From f56e0df969debaea7bd860b89945b03e27116e94 Mon Sep 17 00:00:00 2001
From: Muhamed Fazal PS <muhamedfazalps@gmail.com>
Date: Thu, 16 Jul 2026 19:46:03 +0530
Subject: [PATCH 1/2] fix: improve error message when parametrize values is not
 iterable

When a trailing comma in argnames causes a scalar value to be parsed
as a single-element tuple, TypeError is raised on len(None). Catch
this and show a helpful error message.

Part of #14619
---
 src/_pytest/mark/structures.py | 49 +++++++++++++++++++++-------------
 1 file changed, 30 insertions(+), 19 deletions(-)

diff --git a/src/_pytest/mark/structures.py b/src/_pytest/mark/structures.py
index 5449b17a1c6..958beed868d 100644
--- a/src/_pytest/mark/structures.py
+++ b/src/_pytest/mark/structures.py
@@ -211,26 +211,37 @@ def _for_parametrize(
         parameters = cls._parse_parametrize_parameters(argvalues, force_tuple)
         del argvalues
 
-        if parameters:
-            # Check all parameter sets have the correct number of values.
-            for param in parameters:
-                if len(param.values) != len(argnames):
-                    msg = (
-                        '{nodeid}: in "parametrize" the number of names ({names_len}):\n'
-                        "  {names}\n"
-                        "must be equal to the number of values ({values_len}):\n"
-                        "  {values}"
-                    )
-                    fail(
-                        msg.format(
-                            nodeid=nodeid,
-                            values=param.values,
-                            names=argnames,
-                            names_len=len(argnames),
-                            values_len=len(param.values),
-                        ),
-                        pytrace=False,
+        for param in parameters:
+            try:
+                values_len = len(param.values)
+            except TypeError:
+                fail(
+                    "{nodeid}: in \"parametrize\" cannot determine the number of "
+                    "values: {values!r} is not iterable. This can happen when "
+                    "a trailing comma in the argnames string causes a scalar "
+                    "value to be parsed as a single-element tuple.".format(
+                        nodeid=nodeid,
+                        values=param.values,
                     )
+                )
+                return  # never reached, but appease type checkers
+            if values_len != len(argnames):
+                msg = (
+                    '{nodeid}: in "parametrize" the number of names ({names_len}):\n'
+                    "  {names}\n"
+                    "must be equal to the number of values ({values_len}):\n"
+                    "  {values}"
+                )
+                fail(
+                    msg.format(
+                        nodeid=nodeid,
+                        values=param.values,
+                        names=argnames,
+                        names_len=len(argnames),
+                        values_len=len(param.values),
+                    ),
+                    pytrace=False,
+                )
         else:
             # Empty parameter set (likely computed at runtime): create a single
             # parameter set with NOTSET values, with the "empty parameter set" mark applied to it.

From 93201c9fcce25299e034fc5b0310ffb35e9a63b6 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Thu, 16 Jul 2026 14:16:34 +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 | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/src/_pytest/mark/structures.py b/src/_pytest/mark/structures.py
index 958beed868d..599d252656b 100644
--- a/src/_pytest/mark/structures.py
+++ b/src/_pytest/mark/structures.py
@@ -216,13 +216,10 @@ def _for_parametrize(
                 values_len = len(param.values)
             except TypeError:
                 fail(
-                    "{nodeid}: in \"parametrize\" cannot determine the number of "
-                    "values: {values!r} is not iterable. This can happen when "
+                    f'{nodeid}: in "parametrize" cannot determine the number of '
+                    f"values: {param.values!r} is not iterable. This can happen when "
                     "a trailing comma in the argnames string causes a scalar "
-                    "value to be parsed as a single-element tuple.".format(
-                        nodeid=nodeid,
-                        values=param.values,
-                    )
+                    "value to be parsed as a single-element tuple."
                 )
                 return  # never reached, but appease type checkers
             if values_len != len(argnames):