From ebd40aae0196cec4b6c7f2364655f7fc55024cc0 Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Sun, 16 May 2021 02:32:25 +0200
Subject: [PATCH 01/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 doc/source/whatsnew/v1.3.0.rst                      |  2 +-
 pandas/core/frame.py                                |  2 ++
 pandas/core/series.py                               |  2 ++
 pandas/tests/frame/methods/test_drop_duplicates.py  | 11 +++++++++++
 pandas/tests/series/methods/test_drop_duplicates.py | 11 +++++++++++
 5 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 622029adf357f..e34589a0ae5c5 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -647,7 +647,7 @@ Deprecations
 - Deprecated setting :attr:`Categorical._codes`, create a new :class:`Categorical` with the desired codes instead (:issue:`40606`)
 - Deprecated behavior of :meth:`DatetimeIndex.union` with mixed timezones; in a future version both will be cast to UTC instead of object dtype (:issue:`39328`)
 - Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
-
+- Deprecated passing arguments as positional in :meth:`DataFrame.drop_duplicates` and :meth:`Series.drop_duplicates` (:issue:`41485`)
 .. ---------------------------------------------------------------------------
 
 
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 2941b6ac01904..9f602b643731f 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -77,6 +77,7 @@
     Appender,
     Substitution,
     deprecate_kwarg,
+    deprecate_nonkeyword_arguments,
     doc,
     rewrite_axis_style_signature,
 )
@@ -5953,6 +5954,7 @@ def dropna(
         else:
             return result
 
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
     def drop_duplicates(
         self,
         subset: Hashable | Sequence[Hashable] | None = None,
diff --git a/pandas/core/series.py b/pandas/core/series.py
index c8e9898f9462a..ba6b2ab1e7cf0 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -51,6 +51,7 @@
 from pandas.util._decorators import (
     Appender,
     Substitution,
+    deprecate_nonkeyword_arguments,
     doc,
 )
 from pandas.util._validators import (
@@ -2024,6 +2025,7 @@ def drop_duplicates(self, *, inplace: Literal[True]) -> None:
     def drop_duplicates(self, keep=..., inplace: bool = ...) -> Series | None:
         ...
 
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
     def drop_duplicates(self, keep="first", inplace=False) -> Series | None:
         """
         Return Series with duplicate values removed.
diff --git a/pandas/tests/frame/methods/test_drop_duplicates.py b/pandas/tests/frame/methods/test_drop_duplicates.py
index 10c1f37f4c9ba..7bfdc8bc308e4 100644
--- a/pandas/tests/frame/methods/test_drop_duplicates.py
+++ b/pandas/tests/frame/methods/test_drop_duplicates.py
@@ -471,3 +471,14 @@ def test_drop_duplicates_non_boolean_ignore_index(arg):
     msg = '^For argument "ignore_index" expected type bool, received type .*.$'
     with pytest.raises(ValueError, match=msg):
         df.drop_duplicates(ignore_index=arg)
+
+
+def test_drop_duplicates_pos_args_deprecation():
+    # test deprecation warning message for positional arguments GH#41485
+    df = DataFrame({"a": [1, 1, 2], "b": [1, 1, 3], "c": [1, 1, 3]})
+    msg = (
+        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates except for "
+        r"the argument 'self' will be keyword-only"
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.drop_duplicates(["b", "c"])
\ No newline at end of file
diff --git a/pandas/tests/series/methods/test_drop_duplicates.py b/pandas/tests/series/methods/test_drop_duplicates.py
index dae1bbcd86e81..f24c2c4b85b86 100644
--- a/pandas/tests/series/methods/test_drop_duplicates.py
+++ b/pandas/tests/series/methods/test_drop_duplicates.py
@@ -223,3 +223,14 @@ def test_drop_duplicates_categorical_bool(self, ordered):
         return_value = sc.drop_duplicates(keep=False, inplace=True)
         assert return_value is None
         tm.assert_series_equal(sc, tc[~expected])
+
+
+def test_drop_duplicates_pos_args_deprecation():
+    # test deprecation warning message for positional arguments GH#41485
+    s = Series(['a', 'b', 'c', 'b'])
+    msg = (
+        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates except for "
+        r"the argument 'self' will be keyword-only"
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        s.drop_duplicates("last")
\ No newline at end of file

From 8cb7645502693d1d94a9e2571444efe51f1391de Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Sun, 16 May 2021 02:43:19 +0200
Subject: [PATCH 02/11] leave newline

