Deprecate non-keyword arguments for drop_duplicates.

doc/source/whatsnew/v1.3.0.rst

@@ -647,6 +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`)
 
 .. ---------------------------------------------------------------------------
 

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,

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.

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"])

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")