Inheritance = "Is an" association

Now the school classes have users. But so far a user does not know which school class he is in. This is because the navigation direction goes only from SchoolClass to User:

UML class and object diagrams: SchoolClass, User with direction of navigation

For the realization of both navigation directions an additional property schoolClass of type SchoolClass would be needed on the side of the user:

Realisation of both navigation directions

So far, so good. Unfortunately, this general User would no longer be suitable for teachers or guests of the school, since they do not have a school class! In this case, this attribute would not be required (Bad programming style, because of wasted memory and less readable code).

Inheritance

A way out of this situation is the specialized user Student, which can be easily derived from the class User:

Subclass Student inherits from Superclass User

"Is an" association

Inheritance fits only if the subclass is a kind of superclass type. The Student is an User. Therefore, inheritance can also be considered as an "Is an" association.

Coding in Python

class User:                                                 # superclass "User"
    def __init__(self, surname, firstname, dateOfBirth):    # constructor 
        self._surname = surname                             # protected properties/attributes => only one "_"
        self._firstname = firstname
        self._password = firstname[0] + surname[0] + dateOfBirth  
        self._dateOfBirth = dateOfBirth
        self._username = surname 
        
    def getSurname(self):                                  
        return self._surname
    
    def getFirstname(self):
        return self._firstname

    def getPassword(self):
        return self._password

    def getDateOfBirth(self):
        return self._password

    def getUsername(self):
        return self._password


class Student(User):    # subclass "Student" inherits every property and method from superclass "User"
    def __init__(self, surname, firstname, dateOfBirth, schoolClass):
        User.__init__(self, surname, firstname, dateOfBirth)  # constructor of superclass is invoked
        self.__schoolClass = schoolClass
    def getSchoolClass(self):              
        return self.__schoolClass

Because subclasses inherit every attribute and method of superclasses ("code reuse"), only additional fields (attributes and methods) have to be defined in subclass.

Note: The access modifier private __ in the superclass must be changed to protected _ so that the subclass can also access the properties.