---
 doc/source/whatsnew/v1.3.0.rst                      | 1 +
 pandas/tests/frame/methods/test_drop_duplicates.py  | 2 +-
 pandas/tests/series/methods/test_drop_duplicates.py | 2 +-
 3 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index e34589a0ae5c5..b6849869e9c65 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -648,6 +648,7 @@ Deprecations
 - Deprecated behavior of :meth:`DatetimeIndex.union` with mixed timezones; in a future version both will be cast to UTC instead of object dtype (:issue:`39328`)
 - Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
 - Deprecated passing arguments as positional in :meth:`DataFrame.drop_duplicates` and :meth:`Series.drop_duplicates` (:issue:`41485`)
+
 .. ---------------------------------------------------------------------------
 
 
diff --git a/pandas/tests/frame/methods/test_drop_duplicates.py b/pandas/tests/frame/methods/test_drop_duplicates.py
index 7bfdc8bc308e4..3969a6849fc71 100644
--- a/pandas/tests/frame/methods/test_drop_duplicates.py
+++ b/pandas/tests/frame/methods/test_drop_duplicates.py
@@ -481,4 +481,4 @@ def test_drop_duplicates_pos_args_deprecation():
         r"the argument 'self' will be keyword-only"
     )
     with tm.assert_produces_warning(FutureWarning, match=msg):
-        df.drop_duplicates(["b", "c"])
\ No newline at end of file
+        df.drop_duplicates(["b", "c"])
diff --git a/pandas/tests/series/methods/test_drop_duplicates.py b/pandas/tests/series/methods/test_drop_duplicates.py
index f24c2c4b85b86..7ffa07329a8be 100644
--- a/pandas/tests/series/methods/test_drop_duplicates.py
+++ b/pandas/tests/series/methods/test_drop_duplicates.py
@@ -233,4 +233,4 @@ def test_drop_duplicates_pos_args_deprecation():
         r"the argument 'self' will be keyword-only"
     )
     with tm.assert_produces_warning(FutureWarning, match=msg):
-        s.drop_duplicates("last")
\ No newline at end of file
+        s.drop_duplicates("last")

From fa6574c8426bd082a82f3394e8f0fa2335c6f128 Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Sun, 16 May 2021 14:28:25 +0000
Subject: [PATCH 03/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 pandas/core/frame.py                                | 2 +-
 pandas/tests/frame/methods/test_drop_duplicates.py  | 6 +++---
 pandas/tests/series/methods/test_drop_duplicates.py | 8 ++++----
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 9f602b643731f..eefb3f3d26eee 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -5954,7 +5954,7 @@ def dropna(
         else:
             return result
 
-    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self", "subset"])
     def drop_duplicates(
         self,
         subset: Hashable | Sequence[Hashable] | None = None,
diff --git a/pandas/tests/frame/methods/test_drop_duplicates.py b/pandas/tests/frame/methods/test_drop_duplicates.py
index 3969a6849fc71..29af4c9d3d662 100644
--- a/pandas/tests/frame/methods/test_drop_duplicates.py
+++ b/pandas/tests/frame/methods/test_drop_duplicates.py
@@ -474,11 +474,11 @@ def test_drop_duplicates_non_boolean_ignore_index(arg):
 
 
 def test_drop_duplicates_pos_args_deprecation():
-    # test deprecation warning message for positional arguments GH#41485
+    # GH#41485
     df = DataFrame({"a": [1, 1, 2], "b": [1, 1, 3], "c": [1, 1, 3]})
     msg = (
-        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates except for "
-        r"the argument 'self' will be keyword-only"
+        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates "
+        r"except for the argument 'self' will be keyword-only"
     )
     with tm.assert_produces_warning(FutureWarning, match=msg):
         df.drop_duplicates(["b", "c"])
diff --git a/pandas/tests/series/methods/test_drop_duplicates.py b/pandas/tests/series/methods/test_drop_duplicates.py
index 7ffa07329a8be..b891dff7a5652 100644
--- a/pandas/tests/series/methods/test_drop_duplicates.py
+++ b/pandas/tests/series/methods/test_drop_duplicates.py
@@ -226,11 +226,11 @@ def test_drop_duplicates_categorical_bool(self, ordered):
 
 
 def test_drop_duplicates_pos_args_deprecation():
-    # test deprecation warning message for positional arguments GH#41485
-    s = Series(['a', 'b', 'c', 'b'])
+    # GH#41485
+    s = Series(["a", "b", "c", "b"])
     msg = (
-        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates except for "
-        r"the argument 'self' will be keyword-only"
+        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates "
+        r"except for the argument 'self' will be keyword-only"
     )
     with tm.assert_produces_warning(FutureWarning, match=msg):
         s.drop_duplicates("last")

