DEPR: deprecated nonkeyword arguments for to_string (#54597)

* deprecated nonkeyword arguments

* added test

doc/source/whatsnew/v2.2.0.rst

@@ -95,6 +95,7 @@ Deprecations
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_hdf` except ``path_or_buf``. (:issue:`54229`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_latex` except ``buf``. (:issue:`54229`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_pickle` except ``path``. (:issue:`54229`)
+- Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_string` except ``buf``. (:issue:`54229`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/frame.py

@@ -64,6 +64,7 @@
 from pandas.util._decorators import (
     Appender,
     Substitution,
+    deprecate_nonkeyword_arguments,
     doc,
 )
 from pandas.util._exceptions import find_stack_level
@@ -1229,6 +1230,9 @@ def to_string(
     ) -> None:
         ...
 
+    @deprecate_nonkeyword_arguments(
+        version="3.0", allowed_args=["self", "buf"], name="to_string"
+    )
     @Substitution(
         header_type="bool or list of str",
         header="Write out the column names. If a list of columns "

pandas/tests/io/formats/test_to_string.py

@@ -11,6 +11,7 @@
     option_context,
     to_datetime,
 )
+import pandas._testing as tm
 
 
 def test_repr_embedded_ndarray():
@@ -355,3 +356,15 @@ def test_to_string_string_dtype():
         z     int64[pyarrow]"""
     )
     assert result == expected
+
+
+def test_to_string_pos_args_deprecation():
+    # GH-54229
+    df = DataFrame({"a": [1, 2, 3]})
+    msg = (
+        r"Starting with pandas version 3.0 all arguments of to_string except for the "
+        r"argument 'buf' will be keyword-only."
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        buf = StringIO()
+        df.to_string(buf, None, None, True, True)