Error using parametrize with default arguments

[ndevenish]

With pytest 3.4.0:

import pytest

@pytest.mark.parametrize("something", [True, False])
def test_something(something=False):
  pass

You get the error:

../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/pluggy/__init__.py:617: in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/pluggy/__init__.py:222: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/pluggy/__init__.py:216: in <lambda>
    firstresult=hook.spec_opts.get('firstresult'),
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/_pytest/python.py:200: in pytest_pycollect_makeitem
    res = list(collector._genfunctions(name, obj))
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/_pytest/python.py:378: in _genfunctions
    self.ihook.pytest_generate_tests(metafunc=metafunc)
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/pluggy/__init__.py:617: in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/pluggy/__init__.py:222: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/pluggy/__init__.py:216: in <lambda>
    firstresult=hook.spec_opts.get('firstresult'),
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/_pytest/python.py:125: in pytest_generate_tests
    metafunc.parametrize(*marker.args, **marker.kwargs)
../../base/Python.framework/Versions/2.7/lib/python2.7/site-packages/_pytest/python.py:805: in parametrize
    self.function, name, arg))
E   ValueError: <function test_something at 0x10e4969b0> uses no argument 'something'

It seems pytest is separating the concept of normal python arguments and 'fixtures' here, only classifying only default-free arguments as potential fixtures - and then only considering 'fixtures' as valid targets for parametrisation - but the error message completely obfuscates this: by any regular python interpretation the function definitely has an argument 'something'.

The documentation on this - https://docs.pytest.org/en/latest/parametrize.html doesn't show this as an example but also doesn't mention that it's a constraint; it's only through reading the pytest source that I could work out what the problem was.

Related: The wording of this was changed to 'argument' from 'fixture' in #1539 which makes sense in the context of that error, but only confuses this one.

[RonnyPfannschmidt]

pytest doesnt consider arguments with default values as targets for fixtures/parameterization

[ndevenish]

Yes, that is the root cause. But this isn't a question; my point is that the error is completely confusing and the solution non-obvious unless you read the source code:

pytest doesn't consider arguments with default values

and then complains that there is no argument.

So I'd consider it either a bug with error messages, argument-as-potential-fixture classification (although I assume this was rigorously discussed in the past and is like this for a reason), or a documentation problem.

[RonnyPfannschmidt]

thanks for expressing what you need for better understanding of the issue
i consider it an issue with the error message as well

[feuillemorte]

As I can see pytest ignores arguments with default values:

pytest/_pytest/compat.py

Lines 112 to 115 in e7bcc85

	arg_names = tuple(p.name for p in signature(function).parameters.values()
	if (p.kind is Parameter.POSITIONAL_OR_KEYWORD or
	p.kind is Parameter.KEYWORD_ONLY) and
	p.default is Parameter.empty)

Do we need to raise exception here? In this case all arguments with default value will catch this error

[RonnyPfannschmidt]

that function in particular is working exactly as documented, so just raising an error there is not a good way to sort it out