From d7c341a44f0ede903a0915bbc871b837543358e1 Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Sun, 16 May 2021 14:35:03 +0000
Subject: [PATCH 04/11]  ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 pandas/tests/frame/methods/test_drop_duplicates.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/frame/methods/test_drop_duplicates.py b/pandas/tests/frame/methods/test_drop_duplicates.py
index 29af4c9d3d662..e65240d5acac2 100644
--- a/pandas/tests/frame/methods/test_drop_duplicates.py
+++ b/pandas/tests/frame/methods/test_drop_duplicates.py
@@ -478,7 +478,7 @@ def test_drop_duplicates_pos_args_deprecation():
     df = DataFrame({"a": [1, 1, 2], "b": [1, 1, 3], "c": [1, 1, 3]})
     msg = (
         r"Starting with Pandas version 2\.0 all arguments of drop_duplicates "
-        r"except for the argument 'self' will be keyword-only"
+        r"except for the arguments 'self' and 'subset' will be keyword-only"
     )
     with tm.assert_produces_warning(FutureWarning, match=msg):
-        df.drop_duplicates(["b", "c"])
+        df.drop_duplicates(["b", "c"], "last")

From 19aa58926d7ab45207365f2205508488d59c792d Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Sun, 16 May 2021 15:17:49 +0000
Subject: [PATCH 05/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 doc/source/whatsnew/v1.3.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index b6849869e9c65..b1d644bc8da50 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -647,7 +647,7 @@ Deprecations
 - Deprecated setting :attr:`Categorical._codes`, create a new :class:`Categorical` with the desired codes instead (:issue:`40606`)
 - Deprecated behavior of :meth:`DatetimeIndex.union` with mixed timezones; in a future version both will be cast to UTC instead of object dtype (:issue:`39328`)
 - Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
-- Deprecated passing arguments as positional in :meth:`DataFrame.drop_duplicates` and :meth:`Series.drop_duplicates` (:issue:`41485`)
+- Deprecated passing arguments as positional in :meth:`DataFrame.drop_duplicates` (except for ``subset``) and :meth:`Series.drop_duplicates` (:issue:`41485`)
 
 .. ---------------------------------------------------------------------------
 

From 0d54ca7f00e244cd17a9add0b1540a72a275d1a2 Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Thu, 20 May 2021 18:38:16 +0000
Subject: [PATCH 06/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 pandas/core/frame.py                                |  2 +-
 pandas/core/series.py                               |  2 +-
 pandas/tests/frame/methods/test_drop_duplicates.py  | 13 ++++++++++---
 pandas/tests/series/methods/test_drop_duplicates.py | 12 +++++++++---
 4 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 9b248fe18a88f..79c2f8e3ee60b 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -6004,7 +6004,7 @@ def dropna(
         else:
             return result
 
-    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self", "subset"])
+    @deprecate_nonkeyword_arguments(version=None, allowed_args=["self", "subset"])
     def drop_duplicates(
         self,
         subset: Hashable | Sequence[Hashable] | None = None,
diff --git a/pandas/core/series.py b/pandas/core/series.py
index cf5db51c9b0a6..2f186057efbb5 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -2025,7 +2025,7 @@ def drop_duplicates(self, *, inplace: Literal[True]) -> None:
     def drop_duplicates(self, keep=..., inplace: bool = ...) -> Series | None:
         ...
 
