Improve parametrize error for scalar parameter sets

AUTHORS

@@ -333,6 +333,7 @@ Mihail Milushev
 Mike Fiedler (miketheman)
 Mike Hoyle (hoylemd)
 Mike Lundy
+Mike Ma
 Milan Lesnek
 minbang930
 Miro Hrončok

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.

src/_pytest/mark/structures.py

@@ -214,7 +214,17 @@ 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(
+                        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):
                     msg = (
                         '{nodeid}: in "parametrize" the number of names ({names_len}):\n'
                         "  {names}\n"
@@ -227,7 +237,7 @@ def _for_parametrize(
                             values=param.values,
                             names=argnames,
                             names_len=len(argnames),
-                            values_len=len(param.values),
+                            values_len=values_len,
                         ),
                         pytrace=False,
                     )

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(