deprecate nonkeyword args for bfill

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

pandas/core/generic.py

@@ -6490,6 +6490,7 @@ def bfill(
     ) -> FrameOrSeries | None:
         ...
 
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
     @final
     @doc(klass=_shared_doc_kwargs["klass"])
     def bfill(

pandas/tests/frame/methods/test_fillna.py

@@ -334,6 +334,16 @@ def test_bfill(self, datetime_frame):
             datetime_frame.bfill(), datetime_frame.fillna(method="bfill")
         )
 
+    def test_bfill_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 bfill except "
+            r"for the argument 'self' will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            df.bfill(0)
+
     def test_frame_pad_backfill_limit(self):
         index = np.arange(10)
         df = DataFrame(np.random.randn(10, 4), index=index)

pandas/tests/series/methods/test_fillna.py

@@ -776,6 +776,16 @@ def test_bfill(self):
         ts[2] = np.NaN
         tm.assert_series_equal(ts.bfill(), ts.fillna(method="bfill"))
 
+    def test_bfill_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 bfill except "
+            r"for the argument 'self' will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            ser.bfill(0)
+
     def test_pad_nan(self):
         x = Series(
             [np.nan, 1.0, np.nan, 3.0, np.nan], ["z", "a", "b", "c", "d"], dtype=float