Deprecate non-keyword arguments for rsplit

doc/source/whatsnew/v1.5.0.rst

@@ -705,6 +705,7 @@ Other Deprecations
 - Deprecated the ``closed`` argument in :meth:`interval_range` in favor of ``inclusive`` argument; In a future version passing ``closed`` will raise (:issue:`40245`)
 - Deprecated the methods :meth:`DataFrame.mad`, :meth:`Series.mad`, and the corresponding groupby methods (:issue:`11787`)
 - Deprecated positional arguments to :meth:`Index.join` except for ``other``, use keyword-only arguments instead of positional arguments (:issue:`46518`)
+- Deprecated positional arguments to :meth:`StringMethods.rsplit` and :meth:`StringMethods.split` except for ``pat``, use keyword-only arguments instead of positional arguments (:issue:`47423`)
 - Deprecated indexing on a timezone-naive :class:`DatetimeIndex` using a string representing a timezone-aware datetime (:issue:`46903`, :issue:`36148`)
 - Deprecated the ``closed`` argument in :class:`Interval` in favor of ``inclusive`` argument; In a future version passing ``closed`` will raise (:issue:`40245`)
 - Deprecated the ``closed`` argument in :class:`IntervalIndex` in favor of ``inclusive`` argument; In a future version passing ``closed`` will raise (:issue:`40245`)

pandas/core/strings/accessor.py

@@ -18,7 +18,10 @@
     DtypeObj,
     F,
 )
-from pandas.util._decorators import Appender
+from pandas.util._decorators import (
+    Appender,
+    deprecate_nonkeyword_arguments,
+)
 from pandas.util._exceptions import find_stack_level
 
 from pandas.core.dtypes.common import (
@@ -843,6 +846,7 @@ def cat(
     """,
         }
     )
+    @deprecate_nonkeyword_arguments(version=None, allowed_args=["self", "pat"])
     @forbid_nonstring_types(["bytes"])
     def split(
         self,
@@ -874,6 +878,7 @@ def split(
             "regex_examples": "",
         }
     )
+    @deprecate_nonkeyword_arguments(version=None, allowed_args=["self", "pat"])
     @forbid_nonstring_types(["bytes"])
     def rsplit(self, pat=None, n=-1, expand=False):
         result = self._data.array._str_rsplit(pat, n=n)

pandas/tests/strings/test_split_partition.py

@@ -130,6 +130,23 @@ def test_rsplit_max_number(any_string_dtype):
     tm.assert_series_equal(result, exp)
 
 
+@pytest.mark.parametrize("method", ["split", "rsplit"])
+def test_posargs_deprecation(method):
+    # GH 47423; Deprecate passing n as positional.
+    s = Series(["foo,bar,lorep"])
+
+    msg = (
+        f"In a future version of pandas all arguments of StringMethods.{method} "
+        "except for the argument 'pat' will be keyword-only"
+    )
+
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        result = getattr(s.str, method)(",", 3)
+
+    expected = Series([["foo", "bar", "lorep"]])
+    tm.assert_series_equal(result, expected)
+
+
 def test_split_blank_string(any_string_dtype):
     # expand blank split GH 20067
     values = Series([""], name="test", dtype=any_string_dtype)