-    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
+    @deprecate_nonkeyword_arguments(version=None, allowed_args=["self"])
     def drop_duplicates(self, keep="first", inplace=False) -> Series | None:
         """
         Return Series with duplicate values removed.
diff --git a/pandas/tests/frame/methods/test_drop_duplicates.py b/pandas/tests/frame/methods/test_drop_duplicates.py
index e65240d5acac2..5430584aaeef2 100644
--- a/pandas/tests/frame/methods/test_drop_duplicates.py
+++ b/pandas/tests/frame/methods/test_drop_duplicates.py
@@ -476,9 +476,16 @@ 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 = (
-        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates "
-        r"except for the arguments 'self' and 'subset' will be keyword-only"
+        "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):
-        df.drop_duplicates(["b", "c"], "last")
+        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)
diff --git a/pandas/tests/series/methods/test_drop_duplicates.py b/pandas/tests/series/methods/test_drop_duplicates.py
index b891dff7a5652..efae8367e9391 100644
--- a/pandas/tests/series/methods/test_drop_duplicates.py
+++ b/pandas/tests/series/methods/test_drop_duplicates.py
@@ -228,9 +228,15 @@ def test_drop_duplicates_categorical_bool(self, ordered):
 def test_drop_duplicates_pos_args_deprecation():
     # GH#41485
     s = Series(["a", "b", "c", "b"])
+
     msg = (
-        r"Starting with Pandas version 2\.0 all arguments of drop_duplicates "
-        r"except for the argument 'self' will be keyword-only"
+        "In a future version of pandas all arguments of "
+        "Series.drop_duplicates will be keyword-only"
     )
+
     with tm.assert_produces_warning(FutureWarning, match=msg):
-        s.drop_duplicates("last")
+        result = s.drop_duplicates("last")
+
+    expected = Series(["a", "c", "b"], index=[0, 2, 3])
+
+    tm.assert_series_equal(expected, result)

From 463c37a576efb298e49082d6530d058dcc764ee6 Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Fri, 21 May 2021 21:06:04 +0000
Subject: [PATCH 07/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 doc/source/whatsnew/v1.3.0.rst                |  2 +-
 pandas/core/indexes/base.py                   |  2 ++
 pandas/tests/indexes/multi/test_duplicates.py | 17 +++++++++++++++++
 pandas/tests/indexes/test_base.py             | 17 +++++++++++++++++
 4 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index daa125f034627..89e86fb8d8843 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -649,7 +649,7 @@ Deprecations
 - Deprecated behavior of :meth:`DatetimeIndex.union` with mixed timezones; in a future version both will be cast to UTC instead of object dtype (:issue:`39328`)
 - Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
 - 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``) and :meth:`Series.drop_duplicates` (: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`)
 
 .. ---------------------------------------------------------------------------
 
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index b9fd18dfdce73..4c26ceff5832b 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -54,6 +54,7 @@
 from pandas.util._decorators import (
     Appender,
     cache_readonly,
+    deprecate_nonkeyword_arguments,
     doc,
 )
 
@@ -2633,6 +2634,7 @@ def unique(self: _IndexT, level: Hashable | None = None) -> _IndexT:
         result = super().unique()
         return self._shallow_copy(result)
 
+    @deprecate_nonkeyword_arguments(version=None, allowed_args=["self"])
     @final
     def drop_duplicates(self: _IndexT, keep: str_t | bool = "first") -> _IndexT:
         """
diff --git a/pandas/tests/indexes/multi/test_duplicates.py b/pandas/tests/indexes/multi/test_duplicates.py
index ea59d55989f8b..7aa4c30ebc219 100644
--- a/pandas/tests/indexes/multi/test_duplicates.py
+++ b/pandas/tests/indexes/multi/test_duplicates.py
@@ -306,3 +306,20 @@ def test_duplicated_drop_duplicates():
     assert duplicated.dtype == bool
     expected = MultiIndex.from_arrays(([2, 3, 2, 3], [1, 1, 2, 2]))
     tm.assert_index_equal(idx.drop_duplicates(keep=False), expected)
+
+
+def test_multi_drop_duplicates_pos_args_deprecation():
+    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"
+    )
+
+    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)
diff --git a/pandas/tests/indexes/test_base.py b/pandas/tests/indexes/test_base.py
index 47657fff56ceb..c73bba075cdc6 100644
--- a/pandas/tests/indexes/test_base.py
+++ b/pandas/tests/indexes/test_base.py
@@ -1738,3 +1738,20 @@ def test_construct_from_memoryview(klass, extra_kwargs):
     result = klass(memoryview(np.arange(2000, 2005)), **extra_kwargs)
     expected = klass(range(2000, 2005), **extra_kwargs)
     tm.assert_index_equal(result, expected)
+
+
+def test_drop_duplicates_pos_args_deprecation():
+    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)

