ENH: Deprecate non-keyword arguments for drop_duplicates.

Author: jmholzer

doc/source/whatsnew/v1.3.0.rst

@@ -676,11 +676,9 @@ 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`)
-<<<<<<< HEAD
 - 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`)
-- 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`)
 
 .. ---------------------------------------------------------------------------
 

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:
         """

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)

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)