Improve parametrize error for invalid tuple-style values

AUTHORS

@@ -207,6 +207,7 @@ Hugo van Kemenade
 Hui Wang (coldnight)
 Ian Bicking
 Ian Lesperance
+Ilia Sorokin
 Ilya Abdolmanafi
 Ilya Konstantinov
 Ionuț Turturică

changelog/14619.bugfix.rst

@@ -0,0 +1 @@
+Improved ``parametrize`` collection errors for tuple-style parameter names with invalid scalar values.

src/_pytest/mark/structures.py

@@ -213,8 +213,28 @@ 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):
+            for i, param in enumerate(parameters):
+                try:
+                    values_len = len(param.values)
+                except TypeError:
+                    values_len = None
+
+                if values_len is None:
+                    msg = (
+                        '{nodeid}: in "parametrize" the parameter set at index {index} '
+                        "needs to be a sequence, but got {values!r}"
+                    )
+                    fail(
+                        msg.format(nodeid=nodeid, index=i, values=param.values),
+                        pytrace=False,
+                    )
+
+                if values_len != len(argnames):
+                    values = (
+                        list(param.values)
+                        if isinstance(param.values, str)
+                        else param.values
+                    )
                     msg = (
                         '{nodeid}: in "parametrize" the number of names ({names_len}):\n'
                         "  {names}\n"
@@ -224,10 +244,10 @@ def _for_parametrize(
                     fail(
                         msg.format(
                             nodeid=nodeid,
-                            values=param.values,
+                            values=values,
                             names=argnames,
                             names_len=len(argnames),
-                            values_len=len(param.values),
+                            values_len=values_len,
                         ),
                         pytrace=False,
                     )

testing/python/metafunc.py

@@ -119,6 +119,36 @@ def func(arg):
         assert metafunc._calls[0].params == {"arg": "a"}
         assert metafunc._calls[1].params == {"arg": "b"}
 
+    def test_parametrize_trailing_comma_scalar_parameter_set(self) -> None:
+        def func(arg):
+            pass  # pragma: no cover
+
+        metafunc = self.Metafunc(func)
+
+        with pytest.raises(fail.Exception) as excinfo:
+            metafunc.parametrize("arg,", [None])
+
+        assert str(excinfo.value) == (
+            'mock::nodeid: in "parametrize" the parameter set at index 0 '
+            "needs to be a sequence, but got None"
+        )
+
+    def test_parametrize_string_parameter_set_error_shows_sequence(self) -> None:
+        def func(arg):
+            pass  # pragma: no cover
+
+        metafunc = self.Metafunc(func)
+
+        with pytest.raises(fail.Exception) as excinfo:
+            metafunc.parametrize("arg,", ["foo"])
+
+        assert str(excinfo.value) == (
+            'mock::nodeid: in "parametrize" the number of names (1):\n'
+            "  ['arg']\n"
+            "must be equal to the number of values (3):\n"
+            "  ['f', 'o', 'o']"
+        )
+
     def test_parametrize_error(self) -> None:
         def func(x, y):
             pass