Construction Zone

Announcements

• Problem sets 5 is due tomorrow at 10pm.
• Midterm 2 is on Friday, and it will last for an hour (class time)
  • No assignment this week. Just practice chapter 6 exercises to prepare for the exams
  • It is totally open exam and could be taken from anywhere
• Week 12 is Imageshop project and Week 13 is your personal project
  • You might want to use part of the break to engage with your personal project
• Polling: https://www.polleverywhere.com/agbofred203

Recall!

• We could accomplish a more more general class attributes by passing arguments to our function:

  def create_employee(name, title, salary):
    emp = Employee()
    emp.name = name
    emp.title = title
    emp.salary = salary
    return emp

• We could then use that as:

  clerk = create_employee('Bob Cratchit', 'clerk', 15)
  boss = create_employee(
            'Ebeneezer Scrooge', 'founder', 1000
            )

Constructors

• While the previous method works, it is not ideal
  • Forces the client to tinker with the internal workings of the Employee
  • Details of the data structure are the property of the implementation, not the client
• Better to add a method to the Employee class called a constructor, which is responsible for initializing attributes to a newly created object
  • In Python, a constructor is created by defining a special function named __init__
  • The constructor function is called automatically whenever a new object of that type is created

Know Thy self

An Employee Constructor

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


clerk = Employee('Bob Cratchit', 'clerk', 15)

• Note that you do not need to provide an argument for self when creating the object, Python supplies this reference automatically
• Viewing in PythonTutor can be helpful, as is shown here

Understanding Check

class Car:
    def __init__(self, color, year):
        self.color = color
        self.year = year
        self.make = 'Toyota'

A = Car('blue', 2008)
B = Car('red', 2006)
A.make = 'Honda'
A.year = B.year
print(A.make, A.color, A.year)

What’s your Method?

• Most classes define additional functions called methods to allow clients to read or update attributes or manipulate the object

• Methods look like a normal function definition but will always declare the parameter self at the beginning of the parameter list

  • This is true even if the method has no other parameters

• Methods are defined in the body of the class and would thus look something like:

  def method_name (self, other_parameters):
    ...body of the method...

• For example

  def give_raise(self, amount):
    self.salary += amount

Accessing and Using Methods

• After definition, there are two mains ways you can access and use the method:
  • Dot Notation (Conventional)

    • Python sets self to be a reference to the receiver, which is the object to which the method is applied

      clerk = Employee('Bob', 'clerk', 15)
      clerk.give_raise(15)

  • Function Notation

    • You retrieve the method from the class itself, and then provide self manually

      clerk = Employee('Bob', 'clerk', 15)
      Employee.give_raise(clerk, 15)

Visualization Summary

• To summarize in a visual manner, we can look at everything together on PythonTutor

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 the __str__ or __repr__ method to specify how your object

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