ENH: Deprecate non-keyword arguments for drop_duplicates.

pandas/core/indexes/multi.py

@@ -78,10 +78,8 @@
 from pandas.core.indexes.base import (
     Index,
     _index_shared_docs,
-    _IndexT,
     ensure_index,
     get_unanimous_names,
-    str_t,
 )
 from pandas.core.indexes.frozen import FrozenList
 from pandas.core.indexes.numeric import Int64Index
@@ -3797,8 +3795,8 @@ def isin(self, values, level=None) -> np.ndarray:
             return levs.isin(values)
 
     @deprecate_nonkeyword_arguments(version=None, allowed_args=["self"])
-    def drop_duplicates(self: _IndexT, keep: str_t | bool = "first") -> _IndexT:
-        return super(Index, self).drop_duplicates(keep=keep)
+    def drop_duplicates(self, keep: str | bool = "first") -> MultiIndex:
+        return super().drop_duplicates(keep=keep)
 
     # ---------------------------------------------------------------
     # Arithmetic/Numeric Methods - Disabled

pandas/tests/frame/methods/test_drop_duplicates.py

@@ -476,16 +476,12 @@ def test_drop_duplicates_non_boolean_ignore_index(arg):
 def test_drop_duplicates_pos_args_deprecation():
     # GH#41485
     df = DataFrame({"a": [1, 1, 2], "b": [1, 1, 3], "c": [1, 1, 3]})
-
     msg = (
         "In a future version of pandas all arguments of "
         "DataFrame.drop_duplicates except for the argument 'subset' "
         "will be keyword-only"
     )
-
     with tm.assert_produces_warning(FutureWarning, match=msg):
         result = df.drop_duplicates(["b", "c"], "last")
-
     expected = DataFrame({"a": [1, 2], "b": [1, 3], "c": [1, 3]}, index=[1, 2])
-
     tm.assert_frame_equal(expected, result)

pandas/tests/indexes/multi/test_duplicates.py

@@ -310,18 +310,13 @@ def test_duplicated_drop_duplicates():
 
 def test_multi_drop_duplicates_pos_args_deprecation():
     # GH#41485
-
     idx = MultiIndex.from_arrays([[1, 2, 3, 1], [1, 2, 3, 1]])
-
     msg = (
         "In a future version of pandas all arguments of "
         "MultiIndex.drop_duplicates will be keyword-only"
     )
-
     with tm.assert_produces_warning(FutureWarning, match=msg):
         idx.drop_duplicates("last")
         result = idx.drop_duplicates("last")
-
     expected = MultiIndex.from_arrays([[2, 3, 1], [2, 3, 1]])
-
     tm.assert_index_equal(expected, result)

pandas/tests/indexes/test_base.py

@@ -1742,18 +1742,13 @@ def test_construct_from_memoryview(klass, extra_kwargs):
 
 def test_drop_duplicates_pos_args_deprecation():
     # GH#41485
-
     idx = Index([1, 2, 3, 1])
-
     msg = (
         "In a future version of pandas all arguments of "
         "Index.drop_duplicates will be keyword-only"
     )
-
     with tm.assert_produces_warning(FutureWarning, match=msg):
         idx.drop_duplicates("last")
         result = idx.drop_duplicates("last")
-
     expected = Index([2, 3, 1])
-
     tm.assert_index_equal(expected, result)

pandas/tests/series/methods/test_drop_duplicates.py

@@ -228,15 +228,11 @@ def test_drop_duplicates_categorical_bool(self, ordered):
 def test_drop_duplicates_pos_args_deprecation():
     # GH#41485
     s = Series(["a", "b", "c", "b"])
-
     msg = (
         "In a future version of pandas all arguments of "
         "Series.drop_duplicates will be keyword-only"
     )
-
     with tm.assert_produces_warning(FutureWarning, match=msg):
         result = s.drop_duplicates("last")
-
     expected = Series(["a", "c", "b"], index=[0, 2, 3])
-
     tm.assert_series_equal(expected, result)