BUG: DataFrame GroupBy indexing with single items DeprecationWarning(pandas-dev#23566)

Author: yehoshuadimarsky

pandas/core/groupby/generic.py

@@ -24,6 +24,7 @@
     Union,
     cast,
 )
+import warnings
 
 import numpy as np
 
@@ -325,7 +326,7 @@ def _aggregate_multiple_funcs(self, arg):
         return DataFrame(results, columns=columns)
 
     def _wrap_series_output(
-        self, output: Mapping[base.OutputKey, Union[Series, np.ndarray]], index: Index,
+        self, output: Mapping[base.OutputKey, Union[Series, np.ndarray]], index: Index
     ) -> Union[Series, DataFrame]:
         """
         Wraps the output of a SeriesGroupBy operation into the expected result.
@@ -1574,6 +1575,17 @@ def filter(self, func, dropna=True, *args, **kwargs):
 
         return self._apply_filter(indices, dropna)
 
+    def __getitem__(self, key):
+        # per GH 23566
+        if isinstance(key, tuple):
+            warnings.warn(
+                "Indexing with individual keys or with a tuple of keys "
+                "will be deprecated, use a list instead.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+        return super().__getitem__(key)
+
     def _gotitem(self, key, ndim: int, subset=None):
         """
         sub-classes to define

pandas/tests/groupby/test_grouping.py

@@ -71,14 +71,16 @@ def test_getitem_list_of_columns(self):
         )
 
         result = df.groupby("A")[["C", "D"]].mean()
-        result2 = df.groupby("A")["C", "D"].mean()
-        result3 = df.groupby("A")[df.columns[2:4]].mean()
+        result2 = df.groupby("A")[df.columns[2:4]].mean()
 
         expected = df.loc[:, ["A", "C", "D"]].groupby("A").mean()
 
         tm.assert_frame_equal(result, expected)
         tm.assert_frame_equal(result2, expected)
-        tm.assert_frame_equal(result3, expected)
+
+        # per GH 23566 this should raise a deprecation warning
+        with tm.assert_produces_warning(DeprecationWarning):
+            df.groupby("A")["C", "D"].mean()
 
     def test_getitem_numeric_column_names(self):
         # GH #13731