Updated docstring, added whatsnew

Author: rhshadrach

doc/source/whatsnew/v1.1.0.rst

@@ -721,6 +721,7 @@ Groupby/resample/rolling
 - Bug in :meth:`DataFrame.groupby` where a ``ValueError`` would be raised when grouping by a categorical column with read-only categories and ``sort=False`` (:issue:`33410`)
 - Bug in :meth:`GroupBy.first` and :meth:`GroupBy.last` where None is not preserved in object dtype (:issue:`32800`)
 - Bug in :meth:`Rolling.min` and :meth:`Rolling.max`: Growing memory usage after multiple calls when using a fixed window (:issue:`30726`)
+- Bug in :meth:`DataFrame.groupby` when using ``as_index=False`` would modify the grouping column when used with ``idxmax``, ``idxmin``, ``mad``, ``nunique``, and ``skew`` (:issue:`21090`)
 
 Reshaping
 ^^^^^^^^^

pandas/core/groupby/generic.py

@@ -1846,11 +1846,11 @@ def nunique(self, dropna: bool = True):
         5   ham       5      y
 
         >>> df.groupby('id').nunique()
-            id  value1  value2
+              value1  value2
         id
-        egg    1       1       1
-        ham    1       1       2
-        spam   1       2       1
+        egg        1       1
+        ham        1       2
+        spam       2       1
 
         Check for rows with the same id but conflicting values:
 

pandas/tests/groupby/test_function.py

@@ -585,6 +585,7 @@ def test_ops_general(op, targop):
 
 
 def test_ops_not_as_index(reduction_func):
+    # GH 21090
     # Using as_index=False should not modify grouped column
 
     if reduction_func in ("corrwith",):