BUG: Fix for GH #14848 for groupby().describe() with tuples as the Index

closes #14848

Author: Dr-Irv <irv@princeton.com>

Closes #15110 from Dr-Irv/Issue14848 and squashes the following commits:

c18c6cb [Dr-Irv] Undo change to merge.py and make whatsnew a 2 line comment.
db13c3b [Dr-Irv] Use not is_list_like
fbd20f5 [Dr-Irv] Raise error when creating index of tuples with name parameter a string
f3a7a21 [Dr-Irv] Changes per jreback requests
9489cb2 [Dr-Irv] BUG: Fix issue #14848 groupby().describe() on indices containing all tuples

Author: Dr-Irv

doc/source/whatsnew/v0.20.0.txt

@@ -352,6 +352,8 @@ Bug Fixes
 - Bug in converting object elements of array-like objects to unsigned 64-bit integers (:issue:`4471`, :issue:`14982`)
 - Bug in ``pd.pivot_table()`` where no error was raised when values argument was not in the columns (:issue:`14938`)
 
+- Bug in ``DataFrame.groupby().describe()`` when grouping on ``Index`` containing tuples (:issue:`14848`)
+- Bug in creating a ``MultiIndex`` with tuples and not passing a list of names; this will now raise ``ValueError`` (:issue:`15110`)
 
 
 

pandas/indexes/multi.py

@@ -490,6 +490,10 @@ def _set_names(self, names, level=None, validate=True):
         that it only acts on copies
         """
 
+        # GH 15110
+        # Don't allow a single string for names in a MultiIndex
+        if names is not None and not is_list_like(names):
+            raise ValueError('Names should be list-like for a MultiIndex')
         names = list(names)
 
         if validate and level is not None and len(names) != len(level):

pandas/tests/groupby/test_groupby.py

@@ -1490,6 +1490,19 @@ def test_frame_describe_multikey(self):
         for name, group in groupedT:
             assert_frame_equal(result[name], group.describe())
 
+    def test_frame_describe_tupleindex(self):
+
+        # GH 14848 - regression from 0.19.0 to 0.19.1
+        df1 = DataFrame({'x': [1, 2, 3, 4, 5] * 3,
+                         'y': [10, 20, 30, 40, 50] * 3,
+                         'z': [100, 200, 300, 400, 500] * 3})
+        df1['k'] = [(0, 0, 1), (0, 1, 0), (1, 0, 0)] * 5
+        df2 = df1.rename(columns={'k': 'key'})
+        result = df1.groupby('k').describe()
+        expected = df2.groupby('key').describe()
+        expected.index.set_names(result.index.names, inplace=True)
+        assert_frame_equal(result, expected)
+
     def test_frame_groupby(self):
         grouped = self.tsframe.groupby(lambda x: x.weekday())
 

pandas/tests/indexes/test_multi.py

@@ -2554,3 +2554,12 @@ def test_unsortedindex(self):
 
         with assertRaises(KeyError):
             df.loc(axis=0)['q', :]
+
+    def test_tuples_with_name_string(self):
+        # GH 15110 and GH 14848
+
+        li = [(0, 0, 1), (0, 1, 0), (1, 0, 0)]
+        with assertRaises(ValueError):
+            pd.Index(li, name='abc')
+        with assertRaises(ValueError):
+            pd.Index(li, name='a')