I want to ensure that *no warning at all* is raised in one assertion.

Could not find any explicit answer in [pytest documentation about warnings](https://docs.pytest.org/en/3.2.1/warnings.html#assertwarnings). (dead link, the 3.2.\* doc is not available anymore).

I've tried this, thinking maybe `None` would mean "nothing":

```
def test_AttrStr_parse_warnings():
    """Check _AttrStr.parse() raises proper warnings in proper cases."""
    with pytest.warns(None):
        _AttrStr('').parse()

```

but this assertion is also always correct, for instance, the test does not fail, even if a warning is actually raised:

```
def test_AttrStr_parse_warnings():
    """Check _AttrStr.parse() raises proper warnings in proper cases."""
    with pytest.warns(None):
        _AttrStr('').parse()
        warnings.warn('any message')

```