Moved test, changed whatsnew to api breaking section

Author: rhshadrach

doc/source/whatsnew/v1.1.0.rst

@@ -552,6 +552,34 @@ Assignment to multiple columns of a :class:`DataFrame` when some of the columns
    df[['a', 'c']] = 1
    df
 
+.. _whatsnew_110.api_breaking.groupby_as_index_false:
+
+Using groupby with ``as_index=False``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Using :meth:`DataFrame.groupby` with ``as_index=False`` and the function ``idxmax``, ``idxmin``, ``mad``, ``nunique``, or ``skew`` would modify the grouping column. Now, the grouping column remains unchanged. (:issue:`21090`)
+
+.. ipython:: python
+
+   df = pd.DataFrame({"a": ["x", "x", "y", "y"], "b": [1, 1, 2, 3]})
+   df
+
+*Previous behavior*:
+
+.. code-block:: ipython
+
+   In [3]: df.groupby("a", as_index=False).nunique()
+   Out[4]:
+      a  b
+   0  1  1
+   1  1  2
+
+*New behavior*:
+
+.. ipython:: python
+
+   df.groupby("a", as_index=False).nunique()
+
 .. _whatsnew_110.deprecations:
 
 Deprecations

pandas/tests/groupby/test_groupby.py

@@ -658,6 +658,37 @@ def test_groupby_as_index_agg(df):
         tm.assert_frame_equal(left, right)
 
 
+def test_ops_not_as_index(reduction_func):
+    # GH 21090
+    # Using as_index=False should not modify grouped column
+
+    if reduction_func in ("corrwith",):
+        pytest.skip("Test not applicable")
+
+    if reduction_func in ("nth", "ngroup", "size",):
+        pytest.skip("Skip until behavior is determined (GH #5755)")
+
+    if reduction_func in ("sem", "std"):
+        pytest.skip("Function incorrectly modifies keys (GH #10355)")
+
+    df = DataFrame(np.random.randint(0, 5, size=(100, 2)), columns=["a", "b"])
+    expected = getattr(df.groupby("a"), reduction_func)().reset_index()
+
+    g = df.groupby("a", as_index=False)
+
+    result = getattr(g, reduction_func)()
+    tm.assert_frame_equal(result, expected)
+
+    result = g.agg(reduction_func)
+    tm.assert_frame_equal(result, expected)
+
+    result = getattr(g["b"], reduction_func)()
+    tm.assert_frame_equal(result, expected)
+
+    result = g["b"].agg(reduction_func)
+    tm.assert_frame_equal(result, expected)
+
+
 def test_as_index_series_return_frame(df):
     grouped = df.groupby("A", as_index=False)
     grouped2 = df.groupby(["A", "B"], as_index=False)