You have a list of students:

class Student:
    def __init__(self, name, grade, age):
        self.name = name
        self.grade = grade
        self.age = age
    def __repr__(self):
        return repr((self.name, self.grade, self.age))

student_objects = [
    Student('john', 'A', 15),
    Student('jane', 'B', 12),
    Student('dave', 'B', 10),
]

Please sort them by their age. What if you want a descending order?
?

# ascending order:
student_objects.sort(key=lambda student: student.age) # careful returns none

# descending order:
student_objects.sort(key=lambda student: student.age, reverse=True) # careful returns none