Nitpick: if you're writing a custom assertion function, you may want to pass a message string to the `assertEqual` function as a third argument. By default, if the comparison fails, it will display an error message like "Error: 0 is not equal to 1", which is not very helpful. You may want to pass a string like "ext_user not expected to be enrolled in self.course, but is", so that if/when the test fails, it's easier to understand what is going wrong.