From 2cb482ffac31bc0efdf048c894e2ca3901ffe3b5 Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Sun, 23 May 2021 00:25:47 +0000
Subject: [PATCH 08/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 doc/source/whatsnew/v1.3.0.rst                | 2 +-
 pandas/core/indexes/multi.py                  | 7 +++++++
 pandas/tests/indexes/multi/test_duplicates.py | 4 +++-
 pandas/tests/indexes/test_base.py             | 2 ++
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 38a6d710da6fb..5ad6369c12ecf 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/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`)
 
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index 1a3719233a1da..e60675dcea603 100644
--- a/pandas/core/indexes/multi.py
+++ b/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
 
diff --git a/pandas/tests/indexes/multi/test_duplicates.py b/pandas/tests/indexes/multi/test_duplicates.py
index 7aa4c30ebc219..1a8bbedc7f5c4 100644
--- a/pandas/tests/indexes/multi/test_duplicates.py
+++ b/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):
diff --git a/pandas/tests/indexes/test_base.py b/pandas/tests/indexes/test_base.py
index c73bba075cdc6..3b2ef5d785912 100644
--- a/pandas/tests/indexes/test_base.py
+++ b/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 = (

From 09fe413af8f928f1c5e9f256da4a5d6108e5d7ed Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Sun, 23 May 2021 13:14:24 +0000
Subject: [PATCH 09/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 pandas/core/indexes/base.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 4c26ceff5832b..8aa1bdc99a7a2 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -2635,7 +2635,6 @@ def unique(self: _IndexT, level: Hashable | None = None) -> _IndexT:
         return self._shallow_copy(result)
 
     @deprecate_nonkeyword_arguments(version=None, allowed_args=["self"])
-    @final
     def drop_duplicates(self: _IndexT, keep: str_t | bool = "first") -> _IndexT:
         """
         Return Index with duplicate values removed.

From 03d0330fa69e8ef70f0c910334672b5fe78375f1 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <marcogorelli@protonmail.com>
Date: Sun, 23 May 2021 18:45:47 +0100
Subject: [PATCH 10/11] remove redundant line

---
 pandas/tests/indexes/multi/test_duplicates.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/tests/indexes/multi/test_duplicates.py b/pandas/tests/indexes/multi/test_duplicates.py
index 1a8bbedc7f5c4..676a80ff7495c 100644
--- a/pandas/tests/indexes/multi/test_duplicates.py
+++ b/pandas/tests/indexes/multi/test_duplicates.py
@@ -319,7 +319,6 @@ def test_multi_drop_duplicates_pos_args_deprecation():
     )
 
     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]])

From fbf70a23d21c9782df57b7e40fc3a328d4c88a37 Mon Sep 17 00:00:00 2001
From: jmholzer <jannic.holzer@gmail.com>
Date: Mon, 24 May 2021 20:19:48 +0000
Subject: [PATCH 11/11] ENH: Deprecate non-keyword arguments for
 drop_duplicates.

---
 pandas/core/indexes/multi.py                        | 6 ++----
 pandas/tests/frame/methods/test_drop_duplicates.py  | 4 ----
 pandas/tests/indexes/multi/test_duplicates.py       | 5 -----
 pandas/tests/indexes/test_base.py                   | 5 -----
 pandas/tests/series/methods/test_drop_duplicates.py | 4 ----
 5 files changed, 2 insertions(+), 22 deletions(-)

diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index e60675dcea603..2a03b5696fcc2 100644
--- a/pandas/core/indexes/multi.py
+++ b/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
diff --git a/pandas/tests/frame/methods/test_drop_duplicates.py b/pandas/tests/frame/methods/test_drop_duplicates.py
index 5430584aaeef2..8cbf7bbfe0368 100644
--- a/pandas/tests/frame/methods/test_drop_duplicates.py
+++ b/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)
diff --git a/pandas/tests/indexes/multi/test_duplicates.py b/pandas/tests/indexes/multi/test_duplicates.py
index 1a8bbedc7f5c4..ee0b847dd681f 100644
--- a/pandas/tests/indexes/multi/test_duplicates.py
+++ b/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)
diff --git a/pandas/tests/indexes/test_base.py b/pandas/tests/indexes/test_base.py
index 3b2ef5d785912..f41c79bd09f67 100644
--- a/pandas/tests/indexes/test_base.py
+++ b/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)
diff --git a/pandas/tests/series/methods/test_drop_duplicates.py b/pandas/tests/series/methods/test_drop_duplicates.py
index efae8367e9391..7eb51f8037792 100644
--- a/pandas/tests/series/methods/test_drop_duplicates.py
+++ b/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)