Expanded tests, added reference to GHI

Author: rhshadrach

pandas/tests/groupby/test_function.py

@@ -603,10 +603,15 @@ def test_ops_general(op, targop):
     ],
 )
 def test_ops_not_as_index(op):
+    # GH 10355
+    # Using as_index=False should not modify grouped column
     df = DataFrame(np.random.randint(0, 5, size=(100, 2)), columns=["a", "b"])
+    expected = getattr(df.groupby("a"), op)().reset_index()
 
     result = getattr(df.groupby("a", as_index=False), op)()
-    expected = getattr(df.groupby("a"), op)().reset_index()
+    tm.assert_frame_equal(result, expected)
+
+    result = getattr(df.groupby("a", as_index=False)["b"], op)()
     tm.assert_frame_equal(result, expected)