BUG: Never interpret a tuple as a list of keys

Author: toobaz

doc/source/whatsnew/v0.22.0.txt

@@ -99,7 +99,7 @@ Indexing
 ^^^^^^^^
 
 - Bug in :func:`Series.truncate` which raises ``TypeError`` with a monotonic ``PeriodIndex`` (:issue:`17717`)
-- Bug in :func:`DataFrame.groupby` where key as tuple in a ``MultiIndex`` were interpreted as a list of keys (:issue:`17979`)
+- Bug in :func:`DataFrame.groupby` where tuples were interpreted as lists of keys rather than as keys (:issue:`17979`)
 -
 -
 

pandas/core/groupby.py

@@ -2704,7 +2704,6 @@ 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
@@ -2765,9 +2764,8 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
     elif isinstance(key, BaseGrouper):
         return key, [], obj
 
-    # when MultiIndex, allow tuple to be a key
-    if not isinstance(key, (tuple, list)) or \
-            (isinstance(key, tuple) and is_axis_multiindex):
+    # Everything which is not a list is a key (including tuples):
+    if not isinstance(key, list):
         keys = [key]
         match_axis_length = False
     else: