CLN: Enforce deprecation get_group with tuples of length 1 (#57743)

* CLN: Enforce deprecation get_group with tuples of length 1

* Add in similar deprecation involving iteration

* type ignore

doc/source/whatsnew/v3.0.0.rst

@@ -200,10 +200,12 @@ Removal of prior version deprecations/changes
 - All arguments in :meth:`Series.to_dict` are now keyword only (:issue:`56493`)
 - Changed the default value of ``observed`` in :meth:`DataFrame.groupby` and :meth:`Series.groupby` to ``True`` (:issue:`51811`)
 - Enforced deprecation disallowing parsing datetimes with mixed time zones unless user passes ``utc=True`` to :func:`to_datetime` (:issue:`57275`)
+- Enforced deprecation of :meth:`.DataFrameGroupBy.get_group` and :meth:`.SeriesGroupBy.get_group` allowing the ``name`` argument to be a non-tuple when grouping by a list of length 1 (:issue:`54155`)
 - Enforced deprecation of ``axis=None`` acting the same as ``axis=0`` in the DataFrame reductions ``sum``, ``prod``, ``std``, ``var``, and ``sem``, passing ``axis=None`` will now reduce over both axes; this is particularly the case when doing e.g. ``numpy.sum(df)`` (:issue:`21597`)
 - Enforced deprecation of passing a dictionary to :meth:`SeriesGroupBy.agg` (:issue:`52268`)
 - Enforced silent-downcasting deprecation for :ref:`all relevant methods <whatsnew_220.silent_downcasting>` (:issue:`54710`)
 - In :meth:`DataFrame.stack`, the default value of ``future_stack`` is now ``True``; specifying ``False`` will raise a ``FutureWarning`` (:issue:`55448`)
+- Iterating over a :class:`.DataFrameGroupBy` or :class:`.SeriesGroupBy` will return tuples of length 1 for the groups when grouping by ``level`` a list of length 1 (:issue:`50064`)
 - Methods ``apply``, ``agg``, and ``transform`` will no longer replace NumPy functions (e.g. ``np.sum``) and built-in functions (e.g. ``min``) with the equivalent pandas implementation; use string aliases (e.g. ``"sum"`` and ``"min"``) if you desire to use the pandas implementation (:issue:`53974`)
 - Passing both ``freq`` and ``fill_value`` in :meth:`DataFrame.shift` and :meth:`Series.shift` and :meth:`.DataFrameGroupBy.shift` now raises a ``ValueError`` (:issue:`54818`)
 - Removed :meth:`.DataFrameGroupBy.quantile` and :meth:`.SeriesGroupBy.quantile` supporting bool dtype (:issue:`53975`)

pandas/core/groupby/groupby.py

@@ -919,17 +919,9 @@ def get_group(self, name) -> DataFrame | Series:
         ):
             # GH#25971
             if isinstance(name, tuple) and len(name) == 1:
-                # Allow users to pass tuples of length 1 to silence warning
                 name = name[0]
-            elif not isinstance(name, tuple):
-                warnings.warn(
-                    "When grouping with a length-1 list-like, "
-                    "you will need to pass a length-1 tuple to get_group in a future "
-                    "version of pandas. Pass `(name,)` instead of `name` to silence "
-                    "this warning.",
-                    FutureWarning,
-                    stacklevel=find_stack_level(),
-                )
+            else:
+                raise KeyError(name)
 
         inds = self._get_index(name)
         if not len(inds):
@@ -1015,18 +1007,11 @@ def __iter__(self) -> Iterator[tuple[Hashable, NDFrameT]]:
         keys = self.keys
         level = self.level
         result = self._grouper.get_iterator(self._selected_obj)
-        # error: Argument 1 to "len" has incompatible type "Hashable"; expected "Sized"
-        if is_list_like(level) and len(level) == 1:  # type: ignore[arg-type]
-            # GH 51583
-            warnings.warn(
-                "Creating a Groupby object with a length-1 list-like "
-                "level parameter will yield indexes as tuples in a future version. "
-                "To keep indexes as scalars, create Groupby objects with "
-                "a scalar level parameter instead.",
-                FutureWarning,
-                stacklevel=find_stack_level(),
-            )
-        if isinstance(keys, list) and len(keys) == 1:
+        # mypy: Argument 1 to "len" has incompatible type "Hashable"; expected "Sized"
+        if (
+            (is_list_like(level) and len(level) == 1)  # type: ignore[arg-type]
+            or (isinstance(keys, list) and len(keys) == 1)
+        ):
             # GH#42795 - when keys is a list, return tuples even when length is 1
             result = (((key,), group) for key, group in result)
         return result

pandas/tests/groupby/test_categorical.py

@@ -252,11 +252,7 @@ def test_level_get_group(observed):
             names=["Index1", "Index2"],
         ),
     )
-    msg = "you will need to pass a length-1 tuple"
-    with tm.assert_produces_warning(FutureWarning, match=msg):
-        # GH#25971 - warn when not passing a length-1 tuple
-        result = g.get_group("a")
-
+    result = g.get_group(("a",))
     tm.assert_frame_equal(result, expected)
 
 

pandas/tests/groupby/test_groupby.py

@@ -2529,19 +2529,14 @@ def test_groupby_string_dtype():
 @pytest.mark.parametrize(
     "level_arg, multiindex", [([0], False), ((0,), False), ([0], True), ((0,), True)]
 )
-def test_single_element_listlike_level_grouping_deprecation(level_arg, multiindex):
+def test_single_element_listlike_level_grouping(level_arg, multiindex):
     # GH 51583
     df = DataFrame({"a": [1, 2], "b": [3, 4], "c": [5, 6]}, index=["x", "y"])
     if multiindex:
         df = df.set_index(["a", "b"])
-    depr_msg = (
-        "Creating a Groupby object with a length-1 list-like "
-        "level parameter will yield indexes as tuples in a future version. "
-        "To keep indexes as scalars, create Groupby objects with "
-        "a scalar level parameter instead."
-    )
-    with tm.assert_produces_warning(FutureWarning, match=depr_msg):
-        [key for key, _ in df.groupby(level=level_arg)]
+    result = [key for key, _ in df.groupby(level=level_arg)]
+    expected = [(1,), (2,)] if multiindex else [("x",), ("y",)]
+    assert result == expected
 
 
 @pytest.mark.parametrize("func", ["sum", "cumsum", "cumprod", "prod"])
@@ -2880,22 +2875,18 @@ def test_groupby_series_with_datetimeindex_month_name():
     "kwarg, value, name, warn",
     [
         ("by", "a", 1, None),
-        ("by", ["a"], 1, FutureWarning),
         ("by", ["a"], (1,), None),
         ("level", 0, 1, None),
-        ("level", [0], 1, FutureWarning),
         ("level", [0], (1,), None),
     ],
 )
-def test_depr_get_group_len_1_list_likes(test_series, kwarg, value, name, warn):
+def test_get_group_len_1_list_likes(test_series, kwarg, value, name, warn):
     # GH#25971
     obj = DataFrame({"b": [3, 4, 5]}, index=Index([1, 1, 2], name="a"))
     if test_series:
         obj = obj["b"]
     gb = obj.groupby(**{kwarg: value})
-    msg = "you will need to pass a length-1 tuple"
-    with tm.assert_produces_warning(warn, match=msg):
-        result = gb.get_group(name)
+    result = gb.get_group(name)
     if test_series:
         expected = Series([3, 4], index=Index([1, 1], name="a"), name="b")
     else: