Spurious comma in `parametrize` causes obfuscated crash

[crusaderky]

This is a regression in pytest 9.1.

@pytest.mark.parametrize("x,", [None, "foo", "bar"])  # Spurious comma after parameter name
def test1(x):
    pass

output:

________________________________________________________________ ERROR collecting test.py ________________________________________________________________
.pixi/envs/py314/lib/python3.14/site-packages/pluggy/_hooks.py:512: in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.pixi/envs/py314/lib/python3.14/site-packages/pluggy/_manager.py:120: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.pixi/envs/py314/lib/python3.14/site-packages/_pytest/python.py:250: in pytest_pycollect_makeitem
    return list(collector._genfunctions(name, obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.pixi/envs/py314/lib/python3.14/site-packages/_pytest/python.py:476: in _genfunctions
    self.ihook.pytest_generate_tests.call_extra(methods, dict(metafunc=metafunc))
.pixi/envs/py314/lib/python3.14/site-packages/pluggy/_hooks.py:573: in call_extra
    return self._hookexec(self.name, hookimpls, kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.pixi/envs/py314/lib/python3.14/site-packages/pluggy/_manager.py:120: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.pixi/envs/py314/lib/python3.14/site-packages/_pytest/python.py:124: in pytest_generate_tests
    metafunc.parametrize(*marker.args, **marker.kwargs, _param_mark=marker)
.pixi/envs/py314/lib/python3.14/site-packages/_pytest/python.py:1316: in parametrize
    argnames, parametersets = ParameterSet._for_parametrize(
.pixi/envs/py314/lib/python3.14/site-packages/_pytest/mark/structures.py:218: in _for_parametrize
    if len(param.values) != len(argnames):
       ^^^^^^^^^^^^^^^^^
E   TypeError: object of type 'NoneType' has no len()

As there is no hint as to where the offending test is, the only way to find it is to bisect the whole module.

If the first value is a string, pytest cleanly points at the offending test, but IMHO the error message is improvable as it could better clarify that it's treating the first string as a sequence:

@pytest.mark.parametrize("x,", ["foo", "bar"])
def test1(x):
    pass

output:

test.py::test1: in "parametrize" the number of names (1):
  ['x']
must be equal to the number of values (3):
  foo

suggested output:

test.py::test1: in "parametrize" the number of names (1):
  ['x']
must be equal to the number of values (3):
  ['f', 'o', 'o']

[bluetech]

Probably the trailing comma difference is due to 3fc824a.

However, the bad error message happens also on 9.0 with a slightly different argnames e.g. x,y.

I agree we should improve the error message.