ENH: Deprecate non-keyword arguments for drop_duplicates.

Author: jmholzer

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,

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.

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)

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)