DEPR: Positional arguments in Series.to_string (pandas-dev#57375)

* Adding deprecation, documentation, and unit test.

* Update series.py

* Updating unit tests.

* Updating implementation based on reviewer feedback.

* Updating implementation based on reviewer feedback.

* Updating implementation based on reviewer feedback.

* Updating implementation based on reviewer feedback.

* Updating implementation based on reviewer feedback.

doc/source/whatsnew/v3.0.0.rst

@@ -102,6 +102,7 @@ Deprecations
 ~~~~~~~~~~~~
 - Deprecated :meth:`Timestamp.utcfromtimestamp`, use ``Timestamp.fromtimestamp(ts, "UTC")`` instead (:issue:`56680`)
 - Deprecated :meth:`Timestamp.utcnow`, use ``Timestamp.now("UTC")`` instead (:issue:`56680`)
+- Deprecated allowing non-keyword arguments in :meth:`Series.to_string` except ``buf``. (:issue:`57280`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/series.py

@@ -46,6 +46,7 @@
 from pandas.util._decorators import (
     Appender,
     Substitution,
+    deprecate_nonkeyword_arguments,
     doc,
 )
 from pandas.util._exceptions import find_stack_level
@@ -1488,6 +1489,7 @@ def __repr__(self) -> str:
     def to_string(
         self,
         buf: None = ...,
+        *,
         na_rep: str = ...,
         float_format: str | None = ...,
         header: bool = ...,
@@ -1504,6 +1506,7 @@ def to_string(
     def to_string(
         self,
         buf: FilePath | WriteBuffer[str],
+        *,
         na_rep: str = ...,
         float_format: str | None = ...,
         header: bool = ...,
@@ -1516,6 +1519,9 @@ def to_string(
     ) -> None:
         ...
 
+    @deprecate_nonkeyword_arguments(
+        version="3.0.0", allowed_args=["self", "buf"], name="to_string"
+    )
     def to_string(
         self,
         buf: FilePath | WriteBuffer[str] | None = None,

pandas/tests/io/formats/test_to_string.py

@@ -35,6 +35,16 @@ def _three_digit_exp():
 
 
 class TestDataFrameToStringFormatters:
+    def test_keyword_deprecation(self):
+        # GH 57280
+        msg = (
+            "Starting with pandas version 3.0.0 all arguments of to_string "
+            "except for the argument 'buf' will be keyword-only."
+        )
+        s = Series(["a", "b"])
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            s.to_string(None, "NaN")
+
     def test_to_string_masked_ea_with_formatter(self):
         # GH#39336
         df = DataFrame(