Prevent pytest.raises from expecting an exception

[tucked]

Here is an example of a case where it would be really handy to be able to tell pytest.raises not to expect an exception:

@pytest.mark.parametrize(
    'exception_for_object',
    [
        SomeError, some_obj1,
        OtherError, some_obj2,
        (), some_obj3,  # does not raise an exception
    ],
)
def test_thing(exception_for_object):
    exception, obj = exception_for_object
    with pytest.raises(exception):
        obj.thing()

The result, though, is this:

>           obj.thing()
E           Failed: DID NOT RAISE ()

This behavior is not useful because obj.thing couldn't raise () anyway:

>>> raise ()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions must derive from BaseException

Not even in Python 2:

>>> raise ()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions must be old-style classes or derived from BaseException, not tuple

Note that None doesn't work instead of ().

>           obj.thing()
E           TypeError: issubclass() arg 2 must be a class or tuple of classes

followed by

>           obj.thing()
E           Failed: DID NOT RAISE None

---

Tox dump of pip list (running on CentOS 7.6.1810):

atomicwrites==1.3.0,attrs==19.1.0,certifi==2019.3.9,chardet==3.0.4,coverage==4.5.3,funcsigs==1.0.2,future==0.16.0,idna==2.8,isi-sdk-7-2==0.2.5,isi-sdk-8-0==0.2.5,isi-sdk-8-0-1==0.2.5,isi-sdk-8-1-0==0.2.5,isi-sdk-8-1-1==0.2.5,isilon-hadoop-tools==2.0.0,mock==2.0.0,more-itertools==5.0.0,pathlib2==2.3.3,pbr==5.1.3,pluggy==0.9.0,py==1.8.0,pytest==4.3.1,pytest-cov==2.6.1,pytest-randomly==1.2.3,python-dateutil==2.8.0,python-kadmin==0.1.2,python-kadmin-local==0.1.2,requests==2.21.0,scandir==1.10.0,six==1.12.0,urllib3==1.21.1

[tucked]

Here's a workaround:

def test_thing(exception_for_object):
    exception, obj = exception_for_object
    try:
        obj.thing()
    except exception:
        pass
    else:
        assert not exception

[blueyed]

Why not:

def test_thing(exception_for_object):
    exception, obj = exception_for_object
    if exception:
        with pytest.raises(exception):
            obj.thing()
    else:
        obj.thing()

But check out also https://github.com/blueyed/pytest/blob/253415cf397fd3d73d0dac0478d27075bdcea1f6/doc/en/example/parametrize.rst#parametrizing-conditional-raising

[blueyed]

However, I can see that pytest.raises(False) (or better None I guess) would be nice to have.

[tucked]

@blueyed I try to avoid splitting tests like that in favor of just having 2 tests. That link is pretty cool, though! Hadn't thought of that...

[RonnyPfannschmidt]

Im on mobile, bit this has been discussed before, use a different context manager

[asottile]

#1830 #4324 #4682