deprecate default args as positional in set_index

Author: MarcoGorelli

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 (apart from ``keys``) as positional in :meth:`DataFrame.set_index` (:issue:`41485`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/frame.py

@@ -77,6 +77,7 @@
     Appender,
     Substitution,
     deprecate_kwarg,
+    deprecate_nonkeyword_arguments,
     doc,
     rewrite_axis_style_signature,
 )
@@ -5301,6 +5302,7 @@ def shift(
             periods=periods, freq=freq, axis=axis, fill_value=fill_value
         )
 
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self", "keys"])
     def set_index(
         self,
         keys,

pandas/tests/frame/methods/test_set_index.py

@@ -705,3 +705,13 @@ def test_set_index_periodindex(self):
         tm.assert_index_equal(df.index, idx1)
         df = df.set_index(idx2)
         tm.assert_index_equal(df.index, idx2)
+
+    def test_drop_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 set_index except for "
+            r"the arguments 'self' and 'keys' will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            df.set_index("a", True)