicated names contained in the index these names get suffixed by there
corresonding position (i.e. [name,name] => [name0,name1])

Author: guenteru

pandas/core/groupby/groupby.py

@@ -7,6 +7,7 @@
 import copy
 from textwrap import dedent
 from contextlib import contextmanager
+import pdb
 
 from pandas.compat import (
     zip, range, lzip,
@@ -2298,7 +2299,18 @@ def levels(self):
 
     @property
     def names(self):
-        return [ping.name for ping in self.groupings]
+        # GH 19029
+        # add suffix to level name in case they contain duplicates (GH 19029):
+        orig_names =  [ping.name for ping in self.groupings]
+        # if no names were assigned return the original names 
+        if all(x is None for x in orig_names):
+            return orig_names
+        # in case duplicates are contained rename all of them
+        if len(set(orig_names)) < len(orig_names):
+            orig_names = [''.join([str(x),str(i)])
+                                for i,x in enumerate(orig_names)]
+
+        return orig_names
 
     def size(self):
         """

pandas/tests/groupby/test_categorical.py

@@ -558,9 +558,15 @@ def test_as_index():
     result = df.groupby(['cat', s], as_index=False, observed=True).sum()
     tm.assert_frame_equal(result, expected)
 
-    # GH18872: conflicting names in desired index
-    with pytest.raises(ValueError):
+    # GH 19029: conflicitng names should not raise a value error anymore
+    raised=False
+    try:
         df.groupby(['cat', s.rename('cat')], observed=True).sum()
+    except ValueError as e:
+        raised = True
+    assert raised == False
+        
+         
 
     # is original index dropped?
     group_columns = ['cat', 'A']