Deprecate passing args as positional in sort_index (#41506)

doc/source/user_guide/io.rst

@@ -1897,7 +1897,7 @@ Writing in ISO date format:
 
    dfd = pd.DataFrame(np.random.randn(5, 2), columns=list("AB"))
    dfd["date"] = pd.Timestamp("20130101")
-   dfd = dfd.sort_index(1, ascending=False)
+   dfd = dfd.sort_index(axis=1, ascending=False)
    json = dfd.to_json(date_format="iso")
    json
 

doc/source/whatsnew/v1.3.0.rst

@@ -680,6 +680,7 @@ Deprecations
 - Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
 - Deprecated special treatment of lists with first element a Categorical in the :class:`DataFrame` constructor; pass as ``pd.DataFrame({col: categorical, ...})`` instead (:issue:`38845`)
 - Deprecated passing arguments as positional (except for ``"method"``) in :meth:`DataFrame.interpolate` and :meth:`Series.interpolate` (:issue:`41485`)
+- Deprecated passing arguments as positional in :meth:`DataFrame.sort_index` and :meth:`Series.sort_index` (:issue:`41485`)
 - Deprecated passing arguments as positional in :meth:`DataFrame.drop_duplicates` (except for ``subset``), :meth:`Series.drop_duplicates`, :meth:`Index.drop_duplicates` and :meth:`MultiIndex.drop_duplicates`(:issue:`41485`)
 - Deprecated passing arguments (apart from ``value``) as positional in :meth:`DataFrame.fillna` and :meth:`Series.fillna` (:issue:`41485`)
 - Deprecated construction of :class:`Series` or :class:`DataFrame` with ``DatetimeTZDtype`` data and ``datetime64[ns]`` dtype.  Use ``Series(data).dt.tz_localize(None)`` instead (:issue:`41555`,:issue:`33401`)

pandas/core/frame.py

@@ -6292,6 +6292,7 @@ def sort_values(  # type: ignore[override]
         else:
             return result.__finalize__(self, method="sort_values")
 
+    @deprecate_nonkeyword_arguments(version=None, allowed_args=["self"])
     def sort_index(
         self,
         axis: Axis = 0,

pandas/core/series.py

@@ -3468,6 +3468,7 @@ def sort_values(
         else:
             return result.__finalize__(self, method="sort_values")
 
+    @deprecate_nonkeyword_arguments(version=None, allowed_args=["self"])
     def sort_index(
         self,
         axis=0,

pandas/tests/frame/methods/test_sort_index.py

@@ -867,3 +867,15 @@ def test_sort_index_multiindex_sparse_column(self):
         result = expected.sort_index(level=0)
 
         tm.assert_frame_equal(result, expected)
+
+    def test_sort_index_pos_args_deprecation(self):
+        # https://github.com/pandas-dev/pandas/issues/41485
+        df = DataFrame({"a": [1, 2, 3]})
+        msg = (
+            r"In a future version of pandas all arguments of DataFrame.sort_index "
+            r"will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = df.sort_index(1)
+        expected = DataFrame({"a": [1, 2, 3]})
+        tm.assert_frame_equal(result, expected)

pandas/tests/indexing/test_loc.py

@@ -1352,9 +1352,9 @@ def test_loc_setitem_unsorted_multiindex_columns(self, key):
         expected = DataFrame([[0, 2, 0], [0, 5, 0]], columns=mi)
         tm.assert_frame_equal(obj, expected)
 
-        df = df.sort_index(1)
+        df = df.sort_index(axis=1)
         df.loc[:, key] = np.zeros((2, 2), dtype=int)
-        expected = expected.sort_index(1)
+        expected = expected.sort_index(axis=1)
         tm.assert_frame_equal(df, expected)
 
     def test_loc_setitem_uint_drop(self, any_int_dtype):

pandas/tests/series/methods/test_sort_index.py

@@ -320,3 +320,15 @@ def test_sort_values_key_type(self):
         result = s.sort_index(key=lambda x: x.month_name())
         expected = s.iloc[[2, 1, 0]]
         tm.assert_series_equal(result, expected)
+
+    def test_sort_index_pos_args_deprecation(self):
+        # https://github.com/pandas-dev/pandas/issues/41485
+        ser = Series([1, 2, 3])
+        msg = (
+            r"In a future version of pandas all arguments of Series.sort_index "
+            r"will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = ser.sort_index(0)
+        expected = Series([1, 2, 3])
+        tm.assert_series_equal(result, expected)