DEPR: deprecate obj argument in GroupBy.get_group

doc/source/whatsnew/v2.1.0.rst

@@ -289,6 +289,7 @@ Deprecations
 - Deprecated constructing :class:`SparseArray` from scalar data, pass a sequence instead (:issue:`53039`)
 - Deprecated falling back to filling when ``value`` is not specified in :meth:`DataFrame.replace` and :meth:`Series.replace` with non-dict-like ``to_replace`` (:issue:`33302`)
 - Deprecated option "mode.use_inf_as_na", convert inf entries to ``NaN`` before instead (:issue:`51684`)
+- Deprecated parameter ``obj`` in :meth:`GroupBy.get_group` (:issue:`53545`)
 - Deprecated positional indexing on :class:`Series` with :meth:`Series.__getitem__` and :meth:`Series.__setitem__`, in a future version ``ser[item]`` will *always* interpret ``item`` as a label, not a position (:issue:`50617`)
 - Deprecated the "method" and "limit" keywords on :meth:`Series.fillna`, :meth:`DataFrame.fillna`, :meth:`SeriesGroupBy.fillna`, :meth:`DataFrameGroupBy.fillna`, and :meth:`Resampler.fillna`, use ``obj.bfill()`` or ``obj.ffill()`` instead (:issue:`53394`)
 - Deprecated the ``method`` and ``limit`` keywords in :meth:`DataFrame.replace` and :meth:`Series.replace` (:issue:`33302`)

pandas/core/groupby/groupby.py

@@ -925,6 +925,11 @@ def get_group(self, name, obj=None) -> DataFrame | Series:
             it is None, the object groupby was called on will
             be used.
 
+            .. deprecated:: 2.1.0
+                The obj is deprecated and will be removed in a future version.
+                Do ``df.iloc[gb.indices.get(name)]``
+                instead of ``gb.get_group(name, obj=df)``.
+
         Returns
         -------
         same type as obj
@@ -961,14 +966,21 @@ def get_group(self, name, obj=None) -> DataFrame | Series:
         owl     1  2  3
         toucan  1  5  6
         """
-        if obj is None:
-            obj = self._selected_obj
-
         inds = self._get_index(name)
         if not len(inds):
             raise KeyError(name)
 
-        return obj._take_with_is_copy(inds, axis=self.axis)
+        if obj is None:
+            return self._selected_obj.iloc[inds]
+        else:
+            warnings.warn(
+                "obj is deprecated and will be removed in a future version. "
+                "Do ``df.iloc[gb.indices.get(name)]`` "
+                "instead of ``gb.get_group(name, obj=df)``.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+            )
+            return obj._take_with_is_copy(inds, axis=self.axis)
 
     @final
     def __iter__(self) -> Iterator[tuple[Hashable, NDFrameT]]:

pandas/tests/groupby/test_groupby.py

@@ -695,6 +695,16 @@ def test_as_index_select_column():
     tm.assert_series_equal(result, expected)
 
 
+def test_obj_arg_get_group_deprecated():
+    depr_msg = "obj is deprecated"
+
+    df = DataFrame({"a": [1, 1, 2], "b": [3, 4, 5]})
+    expected = df.iloc[df.groupby("b").indices.get(4)]
+    with tm.assert_produces_warning(FutureWarning, match=depr_msg):
+        result = df.groupby("b").get_group(4, obj=df)
+        tm.assert_frame_equal(result, expected)
+
+
 def test_groupby_as_index_select_column_sum_empty_df():
     # GH 35246
     df = DataFrame(columns=Index(["A", "B", "C"], name="alpha"))