DEPR: Enforce DataFrameGroupBy.__iter__ returning tuples of length 1 (#50064)

Author: rhshadrach

doc/source/whatsnew/v2.0.0.rst

@@ -596,7 +596,7 @@ Removal of prior version deprecations/changes
 - Enforced deprecation of silently dropping nuisance columns in groupby and resample operations when ``numeric_only=False`` (:issue:`41475`)
 - Changed default of ``numeric_only`` in various :class:`.DataFrameGroupBy` methods; all methods now default to ``numeric_only=False`` (:issue:`46072`)
 - Changed default of ``numeric_only`` to ``False`` in :class:`.Resampler` methods (:issue:`47177`)
--
+- When providing a list of columns of length one to :meth:`DataFrame.groupby`, the keys that are returned by iterating over the resulting :class:`DataFrameGroupBy` object will now be tuples of length one (:issue:`47761`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_200.performance:

pandas/core/groupby/groupby.py

@@ -70,7 +70,6 @@ class providing the base-class of operations.
     cache_readonly,
     doc,
 )
-from pandas.util._exceptions import find_stack_level
 
 from pandas.core.dtypes.cast import ensure_dtype_can_hold_na
 from pandas.core.dtypes.common import (
@@ -832,19 +831,11 @@ def __iter__(self) -> Iterator[tuple[Hashable, NDFrameT]]:
         for each group
         """
         keys = self.keys
+        result = self.grouper.get_iterator(self._selected_obj, axis=self.axis)
         if isinstance(keys, list) and len(keys) == 1:
-            warnings.warn(
-                (
-                    "In a future version of pandas, a length 1 "
-                    "tuple will be returned when iterating over a "
-                    "groupby with a grouper equal to a list of "
-                    "length 1. Don't supply a list with a single grouper "
-                    "to avoid this warning."
-                ),
-                FutureWarning,
-                stacklevel=find_stack_level(),
-            )
-        return self.grouper.get_iterator(self._selected_obj, axis=self.axis)
+            # GH#42795 - when keys is a list, return tuples even when length is 1
+            result = (((key,), group) for key, group in result)
+        return result
 
 
 # To track operations that expand dimensions, like ohlc

pandas/tests/groupby/test_groupby.py

@@ -2743,18 +2743,10 @@ def test_groupby_none_column_name():
 
 def test_single_element_list_grouping():
     # GH 42795
-    df = DataFrame(
-        {"a": [np.nan, 1], "b": [np.nan, 5], "c": [np.nan, 2]}, index=["x", "y"]
-    )
-    msg = (
-        "In a future version of pandas, a length 1 "
-        "tuple will be returned when iterating over "
-        "a groupby with a grouper equal to a list of "
-        "length 1. Don't supply a list with a single grouper "
-        "to avoid this warning."
-    )
-    with tm.assert_produces_warning(FutureWarning, match=msg):
-        values, _ = next(iter(df.groupby(["a"])))
+    df = DataFrame({"a": [1, 2], "b": [np.nan, 5], "c": [np.nan, 2]}, index=["x", "y"])
+    result = [key for key, _ in df.groupby(["a"])]
+    expected = [(1,), (2,)]
+    assert result == expected
 
 
 @pytest.mark.parametrize("func", ["sum", "cumsum", "cumprod", "prod"])