Do you know why there is assertRaises(), but there is no assertNotRaises()?

Because assertNotRaises(Exception, callable, *args, **kwargs) would be equivalent to just callable(*args, **kwargs). Call the function, and if is raised an exception, the test will fail.

Similarly, there is no need of assert_not_called_with(). Just define a function which raises an exception when it is called with the specified arguments.