fix: prevent NOTSET sentinel leaking as test ID in parametrize with callable ids

changelog/13235.bugfix.rst

@@ -0,0 +1,5 @@
+Fixed :func:`pytest.mark.parametrize` with an empty ``argvalues`` list and a
+callable ``ids`` parameter leaking the internal ``NOTSET`` string as the test
+ID (e.g. ``test_foo[NOTSET]``). The callable is no longer invoked for the
+placeholder parameterset, and the generated ID now follows the standard
+auto-naming convention (e.g. ``test_foo[x0]``).

src/_pytest/mark/structures.py

@@ -237,9 +237,7 @@ def _for_parametrize(
             # parameter set with NOTSET values, with the "empty parameter set" mark applied to it.
             mark = get_empty_parameterset_mark(config, argnames, func)
             parameters.append(
-                ParameterSet(
-                    values=(NOTSET,) * len(argnames), marks=[mark], id="NOTSET"
-                )
+                ParameterSet(values=(NOTSET,) * len(argnames), marks=[mark], id=None)
             )
         return argnames, parameters
 

src/_pytest/python.py

@@ -1013,6 +1013,8 @@ def _idval_from_function(self, val: object, argname: str, idx: int) -> str | Non
         user-provided id callable, if given."""
         if self.idfn is None:
             return None
+        if val is NOTSET:
+            return None
         try:
             id = self.idfn(val)
         except Exception as e:

testing/python/metafunc.py

@@ -1475,6 +1475,21 @@ def test_temp(temp):
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(["* 1 skipped *"])
 
+    def test_parametrize_empty_argvalues_callable_ids(self, pytester: Pytester) -> None:
+        """Callable ids with empty argvalues must not crash or leak 'NOTSET' (#13235)."""
+        pytester.makepyfile(
+            """
+            import pytest
+
+            @pytest.mark.parametrize("x", [], ids=lambda x: x.id)
+            def test_foo(x):
+                pass
+        """
+        )
+        result = pytester.runpytest("-v")
+        result.stdout.no_fnmatch_line("*NOTSET*")
+        result.stdout.fnmatch_lines(["* 1 skipped *"])
+
     def test_parametrized_ids_invalid_type(self, pytester: Pytester) -> None:
         """Test error with non-strings/non-ints, without generator (#1857)."""
         pytester.makepyfile(