ENH: Deprecate non-keyword arguments for drop_duplicates.

doc/source/whatsnew/v1.3.0.rst

@@ -676,7 +676,7 @@ 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.drop_duplicates` (except for ``subset``), :meth:`Series.drop_duplicates` and :meth:`Index.drop_duplicates` (: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/indexes/multi.py

@@ -41,6 +41,7 @@
 from pandas.util._decorators import (
     Appender,
     cache_readonly,
+    deprecate_nonkeyword_arguments,
     doc,
 )
 
@@ -77,8 +78,10 @@
 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
@@ -3793,6 +3796,10 @@ def isin(self, values, level=None) -> np.ndarray:
                 return np.zeros(len(levs), dtype=np.bool_)
             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)
+
     # ---------------------------------------------------------------
     # Arithmetic/Numeric Methods - Disabled
 

pandas/tests/indexes/multi/test_duplicates.py

@@ -309,11 +309,13 @@ 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 "
-        "Index.drop_duplicates will be keyword-only"
+        "MultiIndex.drop_duplicates will be keyword-only"
     )
 
     with tm.assert_produces_warning(FutureWarning, match=msg):

pandas/tests/indexes/test_base.py

@@ -1741,6 +1741,8 @@ def test_construct_from_memoryview(klass, extra_kwargs):
 
 
 def test_drop_duplicates_pos_args_deprecation():
+    # GH#41485
+
     idx = Index([1, 2, 3, 1])
 
     msg = (