Solution: Extend class "User" to our needs

UML class and object diagram

The code is devided here into two files.
Good practice is to outsource class definitions into a separate file that has the same name as the class itself:

User.py

class User:                                                 # class definition 
    def __init__(self, surname, firstname, dateOfBirth):    # constructor 
        self.__surname = surname                            # private properties/attributes 
        self.__firstname = firstname
        self.__password = firstname[0] + surname[0] + dateOfBirth  # assembling of initial password
        self.__dateOfBirth = dateOfBirth
        self.__username = surname 
        
    def getSurname(self):                                   # getter methods
        return self.__surname
    
    def getFirstname(self):
        return self.__firstname

    def getPassword(self):
        return self.__password

    def getDateOfBirth(self):
        return self.__dateOfBirth

    def getUsername(self):
        return self.__username

The creation of objects and method calls in a second file:

testUser.py

from User import User                       # Import from User.py in same directory the class User

user1 = User("Miller","Sam","22.11.2004")   # Creation of objects user1 and user2
user2 = User("Smith","Jake","13.02.2003")

print(user1.getPassword())                  # call of methods and output of return values
print(user2.getUsername())

The output

SM22.11.2004
Smith