deprecate passing args as positional in dropna

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.dropna` and :meth:`Series.dropna` (:issue:`41485`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/frame.py

@@ -77,6 +77,7 @@
     Appender,
     Substitution,
     deprecate_kwarg,
+    deprecate_nonkeyword_arguments,
     doc,
     rewrite_axis_style_signature,
 )
@@ -5804,6 +5805,7 @@ def notna(self) -> DataFrame:
     def notnull(self) -> DataFrame:
         return ~self.isna()
 
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
     def dropna(
         self,
         axis: Axis = 0,

pandas/core/series.py

@@ -51,6 +51,7 @@
 from pandas.util._decorators import (
     Appender,
     Substitution,
+    deprecate_nonkeyword_arguments,
     doc,
 )
 from pandas.util._validators import (
@@ -5059,6 +5060,7 @@ def notna(self) -> Series:
     def notnull(self) -> Series:
         return super().notnull()
 
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
     def dropna(self, axis=0, inplace=False, how=None):
         """
         Return a new Series with missing values removed.

pandas/tests/frame/methods/test_dropna.py

@@ -231,3 +231,13 @@ def test_dropna_with_duplicate_columns(self):
 
         result = df.dropna(subset=["A", "C"], how="all")
         tm.assert_frame_equal(result, expected)
+
+    def test_dropna_pos_args_deprecation(self):
+        # https://github.com/pandas-dev/pandas/issues/41485
+        df = DataFrame({"a": [1, 2, 3]})
+        msg = (
+            r"Starting with Pandas version 2\.0 all arguments of dropna except for the "
+            r"argument 'self' will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            df.dropna(1)

pandas/tests/series/methods/test_dropna.py

@@ -101,3 +101,13 @@ def test_datetime64_tz_dropna(self):
         )
         assert result.dtype == "datetime64[ns, Asia/Tokyo]"
         tm.assert_series_equal(result, expected)
+
+    def test_dropna_pos_args_deprecation(self):
+        # https://github.com/pandas-dev/pandas/issues/41485
+        ser = Series([1, 2, 3])
+        msg = (
+            r"Starting with Pandas version 2\.0 all arguments of dropna except for the "
+            r"argument 'self' will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            ser.dropna(0)