From 3adc639def37260144e111c30964fb4541fa7640 Mon Sep 17 00:00:00 2001
From: fei <204683769+feiiiiii5@users.noreply.github.com>
Date: Mon, 27 Jul 2026 19:54:05 +0800
Subject: [PATCH] fix(decorators): guard _get_fn_argnames against empty
 positional args (closes #2422)

_get_fn_argnames() accessed arg_spec_args[0] without checking if the
list was empty. Functions with only keyword-only args (def f(*, x)),
variadic args (def f(*args)), or keyword-variadic args (def f(**kwargs))
have an empty args list from inspect.getfullargspec, causing IndexError.

Fix: add an early return when arg_spec_args is empty, before the [0]
access. This is safe because there are no self/cls args to exclude.

Impact: any pandera decorator (@check_input, @check_output, @check_io,
@check_types) applied to a function with only keyword-only parameters
would crash during argument introspection.

Regression tests cover:
- keyword-only args (exact repro from #2422)
- variadic args only (*args)
- kwargs only (**kwargs)
- no args at all
- Normal cases still work (regular function, method excludes self,
  mixed positional + keyword-only)

Signed-off-by: fei <204683769+feiiiiii5@users.noreply.github.com>
---
 pandera/decorators.py                      |  5 ++
 tests/pandas/test_get_fn_argnames_empty.py | 85 ++++++++++++++++++++++
 2 files changed, 90 insertions(+)
 create mode 100644 tests/pandas/test_get_fn_argnames_empty.py

diff --git a/pandera/decorators.py b/pandera/decorators.py
index e6af8c1fc..bdf3a7511 100644
--- a/pandera/decorators.py
+++ b/pandera/decorators.py
@@ -70,6 +70,11 @@ def _get_fn_argnames(fn: Callable) -> list[str]:
 
     arg_spec_args = inspect.getfullargspec(fn).args
 
+    # Functions with only keyword-only args, *args, or **kwargs have
+    # no positional args → nothing to introspect for self/cls.
+    if not arg_spec_args:
+        return arg_spec_args
+
     first_arg_is_self = arg_spec_args[0] == "self"
     first_arg_is_cls = arg_spec_args[0] == "cls"
     is_py_newer_than_39 = sys.version_info[:2] >= (3, 9)
diff --git a/tests/pandas/test_get_fn_argnames_empty.py b/tests/pandas/test_get_fn_argnames_empty.py
new file mode 100644
index 000000000..3265d8fb7
--- /dev/null
+++ b/tests/pandas/test_get_fn_argnames_empty.py
@@ -0,0 +1,85 @@
+"""
+Regression tests for _get_fn_argnames IndexError on keyword-only functions
+(GitHub issue #2422).
+
+_get_fn_argnames() in pandera/decorators.py accessed arg_spec_args[0]
+without checking if the list was empty.  Functions with only keyword-only
+args (``def f(*, x)``), variadic args (``def f(*args)``), or keyword-
+variadic args (``def f(**kwargs)``) have an empty ``args`` list from
+``inspect.getfullargspec``, causing IndexError.
+
+The fix adds an early return when arg_spec_args is empty.
+"""
+
+import pytest
+from pandera.decorators import _get_fn_argnames
+
+
+class TestGetFnArgnamesEmptyArgs:
+    """Functions with no positional args must not crash."""
+
+    def test_keyword_only_args(self):
+        """def f(*, df, output_col) → returns [] without IndexError."""
+
+        def process(*, df, output_col):
+            pass
+
+        result = _get_fn_argnames(process)
+        assert result == []
+
+    def test_variadic_args_only(self):
+        """def f(*args) → returns [] without IndexError."""
+
+        def process(*args):
+            pass
+
+        result = _get_fn_argnames(process)
+        assert result == []
+
+    def test_kwargs_only(self):
+        """def f(**kwargs) → returns [] without IndexError."""
+
+        def process(**kwargs):
+            pass
+
+        result = _get_fn_argnames(process)
+        assert result == []
+
+    def test_no_args_at_all(self):
+        """def f() → returns [] without IndexError."""
+
+        def process():
+            pass
+
+        result = _get_fn_argnames(process)
+        assert result == []
+
+
+class TestGetFnArgnamesNormalCases:
+    """Normal functions must still work correctly."""
+
+    def test_regular_function(self):
+        def process(df, output_col):
+            pass
+
+        result = _get_fn_argnames(process)
+        assert result == ["df", "output_col"]
+
+    def test_method_excludes_self(self):
+        class MyClass:
+            def process(self, df, output_col):
+                pass
+
+        obj = MyClass()
+        result = _get_fn_argnames(obj.process)
+        assert result == ["df", "output_col"]
+        assert "self" not in result
+
+    def test_mixed_positional_and_keyword_only(self):
+        """def f(a, b, *, c) → returns ['a', 'b'] (keyword-only excluded)."""
+
+        def process(a, b, *, c):
+            pass
+
+        result = _get_fn_argnames(process)
+        assert result == ["a", "b"]