Revise paste student_testing_exercise

class Student:
    def __init__(self, name: str, courses=None):
        if courses is None:
            courses = {}
        self.name = name
        self.courses = courses  # {course_name: [notes]}

def enroll(self, course_name: str, notes, add_course_notes: str = ""):
        if course_name in self.courses.keys():
            [self.courses[course_name].append(x) for x in notes]
            return "Course already added. Notes have been updated."

if add_course_notes == "Y" or add_course_notes == "":
            self.courses[course_name] = notes
            return "Course and course notes have been added."

self.courses[course_name] = []
        return "Course has been added."

def add_notes(self, course_name, notes):
        if course_name in self.courses.keys():
            self.courses[course_name].append(notes)
            return "Notes have been updated"
        raise Exception("Cannot add notes. Course not found.")

def leave_course(self, course_name):
        if course_name in self.courses.keys():
            self.courses.pop(course_name)
            return "Course has been removed"
        raise Exception("Cannot remove course. Course not found.")