fix(decorators): guard _get_fn_argnames against empty positional args (closes #2422)

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)

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"]