From 1a98029feff60d82c0ac467a0e78738821c5c8c9 Mon Sep 17 00:00:00 2001
From: Abdulhamid Rustamov <rustamovabdulhamid976@gmail.com>
Date: Tue, 2 Jun 2026 08:47:57 +0500
Subject: [PATCH 1/2] fix: prevent NOTSET sentinel leaking as test ID in
 parametrize with callable ids

When @pytest.mark.parametrize is used with an empty argvalues list and a
callable ids parameter, pytest internally creates a placeholder ParameterSet
with NOTSET sentinel values. Previously this placeholder used id='NOTSET',
which caused the internal string to leak into the test output as 'test_foo[NOTSET]'.
Additionally, if an ids callable was provided, it would be invoked with the
NOTSET sentinel and crash with an AttributeError or similar.

Fix:
- Use id=None for the placeholder ParameterSet so the standard ID generation
  runs (produces e.g. 'test_foo[x0]' matching the existing workaround behavior)
- Guard _idval_from_function to return None early when val is NOTSET,
  preventing the user's callable from being invoked with the sentinel

Fixes #13235
---
 changelog/13235.bugfix.rst     |  5 +++++
 src/_pytest/mark/structures.py |  2 +-
 src/_pytest/python.py          |  2 ++
 testing/python/metafunc.py     | 15 +++++++++++++++
 4 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 changelog/13235.bugfix.rst

diff --git a/changelog/13235.bugfix.rst b/changelog/13235.bugfix.rst
new file mode 100644
index 00000000000..88ef80a8e2d
--- /dev/null
+++ b/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]``).
diff --git a/src/_pytest/mark/structures.py b/src/_pytest/mark/structures.py
index 35fd24f8f96..f57d4211c45 100644
--- a/src/_pytest/mark/structures.py
+++ b/src/_pytest/mark/structures.py
@@ -238,7 +238,7 @@ def _for_parametrize(
             mark = get_empty_parameterset_mark(config, argnames, func)
             parameters.append(
                 ParameterSet(
-                    values=(NOTSET,) * len(argnames), marks=[mark], id="NOTSET"
+                    values=(NOTSET,) * len(argnames), marks=[mark], id=None
                 )
             )
         return argnames, parameters
diff --git a/src/_pytest/python.py b/src/_pytest/python.py
index ad5a2c6a59b..b8e152a2131 100644
--- a/src/_pytest/python.py
+++ b/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:
diff --git a/testing/python/metafunc.py b/testing/python/metafunc.py
index 026589d65f5..fa91e433d39 100644
--- a/testing/python/metafunc.py
+++ b/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(

From 3945b98f8fafdc6dd13a7387973fc96332fa6be2 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 2 Jun 2026 03:48:50 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 src/_pytest/mark/structures.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/_pytest/mark/structures.py b/src/_pytest/mark/structures.py
index f57d4211c45..6a8f3cd84ed 100644
--- a/src/_pytest/mark/structures.py
+++ b/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=None
-                )
+                ParameterSet(values=(NOTSET,) * len(argnames), marks=[mark], id=None)
             )
         return argnames, parameters