Currently assertRaises(), assertRaisesRegex(), assertWarns() and assertWarnsRegex() have some weird behavior.

    # always success if the callable is None
    self.assertRaises(SomeException, None)

    # keyword arguments except "msg" are ignored
    with self.assertRaises(SomeException, foobar=123):
        ...

    # always success because keyword arguments are ignored
    self.assertRaises(SomeException, callable=func)

Hardly any user code uses these "features" intentionally. More likely such examples are hidden bugs (see for example [1]). A DeprecationWarning is raised in these cases since 3.5 (issue24134), and it is time to make them errors.

[1] https://mail.python.org/pipermail/python-list/2018-July/736363.html