Functional Classes

Happy Wednesday!

Quick Announcements

• Project ImageShop is ongoing and due next week Monday
  • I encourage you NOT to wait until the late hour to start the projects or ask for extension
• Second midterm exam is next week Friday 6th of November.
  • More details next week
  • No Project or homework next week… Use the period to prepare for the midterm 2 exam
• Remember that class attendance is graded as part of the Participation grade component of this class
  • Several unexcused absenses from the class or session meeting will impact your grade significantly.

    • So, PLEASE, show up!!!

Getters and Setters

• In the object-oriented model, the client is not supposed to muck-about with the object internals

• The implementation should therefore provide methods to retrieve desired attributes (called getters) or to make changes to desired attributes (called setters)

• Setting up getters and setters for the attribute salary might look like:

  def get_salary(self):
    return self.salary
  
  def set_salary(self, new_salary):
    self.salary = new_salary

• Getters are far more common than setters, as you don’t always want the client to have the freedom to change attributes on a whim

Representation

• Printing out an object that you just created as an instance of a custom class will look ugly:

  >>> C = Employee('Bob', 'clerk', 15)
  >>> print(C)
  <__main__.Employee object at 0x7f942ba13550>

• You can define special methods for a class that specify how your object should be converted to a string (or anything else really)

  • All these special methods have double underscores before and after, and hence are frequently termed “dunder” (double underscore) methods
  • You can use the __str__ or __repr__ method to specify how your object might be printed

A Good Employee

class Employee:
    def __init__(self, name, title, salary):
        self.name = name
        self.title = title
        self.salary = salary

    def __str__(self):
        return f"{self.name} ({self.title}): {self.salary}"

    def get_salary(self):
        return self.salary

    def set_salary(self, new_salary):
        self.salary = new_salary

Daily LO’s

• Practice defining instance methods
• Use self properly within our method definitions
• Write some getters and setters
• Print out our custom objects in a useful way

Problem 1: Demo Tracing

class Demo:
    def __init__(self):
        self.x = []

    def add(self, v):
        self.x.append(v)

    def get_x(self):
        return self.x

A, B = Demo(), Demo()
C = B.get_x()
A.add(3)
B.add(3)
C.append(A)
print(A.get_x() == B.get_x())

Problem 2

class Pet:
    def __init__(self, name, color, age):
        self.name = name
        self.color="black"
        self.age = age

    def bmi(self, weight, height):
        if weight/height > 2:
            self.age = self.age*2
        else:
            self.age += 1
dog = Pet("Loly", "brown", 2)
dog.bmi(180,45)
print(f'{dog.name}, {dog.color}, {dog.age}')

Activity 3A: Banking Transaction

• Create a BankAccount class with the following requirements:
  • Private attributes: _account_number, _balance, _owner_name
  • Constructor to initialize these attributes
  • Getter and setter methods for all attributes

Activity 3B: Banking Transaction

• Implement the normal transactions but with security check to be sure the transaction is valid!
  • Method to deposit money
  • Method to withdraw money
  • Method to display account information

Activity 3C: Banking Transaction

• Display the account information and with some valid transactions: the formatting string will be your friend here!
  • Implement a method to display account information