Courses 20/100 solution

def register(data):
    courses = {}
    students = {}
    result = ""

    while data != "end":
        course_name, student_name = data.split(" : ")

        if course_name not in courses:
            courses[course_name] = 1
        elif course_name in courses:
            courses[course_name] += 1

        students[student_name] = course_name

        data = input()

    ordered_courses = sorted(courses.items(), key=lambda x: (-x[1], x[0]))
    ordered_students = sorted(students.items())

    for course, count in ordered_courses:
        result += f"{course}: {count}\n"
        for student, subject in ordered_students:
            if subject == course:
                result += f"-- {student}\n"

    return result


action = input()
print(register(action))