def METHOD_NAME(self):
    form = AptNumberWithConfirmationForm(data={"apt_number": "3B", "no_apt_number": True})
    form.full_clean()
    assert form.errors == {
        "__all__": [
            "Please either provide an apartment number or check "
            'the "I have no apartment number" checkbox (but not both).'
        ]
    }