Moved notes to 0.22; created is_axis_multiindex var - pending internal use

Author: GuessWhoSamFoo

doc/source/whatsnew/v0.21.0.txt

@@ -1094,7 +1094,6 @@ Groupby/Resample/Rolling
 - Bug in ``DataFrame.groupby`` where a single level selection from a ``MultiIndex`` unexpectedly sorts (:issue:`17537`)
 - Bug in ``DataFrame.groupby`` where spurious warning is raised when ``Grouper`` object is used to override ambiguous column name (:issue:`17383`)
 - Bug in ``TimeGrouper`` differs when passes as a list and as a scalar (:issue:`17530`)
-- Bug in ``DataFrame.groupby`` where key as tuple in a ``MultiIndex`` were interpreted as a list of keys (:issue:`17979`)
 
 Sparse
 ^^^^^^

doc/source/whatsnew/v0.22.0.txt

@@ -88,6 +88,7 @@ Bug Fixes
 ~~~~~~~~~
 
 - Bug in ``pd.read_msgpack()`` with a non existent file is passed in Python 2 (:issue:`15296`)
+- Bug in ``DataFrame.groupby`` where key as tuple in a ``MultiIndex`` were interpreted as a list of keys (:issue:`17979`)
 
 Conversion
 ^^^^^^^^^^

pandas/core/groupby.py

@@ -2705,6 +2705,7 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
 
     """
     group_axis = obj._get_axis(axis)
+    is_axis_multiindex = isinstance(obj._info_axis, MultiIndex)
 
     # validate that the passed single level is compatible with the passed
     # axis of the object
@@ -2766,8 +2767,8 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
         return key, [], obj
 
     # when MultiIndex, allow tuple to be a key
-    if not isinstance(key, (tuple, list)) or isinstance(key, tuple) and \
-            isinstance(obj._info_axis, MultiIndex):
+    if not isinstance(key, (tuple, list)) or \
+            (isinstance(key, tuple) and is_axis_multiindex):
         keys = [key]
         match_axis_length = False
     else:
@@ -2871,7 +2872,6 @@ def is_in_obj(gpr):
 
     # create the internals grouper
     grouper = BaseGrouper(group_axis, groupings, sort=sort, mutated=mutated)
-
     return grouper, exclusions, obj