class Course:
    '''Class representing a course in which students
    can enroll.'''
    
    def __init__(self, department, number, name, limit):
        '''Create a new course with specified dept, num, name, 
        enrollment limit.'''
        self.department = department
        self.number = number
        self.name = name
        self.limit = limit
        self.enrolled_students = set()
    
    def size(self):
        '''return the number of enrolled students in the course'''
        return len(self.enrolled_students)
    
    def add_student(self, new_student):
        '''adds new_student to the course'''
        if self.size() == self.limit:
            # can't add, at capacity
            print("Course is at capacity")
            return False
        else:
            # can add
            self.enrolled_students.add(new_student)
            print("Enrolled " + str(new_student) + " in " + self.name)
            return True


class Student:
    '''Class representing a student with a name,
    an age, and a major.'''
    
    def __init__(self, name, age, major):
        '''Initializes a new student object.'''
        self.name = name
        self.age = age
        self.major = major
        self.courses = set()
    
    def add_course(self, course):
        '''Tries to enroll in course and returns whether
        enrollment was successful.'''
        if course in self.courses:
            print(self.name + " already enrolled in " + course.name)
            return False
        elif course.add_student(self): # try to enroll (may fail)
            self.courses.add(course)
            print(self.name + " is now enrolled in " + course.name)
            return True
        else: # tried to enroll, but failed
            print(self.name + " could not enroll in " + course.name)
            return False
        
    def drop_course(self, course_name):
        '''Tries to drop course_name and returns whether the drop was successful.'''
        if course_name in self.courses:
            self.courses.remove(course_name)
            return True
        else:
            return False

    def birthyear(self):
        '''Returns the birth year of the student.'''
        return 2015 - self.age
    
    def switch_major(self, new_major):
        self.major = new_major
        print(self.name + " changed major to " + self.major)
        
    def __str__(self):
        '''returns a textual representation of the student object'''
        return self.name + ", age " + str(self.age)
        

course1 = Course('Biology', 423, 'Evolution of Squirrels', 30)
course2 = Course('History', 270, 'Computers of the 12th Century', 20)
course3 = Course('Government', 352, 'Galactic Empires', 1)

student1 = Student('Alice', 19, 'math')
student2 = Student('Bob', 18, 'english')
student3 = Student('Charlie', 20, 'biology')

print(student1) # calls __str__ method to convert to a string

student1.add_course(course1)
student1.add_course(course2)
# ... etc ...