From e426904e089024aadf82ca9410db648351ff9dca Mon Sep 17 00:00:00 2001
From: Mike Ma <mike.ma@lmctl.com>
Date: Wed, 17 Jun 2026 13:06:52 -0500
Subject: [PATCH 1/2] Improve parametrize error for scalar parameter sets

Tuple-style parametrization expects each parameter set to be a sequence of values. When a scalar like None was passed, pytest crashed while trying to format the validation error because it called len() on the scalar.

Report a normal collection error instead, pointing at the offending test.

Tested: PYTHONPATH=src python3 -m pytest testing/test_mark.py::test_parametrized_collect_with_non_sequence_values testing/test_mark.py::test_parametrized_collect_with_wrong_args -q

Tested: PYTHONPATH=src python3 -m pytest testing/test_mark.py -q

Tested: git diff --check -- src/_pytest/mark/structures.py testing/test_mark.py changelog/14619.bugfix.rst AUTHORS

Tested: python3 -m py_compile src/_pytest/mark/structures.py testing/test_mark.py

Not-tested: testing/python/metafunc.py because hypothesis is not installed in this environment.

Co-authored-by: OpenAI Codex <codex@openai.com>
---
 AUTHORS                        |  1 +
 changelog/14619.bugfix.rst     |  1 +
 src/_pytest/mark/structures.py | 18 ++++++++++++++++--
 testing/test_mark.py           | 22 ++++++++++++++++++++++
 4 files changed, 40 insertions(+), 2 deletions(-)
 create mode 100644 changelog/14619.bugfix.rst

diff --git a/AUTHORS b/AUTHORS
index 972f39aa45e..94a70b16a2e 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -333,6 +333,7 @@ Mihail Milushev
 Mike Fiedler (miketheman)
 Mike Hoyle (hoylemd)
 Mike Lundy
+Mike Ma
 Milan Lesnek
 minbang930
 Miro Hrončok
diff --git a/changelog/14619.bugfix.rst b/changelog/14619.bugfix.rst
new file mode 100644
index 00000000000..6208677f843
--- /dev/null
+++ b/changelog/14619.bugfix.rst
@@ -0,0 +1 @@
+Fixed a crash in :func:`pytest.mark.parametrize` when a non-sequence value is passed for a parameter set. It now provides a clear error message during collection.
diff --git a/src/_pytest/mark/structures.py b/src/_pytest/mark/structures.py
index 5449b17a1c6..875d43d59cf 100644
--- a/src/_pytest/mark/structures.py
+++ b/src/_pytest/mark/structures.py
@@ -214,7 +214,21 @@ def _for_parametrize(
         if parameters:
             # Check all parameter sets have the correct number of values.
             for param in parameters:
-                if len(param.values) != len(argnames):
+                try:
+                    values_len = len(param.values)
+                except TypeError:
+                    values_len = None
+                if values_len is None:
+                    fail(
+                        '{nodeid}: in "parametrize" expected a sequence of values, got {values_type}:\n'
+                        "  {values}".format(
+                            nodeid=nodeid,
+                            values=param.values,
+                            values_type=type(param.values).__name__,
+                        ),
+                        pytrace=False,
+                    )
+                if values_len != len(argnames):
                     msg = (
                         '{nodeid}: in "parametrize" the number of names ({names_len}):\n'
                         "  {names}\n"
@@ -227,7 +241,7 @@ def _for_parametrize(
                             values=param.values,
                             names=argnames,
                             names_len=len(argnames),
-                            values_len=len(param.values),
+                            values_len=values_len,
                         ),
                         pytrace=False,
                     )
diff --git a/testing/test_mark.py b/testing/test_mark.py
index 253cda94503..5c8b9f9f967 100644
--- a/testing/test_mark.py
+++ b/testing/test_mark.py
@@ -495,6 +495,28 @@ def test_func(foo, bar):
     )
 
 
+def test_parametrized_collect_with_non_sequence_values(pytester: Pytester) -> None:
+    """Test collect parametrized func with tuple-style argnames and scalar values."""
+    py_file = pytester.makepyfile(
+        """
+        import pytest
+
+        @pytest.mark.parametrize("x,", [None])
+        def test_func(x):
+            pass
+    """
+    )
+
+    result = pytester.runpytest(py_file)
+    result.stdout.fnmatch_lines(
+        [
+            "test_parametrized_collect_with_non_sequence_values.py::test_func: "
+            'in "parametrize" expected a sequence of values, got NoneType:',
+            "  None",
+        ]
+    )
+
+
 def test_parametrized_with_kwargs(pytester: Pytester) -> None:
     """Test collect parametrized func with wrong number of args."""
     py_file = pytester.makepyfile(

From bc13fe923e5c8fc5e85434ff2973f2c239b37def Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 17 Jun 2026 18:07:59 +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 | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/src/_pytest/mark/structures.py b/src/_pytest/mark/structures.py
index 875d43d59cf..546e258139b 100644
--- a/src/_pytest/mark/structures.py
+++ b/src/_pytest/mark/structures.py
@@ -220,12 +220,8 @@ def _for_parametrize(
                     values_len = None
                 if values_len is None:
                     fail(
-                        '{nodeid}: in "parametrize" expected a sequence of values, got {values_type}:\n'
-                        "  {values}".format(
-                            nodeid=nodeid,
-                            values=param.values,
-                            values_type=type(param.values).__name__,
-                        ),
+                        f'{nodeid}: in "parametrize" expected a sequence of values, got {type(param.values).__name__}:\n'
+                        f"  {param.values}",
                         pytrace=False,
                     )
                 if values_len != len(argnames):